December 11,2014 

Mr. Joseph M. Kellmeyer 

One U.S. Bank Plaza 

St Louis, Missouri 63101 

Re: Evaluation of the Xact® Multi-metals CEMS 

Mr. Kellmeyer: 

Pursuant to your request, I have evaluated the use of the Xact® Multi-metals Continuous 

Emissions Monitoring System (Xact CEMS) as set forth in the October, 2014 Draft Air Pollution 

Control Title V permit to Operate for Veolia ES Technical Solutions, L.L.C. in Sauget, Illinois. 

As you are aware, I have management and engineering experience in waste incineration, 

chemical manufacturing operations and environmental remediation dating back to 1980. My 

experience includes work in hazardous waste incineration, hazardous waste fuels management 

and combustion, uranium recovery from phosphoric acid, and the production of sulfuric acid, 

phosphoric acid, nitric acid, ammonia, ammonium nitrates, granular phosphate fertilizers, and 

nitrogen fertilizers. In recent years and through my employment at B3 Systems, Inc., I have 

become extremely familiar with Particulate Monitors (PM) for EPA MACT Compliance and 

Process Control. 

In the October, 2014 Draft Air Pollution Control Title V permit to Operate for Veolia, USEPA 

Region V proposes that Veolia "install, calibrate, maintain and operate an x-ray flourescence 

multi-metals CEMS for use as a continuous parametric monitoring system (CPMS) on Units 2, 3 

and 4 within 12 months (365 days) after this permit becomes effective..." Draft Title V Permit 

at 34. In footnote 46 of the Statement of Basis, USEPA states that it is "aware of one 

commercially available multi-metals CEMS - the Xact® Multi-metals CEMS manufactured and 

distributed by Cooper Environmental Services, LLC ("Cooper"). 

In order to provide you with my opinion, I have extensively investigated the Xact CEMS. Based 

upon my investigation and my knowledge of PM Monitors and stack emissions, I have 

concluded that if the Xact CEMS were installed and operated as proposed in the draft Title V 

permit, the Xact CEMS would not provide an accurate indication of metals emissions. Due to my 

experience with PM CEMS and knowledge of the dynamic PM flow in ducts and stacks, I do not 

believe the Xact CEMS will produce results with the accuracy indicated by EPA and the previous 

validation work. Further, I believe that Method 301 work is biased positive by the testing 

scenario (e.g, using the QAG and not Method 29 under process conditions). 

Key Issues 

The following is a summary of key items raised about the Xact CEMS: 

Draft Permit Language & Statement of Basis [8] [11] 

• USEPA states the Xact CEMS is being installed as a continuous parametric monitoring 

system ("CPMS"). However, a CPMS limit on an operational combustor is always 

6073833.2 

VES016997 
established during CPT testing in order to establish a known relationship, if any, between 

the CPMS and the parameter that is sought to be monitored. 

• USEPA states the Xact CEMS is being established as a CPMS on a temporary basis. 

However, this temporary CPMS is being used as an absolute emission rate for providing 

evidence of deviations that indicate that Veolia may not be in compliance with a HWC 

MACT metals emissions limit. Based on my experience, the HWC MACT recognized 

feedstream analysis plan is used as evidence of compliance - a temporary CPMS is never 

used as evidence of compliance. Draft Permit at 55. 

• Based on my experience, a temporary CPMS that has not been established through CPT 

testing should not have any impact on a feedstream analysis plan that is permanent and 

was established through CPT testing. 

Process Conditions 

• The majority of the metals from the dry process that Veolia uses are in the form of 

particulate matter (PM). 

• The biggest concern for any device monitoring PM, including a Xact CEMS that purports 

to monitor multi-metal particulates, is obtaining a representative sample at the detector. 

• As a result of variations during normal operation, the PM emitted is not size selective. 

• Further, industry and academia provides several references and studies for pneumatic 

conveying of solids such as multi-metal PM found on stacks. After a bend, the majority 

of studies document "roping" or high concentration of PM on outer wall. Velocity profile 

normalizes, but higher PM concentrations stay closer to the wall and can be shifted 

around based on the gas flow dynamics that can vary during normal operation. 

• In Veolia's dry process, the induced draft (ID) fan defines the PM mixing and dispersion 

in the downstream duct/stack. 

• PM stratification in systems such as Veolia is part of a dynamic process that varies during 

normal operations. The dynamic changes in a system (flow, temperature, particle size, 

particle density, etc.) will also change the PM dispersion. 

• Based on a review of stack velocities at Veolia from the 2013 CPT results, the gas 

velocities are high enough to easily carry particle sizes up to 100-150 microns. 

Xact CEMS 

• The Xact CEMS uses Other Test Method 16 (OTM16) which Cooper developed by which 

the Xact CEMS purports to obtain a discrete, quantifiable reading for each of the multimetals that it tests. By comparison, PM CEMS use Performance Specification 11 ("PS 

11") and do not use OTM 16. Further, PM CEMS result is merely a "relative" output and 

6073833.2 -2-

VES016998 
requires Method 5 (M5) to calibrate it. PM CEMS do not obtain a discrete, quantifiable 

reading. 

• The Xact CEMS has been validated, using M301, against an early version of the 

quantitative aerosol generator (QAG). The early versions of the QAG generated particles 

approximately 1 micron in size or less which are particles that transport well. 

Unfortunately, the QAG is not representative of Veolia's process conditions which may 

consist of larger particles. 

• The Xact CEMS works from a single point sampling location. A single point sampling 

location cannot obtain a sample as representative as a traversing sampling collection 

system under changing process conditions. 

Overview and Further Discussion 

Based on my experience and the documents/information reviewed, the performance 

specifications written for the Xact CEMS are similar to those written for gaseous CEMS 

applications. However, an Xact CEMS functions more like a PM CEMS and gas CEMS (e.g., 

CO, 02, THC, etc.) in the fact that it must sample particulate matter. Therefore, based on 

experience and knowledge that B3 Systems has gained from PM CEMS applications, basic 

performance specification requirements developed for the Xact CEMS are not valid for 

transporting multi-metal PM for quantification. 

Based on our experience and research, the biggest issue with the Xact CEMS is getting a 

representative sample to the detector. Transporting PM is not comparable to transporting a gas. 

[1]. 

Work with PM CEMS demonstrates that systems can and do have PM stratification issues. Veolia 

uses a baghouse for PM removal. However, based upon my review of the 2009 and 2013 CPT 

test data, particle size may vary depending upon operational conditions and dry particles of all 

sizes may be transferred into the clean-gas side of the baghouse and carried to the stack. Given 

this fact, the representativeness of a single point source such as the source the Xact CEMS uses 

to sample is very questionable. I have observed while working with PM CEMS that a change in 

particle density can and does affect the accuracy of the results of the single point sampling 

systems such as the sampling system which exists in the Xact CEMS. 

PM CEMS vs Xact CEMS 

The PM CEMS and the Xact CEMS sample particulate matter, with the PM CEMS indicating 

total particulate matter and the Xact CEMS measuring particulate matter for selective multimetal content. USEPA Performance Specification 11 (PS 11) defines the procedures for PM 

CEMS. PS11 uses an EPAManual Reference Method (RM) to calibrate the PM CEMS under 

three (3) levels of PM concentration. This means that if the monitoring location is not ideal, then 

6073833.2 - 3 -

VES016999 
the data from the traversing RM is used to calibrate the single point sampling CEMS. 1

 The RM 

results compensate for the imperfection in the single point sampling location. 

The Xact CEMS on the other hand measure metals in absolute terms and purports to obtain a 

representative sample from a single sampling point. Therefore, if the Xact CEMS fails to obtain 

a representative sample from the stack to be examined by the detector, the output of the Xact 

CEMS will be incorrect. Based on the actual sampling location ofthe Xact CEMS, there is a 

very high possibility that the Xact CEMS output will not match the actual emissions since it does 

not have any way of compensating for PM stratification under changing process conditions. 

Depending on the stack gas sampling location, the sample collected could be higher or lower in 

metals/PM than the overall average concentration. 

Sample Transport 

The Xact® has been challenged against the QAG. 1 think the QAG is an excellent tool to verify 

the detector, similar to a calibration gas for other CEMS since it generates a known concentration 

of metal. However I feel that the QAG does not represent reality when it comes to representing 

process conditions at Veolia. Only recently, and based on work for PM CEMS calibration, has 

Cooper Environmental Services (CES) created a version ofthe QAG that create particle sizes 

much greater than 1 micron [4]. During the time that the Xact CEMS was being validated, the 

QAG being used to validate the Xact® could have only produced metal salts in the resulting gas 

stream that were approximately 1 micron or less. Particles being emitted from industrial 

processes are not controlled or defined by approximately 1 micron or less. Based on data from 

Veolia's 2013 CPT testing, stack velocities across all sampling trains on Units 2, 3 and 4 

averaged 31.2 ft/sec 30.8 ft/sec and 56.8 ft/sec, respectively. At 30 ft/sec (9.1 m/s), large 

particles, 100 micron and greater, can be transported up the stack based on a review of terminal 

settling velocities. [3]. 

Particles of almost any size are denser than their surrounding fluid (air) and therefore have more 

mass. This extra mass means they do not act like a gas, especially once the particle size starts to 

be greater than 5-10 microns. I have reviewed several studies [4] [5] [6] on pneumatic conveying 

of particles. In almost every case, the research discovered that after the particles made a turn, 

such as transitioning from a horizontal duct to a stack, the particles formed a rope, "horse shoe" 

or "U" pattern on the opposite side of the vertical wall. This means that the particles on the wall 

are not in the Centroid (i.e. not in the center of the stack) that OTM-16 requires for sampling. 

The sample required by OTM-16 is therefore not representative ofthe conditions in the stack. 

This error is compounded when the Xact® takes its sub-sample since it is very probable that 

larger particles could still be near the wall and therefore the sub-sample would miss them. 

The particles generated by the QAG in use at the time the Xact CEMS was being validated were 

very fine and would be less affected by these conditions. Therefore, I do not believe that the 

QAG is representative of actual process conditions at Veolia and will not provide representative 

results when auditing the Xact CEMS transport system and/or validating the overall system at 

Veolia. 

1 Traversing in its simplest meaning is taking samples from multiple locations in the stack and 

intergrating these into one sample. 

6073833.2 - 4-

VES017000 
PM transportation is easiest in a downward, vertical direction due to gravity or in an upward, 

vertical direction due to the force ofthe gases working against the gravitational pull. Particle 

transportation is most difficult for horizontal and inclined runs. EPA reference methods use 

sampling probes (i.e. horizontal transport runs). Although the stack gases are being sampled 

isokinetically (i.e. at the same velocity as the stack flow), based on the nozzle diameter, the 

actual flow rate in the probe cannot carry the larger particles through the probe and to the filter. 

This dropping out of the PM is known as "saltation" and accounts for as much as 60% to 80% of 

the PM ending up in the probe. This is easily seen by reviewing the detailed PM results from the 

CPT test, Tables 4-14,4-15 & 4-16 ofthe CPT Report. This is further supported by detailed 

analysis of the metals sampling trains in Tables 4-20,4-22 & 4-24. By reviewing the total metals 

catch in the probe & nozzle rinse (PNR) to the total you can see that the large portion ofthe 

metals show up in the probe, with the exception of mercury. EPA reference methods require the 

recovery of the PM/metals from the probe and nozzle for analysis. The Xact CEMS does not 

require recovery of the multi-metals PM from the probe and umbilical. If the PM does not make 

it to the filter of the Xact®, then the results will not be analyzed and the Xact CEMS will not 

produce results consistent with the actual stack gases. Likewise, if accumulated PM in the probe 

nozzle, and umbilical break free during operation, those PM - potentially carrying metals - would 

be analyzed by the Xact CEMS even though they are not related to current stack conditions. 

In summary, the Xact system has multiple opportunities for sample transport related issues. The 

first is obtaining a representative sample from the stack. The second is transporting the stack 

sample through the large diameter stack probe [10] and umbilical to the stilling chamber. The 

third is taking a representative isokinetic subsample [12] from the stilling well. The final 

transport related issue is getting the subsample to the filter tape for analysis. Based on my 

knowledge and experience with PM CEMS and solids transport, I do not think the Xact can 

obtain an absolute representative sample at the detector versus the EPA reference method (M29). 

I think the dynamic conditions in the stack gases combined with the sample transport issues 

discussed above are impossible to overcome for a single point sampling system where a large 

majority ofthe metals are in the form of PM. 

Cooper and USEPA make much of Xact CEMS use for a brief period of time at the Eli Lilly 

facility in Indiana. The Eli Lilly facility in Indiana used a wet scrubber, not the dry baghouse 

system utilized by Veolia in Sauget, Illinois. The systems at the two facilities are completely 

different and therefore produce PM including multi-metal PM that are of a completely different 

type, size and characteristic. Thus, whether the Xact CEMS worked and produced accurate 

results at Eli Lilly is not predictive as to whether the XAC T CEMS will work and produce 

accurate results at Veolia. 

Forthe reasons set forth above, I do not believe the Xact CEMS will produce accurate results if 

employed at the Veolia Sauget Illinois facility. 

6073833.2 - 5 -

VES017001 
References 

1. The one exception is that small PM particles may "act like a gas", but primarily for submicron particles (< 1 micron) and specifically where particle approach molecular size 

(~0.1 micron) "Fundamentals of Gas Solids/Liquids Separation, Mueller Environmental 

Designs, Inc. 

2. "An Investigation of the Particle Dynamics of a Multi-component Solid Phase in a Dilute 

Phase Pneumatic Conveying System", Lu, Tong, Thesis submitted for the degree of 

Doctor of Philosophy, The University of Edinburgh, March 09 

3. "Perry's Chemical Engineering Handbook", Fourth Edition, Figure 5-74, Pg 5-62 

4. "Quantitative Aerosol Generator (QAG) for PM CEMS Calibration", Presentation at 

EUEC 2014 

5. "Mixing and dispersion of particle ropes in lean phase pneumatic conveying", Harun 

Bilirgen, Edward K. Levy, Energy Research Center, Lehigh University, 117 ATLSS 

Drive, Bethlehem, PA 18015, USA 

6. "Experimental and Numerical Study of Dilute Gas-Solid Flow inside a 90° Horizontal 

Square Pipe Bend", Walid Aniss Aissal, Tarek Abdel Malak Mekhaill, Soubhi Ali 

Hassanein, Osama Hamdy, Open Journal of Fluid Dynamics, 2013, 3, 331-339 

7. "Validation of Three New Methods for Determination of Metals Emissions Using a 

Modified Environmental Protection Agency Method 301", Journal of Air and Waste 

Management Association, 2006 

8. Statement of Basis for DRAFT Title V Permit No. V-IL-1716300103-2014-10, October 

2014 

9. Comprehensive Performance Test Report for Fixed Hearth Incinerator - Unit 2, Fixed 

Hearth Incinerator - Unit 3, Rotary Kiln Incinerator - Unit 4, January 2014 

10. Review of Monitoring Experience with the Xact Multi-Metals CEMS, Krag Peterson, 

Cooper Environmental, IT3 2014 Paper & Presentation, Paper 29 

11. DRAFT Title V Permit No. V-IL-1716300103-2014-10, October 2014 

12. XACT TM 640 Monitoring System Spec Sheet, Cooper Environmental 

6073833.2 - 6 -
VES017003-017018.pdf 
^^^ J FUNDAMENTALS OF 

^ c - fwd GAS SOLIDS/LIQUIDS 

m i

 SEPARATION 

Mueller 

Environmental 

Designs, Inc 

NoiselContro l 

Mis t Eliminatio n 

VES017003 
Fundamentals of Gas Solids/Liquids Separation 

Many process operations require the removal of entrained non-gas particles from multi-phase gas 

streams. The removal of these non-gas particles is the process in itself (capture of a valuable 

product) or the process of cleaning a gas stream in order to protect either stationary or rotating 

equipment from the harmful effects due to non-gas particles entering those devices . 

The removal of entrained non-gas particles from a multi-phase gas stream is a separation process 

involving the removal of: 

Liquid particles from a gas or vapor stream; 

Solid particles from a gas or vapor stream; 

Combination of both liquid and solid particles from a gas or vapor stream. 

In order to understand mechanical separation concepts, a basic understanding of particle formation, 

physics and motion is required. 

Particle Formation 

Non gas particles are generated and entrained in the gas stream (spray regime) or are converted 

to a sheet flow (annular regime) and carried on the fluid stream boundary. These liquid non-gas 

particles can be generated from a pure gas due to a gas-liquid phase change occurring within 

a state change of the gas. This change of state is commonly known as condensation. The 

following graphic is the pressure-volume chart for a typical vapor. 

It is only in the gas liquid multi-phase 

zone that liquid non gas particles can 

exist. 

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Figure 1 Pressure - Volume Relation For Typical Real Gas 

300 

The critical point "C " (See Pressure -

Volume Chart Graphic) is the state point 

at which the liquid and gas phases are 

identical. At higher pressures along the 

critical isotherm, a slight increase or 

decreas e in temperatur e wil l 

instantaneously produce a complete 

phase change. At a temperature higher 

than the critical isotherm, no liquid phase 

will be encountered. At temperatures 

below the critical isotherm gas can exist 

as both a gas or liquid. The amount of 

liquid per unit volume of mixture ca n be 

estimated using the known physical 

properties of the mixture. 

Liquid particles can be formed in mid 

stream by condensing initially at the 

molecular size level. These particles then 

can be carried by the fluid stream . 

Condensatio n on cool surface s will 

produce a liquid film which is swept off 

the surface by drag force due to the gas 

stream velocity being greater than the 

surface tension on the liquid. 

Liquids injected into the stream may be 

borne along in the stream if in smal l 

VES017004 
particle form. The liquid stream injected will be 

disbursed initially due to that force overcoming 

the surface tension forces. The liquid will become 

part of the mixture and has the potential to also 

change state. 

Small liquid particles of like substances will join 

with one another upon collision in what is called 

coalescence. The rate at which the particles with 

no net movement will join together is dependent 

on gas viscosity and temperature and the number 

of particles present. Higher temperatures, lower 

viscosities, and flow stream agitation will provide 

higher coalescence rate. Liquid present within 

the gas stream will flow in one of several flow 

regimes. 

Figure 2 depicts the various flow regimes that 

occur within multi-phase gas streams. The two 

flow regimes that provide the lowest liquid to gas 

weight ratio are annular and spray flow. Annular 

flow is characterized by finite liquid amounts 

carried in suspension with significant amounts along the pipe wall. Spray flow regime is 

characterized by particles in suspension in the gas with negligible amounts of liquids on the pipe 

wall. Slug flow regime is often encountered in gas streams with greater liquid to gas weight ratio 

than annular or spray regimes. 

Liquid particles are generally idealized as being spherical in shape since this is the shape surface 

tension forces impart. Liquid particles might have shapes other than this depending on other 

forces such as gravity and drag. Solid particles are also idealized as being spherical, though 

in reality have irregular shapes. 

Black Powder 

What is it? Black powder is a catchall term that describes material that collects in gas pipe 

lines and creates wear and reduced compressor efficiency, clogged instrumentation and 

valves, and flow losses in long pipe lines. The material can be wet, with a tar-like appearance, 

or dry and be a fine powder. Chemical analyses reveal it is any of several forms of iron sulfide 

and iron oxide. Further, it can be mechanically mixed or chemically combined with any number 

of contaminants, such as water, liquid hydrocarbons, salts, chlorides, sand or dirt. Some pipe 

lines have black-powder problems and others do not. It appears those lines closer to the gas 

gathering end of the system have problems, while those at the distribution end, with relatively 

small systems, do not. Black powder is found in both "dry" and "wet" lines. One parallel line 

can have a problem while the other does not. No pipe line has been identified to date that 

has been able to eliminate the problem once it starts. 

Flow Regimes 

GAS—* ) 

(a) Bubble (e) Slug 

GAS 

(b) Plug (f) Semi-annular 

GAS 

(c) Stratified 

GAS 

(d) Wavy 

GAS 

(g) Annular 

(h) Spray 

Figure 2 

VES017005 
Particle Physics 

The forces impose d on particles in 

mechanical separators are gravity, drag 

and inertial forces. 

Gravity forces act vertically downward. 

Drag forces are resistive forces. Inertial 

forces are a result of particle circular 

motion. Particle circular motion generates 

centripetal force; a force that is directed 

towards the center of the circular motion. 

Opposite to centripetal force is centrifugal 

force and is due to a forced turning of the 

gas stream, and acts at right angles to the 

particle angular motion. 

Centrifuga l forc e is the principl e 

determinant in the operation of cyclone 

and impingement type separators. This 

action is best described in a rectangular 

vector coordinates as shown in figure 3. 

The particle in motion, if not at the wall, 

will accelerate radially to the wall, and in 

the process absorb some of the centrifugal 

force. 

The particle drag forces act in the direction 

ofthe particle motion. Drag forces are in 

response to the viscous effects of the gas 

opposing the particle motion. Drag forces are composed of pressure differences and shearing 

stresses on the surface of the particle. 

Additionally, a buoyant force is present due to displacement of the gas by the particle. This force 

is insignificant on particles greater than 0.1 microns. Particles 0.1 microns and smaller approach 

molecular size where buoyant forces become considerable in relation to particle size and become 

a factor in Brownian Movement. Brownian Movement is the motion exhibited by small particles 

that move non-uniformly along the gas streamlines due to collisions with gas molecules (additional 

information regarding Brownian Movement will follow). 

Significant complex particle motion (rectilinear and curvilinear) translation exists about the axis 

of the particle. Pure particle motion about the rotational axis exists but is not as significant as 

the complex particle motion translation. 

On the particle boundary layer shear stresses of the gas stream are opposed by the surface 

tension ofthe gas-liquid interface. The liquid particle system may change relative to the magnitude 

of the shear stress and surface tension; such as in the instance of liquid particles ma y be 

generated from sheet flow (stream of liquid) and large particles may be broken into smaller 

particles if the shear stress is larger than the surface tension. 

As previously established, the physics that govern separator operation are those which determine 

the forces and stresses acting on particles in the gas stream. The physical phenomena that 

generate drag forces are dependent on particle size, shape, velocity, and on the gas stream's 

density and viscosity. 

For small particles or low velocities, viscous forces are dominant. For larger particles and higher 

velocities inertial forces are dominant. This dynamic ratio of inertial to viscous forces is a 

dimensionless number referred to as the Reynolds Number abbreviated - Re. The Reynolds 

Nurnber provides a determination of which regime, inertial or viscous, is dominant. This dynamic 

ratio is graphically depicted in Figure 4. 

Gas Motion 

Particle being propelled by gas, with the gas velocity 

exceeding the particle velocity. 

The particle is said to "drag" behind the gas, and this 

retarding (slippage) is caused by the drag force being slightly 

less than the gas impact force on the particle. 

Gas Rotation 

Note: ~--i<'' 

Fc - Centrifugal 

Fd - Drag 

Fg - Gravity 

EF - Total 

Forces acting on a particle due to circular acceleration 

generates particle motion along vector EF. 

VES017006 
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Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

\ 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

• — 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

1

 V" 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

\ 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

s Place of the English Units Specified) : 

Symbols and Legend 

A p = Area of Particle Projected on Plane Normal to 

direction of Flow or Motion, sq. ft. 

C = Overall Drag Coefficient, Dimensionless 

Dp = Diameter of Particle, ft. 

F d = Drag or Resistance to Motion of Body in Fluid, Poundals 

N R e - Reynolds Number, Dimensionless 

u = Relative Velocity Between Particle and Main Body of 

Fluid, ft./sec, 

^ = Fluid Viscosity, {lb.mass)/{ft.)(sec.)-Centiposis 1488 

P = Fluid Density, (ib.mass)/(cu.ft.) 

(any Consistent System of Units may be Employed in 

Place of the English Units Specified) 

\ { Spheres 

• Disks 

. Cylinders 

1 

, \ Spheres 

• Disks 

. Cylinders 

i -

Spheres 

• Disks 

. Cylinders \-

rt 

J _ 4 J J i , 

| i 1 H-H - —-1 1 

—Stokes' Law- Ir t< ed a te Law-* -Newton's Law 

- • 

L 

IIII 1 1 III I IIII I 1 

JL 

1 till 1 1 

0.0001 0.0O1 0.01 0.1 1.0 10 100 

Reynolds Number, Np e = — — 

1,000 10,000 100,000 1,000,000 

Drag Coefficients for Spheres, Disks, And Cylinders and Any Fluid. From Perry, R. H., Ed., Chemical Engineers Handbook, 7th. ed.. 1997, 

McGraw Hill Company, Inc. 

Figure 4 

Drag force on spherical particles 3 to 100 micron is directly proportional to velocity in low Reynold's 

number region. The applicable physical law is Stokes' Law and at Re < 2.0 the flow is laminar 

and streamlines are smooth, it is considered to be "creeping flow" around the spherical particle. 

Particles less than 3 microns in the Re < 2.0 regime are subject to Brownian Movement and 

Stokes' - Cunningham Law. See Figure 5. 

Gas Velocity Exceeds Particle Velocity 

Creeping Flow 

At Re < 2 flow is laminar and streamlines smooth 

Figure 6 

When a particle falls under the influence of gravity it will accelerate until the frictional drag in the 

fluid balances the gravitational forces. At this point it will continue to fall at a constant velocity. 

This is the terminal velocity. For spherical particles between 3 and 100 microns and 

0.0001 < Re < 2 the applicable physical law that applies is Stokes'. The formula is stated in 

Figure 6. 

ut = g L D p 

2 (PS -P) 

18n 

ut Terminal Settling Velocity of Particle Under Action 

of Gravity, ft/sec. 

gi_ = Local Acceleration due to Gravity,(ft.)/(sec.) (sec.) 

Diameter of Spherical Particle, ft. 

P = Fluid Density, Ib.mass/cu.ft. 

r

1

 = Fluid Viscosity, (lb.mass)/(ft.)(sec.) 

D 

Figure 6 

VES017007 
The following examples illustrate the extremes in particle sizes and velocities in relation to the 

Reynolds Number: 

Particles with a Reynolds Number of 1.7 

An 80 micron particle in air at 70 °F and 14.7 PSIA with a density of 100 lbs / cu. ft. has terminal 

velocity of 62 FPM . While a 1 micron particle in air at 70 °F and 14.7 PSIA with a density of 100 

lbs I cu. ft. has terminal velocity of 5,0000 FPM . 

Particles with a Reynolds Number of 3.24 X 10-3 

A10 micron particle in air at 70 °F and 14.7 PSIA with a density of 100 lbs / cu. ft. has terminal 

velocity of 0.96 FPM . While a particle less than 1 micron (1.9 X 10-3 to be exact) in air at 70 °F 

and 14.7 PSIA with a density of 100 lbs / cu. ft. has terminal velocity of 5,0000 FPM . 

Particle Size 

Particle size is commonly defined by its diameter in micrometers, more commonly called microns. 

How Small is a Micron? 

1 Micron is = 1/25,400 of an inch (3.937 x 10-5 inches) 

318 Human Hairs = 1 inch 

Pin Head = 1500 microns 

84,667 Smoke Particles = 1 inch 

33 Smoke Particles =10 microns 

For proper separator design, particle size analysis is important for correct equipment selection. 

A convenient index for determination of particle size spectrum is the Reynolds number. Thes e 

dimensionless numbers provide a measure of the ratio between viscous forces and inertial forces. 

Three equations are generally used to describe the action of mechanical separation, each being 

limited to definite particle spectrum. These equations have the following ranges of particle 

spectru m whe n applie d to wate r droplet s in air at atmospheri c conditions : 

1. Newton's Law has a particle spectrum from 15,000 to 100,000 microns. 

2. Th e Intermediate Law particle spectrum ranges from 100 to 15,000 microns . 

3. Stokes ' Law encompasse s particles from 3 to 100 microns . 

Below 3 micron particle size, Stokes'-Cunningham Law applies. Below 0.1 micron, Brownian 

Movement becomes dominant. Brownian Movement is a random motion of particles caused by 

collisions with gas molecules. Particles within Brownian Movement range approach molecular 

size. Newton's and the Intermediate Law equations are generally applied to knock out drums 

and gravity settling separators. Whereas Stokes' and Cunningham's Law equations apply to 

centrifugal, impingement, and filtering type separators. 

Determination of particle size in a system is relatively easy with solids. However, with liquids it 

is not. Liquids are in continuous change of state, subsequently particle size of liquids will vary 

depending upon the source and nature of the operation generating the particular particles. 

For the applicable physical law for particles in relation to the Reynolds Number see Table 1. 

VES017008 
Characteristics Of Dispersed Particles 

ClrlNVliUltlCl! 

1100,000 

5 

2 

10,000 

5 

2 

LOCO 

5 

1 in. 

-1 cm.-

C'lMinorcial 

E()ui:)mei!] fol 

CulliiMion nr 

Uenvjviil of 

Policies from 

a Gas 

0.1 

5 

2 

001 

5

! 

2 

0.031 

.100A, 

A—:= 

Gravity Settling of Spheres in Stil! Fluid 

Spikes of 

Unit Oerraly 

in Air 

Diameler. 

Micois 

100,000 

Reynolds 

Number L N R e _ 

200,000-

10,000 

-1,000-

-100-

Spheres of 

Any Density 

in Any Fluid 

-500-

* Furnishes Average Particle Diameter, but no Size Distribution] 

x Size Distribution may be Obtained by Special Calibration 

From Kinetic Theory of Gases: 

Xm = 3p/p 9 

. RT / n M 

-1.0-

-0.1-

-0.01-

"0.0001-

Laws of Settling 

Newton's Law 

C = 0.44 

(Ps-P) 

Intermediate Law 

C = 18.5NR ° e

6 

0.153 g^D. " 

f ,0.29^0 43 

Stokes' Law 

C = 24N R e

1 

9l

 D p <Ps-p) 

18 ft 

;

"ts 

Stokes - Cunningham Law 

u

r

K mu

t s 

Km " 1 + Kme (*Tn ' 

Critical Particle Diameter 

Above Which Law Will Mot 

Apply 

D

p,crit

 = Kcr 

9L p(fh-f') 

Kc r = 43.5 for 

Intermediate Law 

KCr = 33 for Stokes' Law 

The Cunningham Correction on Slokes 

Law Becomes important for Particles of 

Diameters Under 3 Microns for Settling 

in Gases and Under 0.01 Micron for 

Settling ;n liquids. 

The Value Kme has Experimentally been shown to lie between 1.3 and 

2.3 for Different Gases, Particle Sizes, and Materials (Wasser, 

Physik.Z.,34,257-278[1933Tj. An Approximate Average Value Based on 

the Data of Millikon is Empirically Given by 

Kme=1.644+0.5S2e -(0.656 Dp/Xm) 

Brownian Movement 

/"3cRTK m t 

(

~ V 3n 2F ND. 

Brownian Movement is a Random 

Motion Superimposed upon the 

Gravitational Settling Velocity of the 

Particle it Becomes Appreciobe for 

Particles under 3 Microns Diameter 

and Becomes Entirely Predominant 

for Particles Under 0.1 Micron 

—0.001-

iMomenclaiure: (Any Self-Consistant System of Units may be Employed; English Units are given by 

way of Example.) 

p crit 

;

 Overall drag Coefficient, Dimensionless 

- Diameter of Spherical Particle, ft. 

= Critical Particle Diameter Above Which Law will 

Not Apply, ft. 

= Conversion Factor, 32.17(lb.mass/lb Force).' 

(ft./sec.) 

= Local Accel oration dug to Gravity,(ft)/(sec.) (sec.) 

= Proportionality Factor, Dimensionless 

:

 Stokes-Cunningham Correction Factor 

Dimensionless 

= Proportionality Factor, Dimensionless 

= Moiecuiar Weight, IbJmole 

-Number of Gas Molecules in a mole, 

2.76x10* Molecules/lb mote. 

f 

I's 

f 

= Reynolds Number. Dimensionless -Dp f'/^E/^ 

= Gas Constant, 1,546(fl-ib.Force)(lb. mo1e)(°F) 

= Time, sec 

= Absolute Gas Temperature. °F abs., Dr °R 

= Terminal Settling Velocity of Panicle Under Action 

of Gravity, ft/sec. 

= Terminal Settling Velocity of Particle as Calculated 

from Stokes' Law, ft./sec. 

= Mean Molecular Speed, ft./sec. 

= Average Linear Amplitude of Displacement of 

Particle In Time t, ft. 

= Fluid Density, Ib.mass/cu.ft. 

= True Density of Particle, Ib.mass/cu.ft. 

= Fluid Viscosity, (lb.mass)/(ft.)(sec.) 

= Mean Free Path of Gas Molecules, ft. 

Characteristics of dispersed particles. 

Table 1 

Perry, J. H., Ed., Chemical Engineers Handbook, 3rd. ed., 1950, McGraw Hill company, Inc. 

VES017009 
Particle Motion 

Mechanical separators utilize particle motion in two ways: 

• Primary separation of solids/liquids 

•Carry off of separated solids/liquids 

All separators have more that one generative force providing particle separation, however, they 

are typically classified by the dominant separation process - gravity settling, impingement, 

centrifugal action, coalescing, or filtering. 

All mechanical separators rely on the principle of impingement. Impingement is the action of 

particles colliding with other particles and/or surfaces. 

All mechanical separators use gravity force to assist in the carry off of separated solids/liquids. 

This force is normally considered the maximum agent available for carry off in mechanical type 

separators with the exception of cyclone type separators. 

There are four basic concepts of mechanical separation: 

•Gravity or Knock Out Drums 

•Centrifugal (Cyclone) 

•Impingement 

• Filters 

Mechanical Separator Limitations 

Mechanical separators are limited in primary separation by geometry, by velocity required to 

induce inertial fields, or by pressure differential due to drag losses. 

Mechanical separators are limited in carry off capabilities of separated solids/liquids due to reentrainment and creeping flow. Re-entrainment is due to drag force pulling solid/liquid off surfaces 

to form globs/droplets which are re-injected into the gas stream. Re-entrainment (carry over) 

can occur in all types of separators, and in the case of cyclone types is due to an intense inner 

vortex creating a velocity field to pull solids/liquids out of the drain sump area and re-inject the 

separated particulate into the gas steam. 

Creeping solid/liquid flow is caused by drag force on solids/liquids deposited on surfaces 

overcoming gravity drain forces and causing a flow of solid/liquid out of the separator into the 

clean gas side. 

Both re-entrainment and creeping flow can be eliminated by correct sizing techniques and product 

design, that control the velocity in relation to the fluid density, viscosity, through the separating 

element. 

The basic information regarding particle formation, physics, motion and limitations have been 

presented. The following information details the four concepts of mechanical separation. 

Gravity or Knock Out Drums 

A gravity or knockout drum typically has 

the inlet and outlet connections located 

on the upper portion of the vessel. The 

force used to separate the solids/liquids 

from the gas is gravity. The primary 

physics involved is the terminal velocity 

ofthe particulate. It can be seen that the 

gas velocity must be very low in order 

for separation to occur. 

VES017010 
A 

Centrifugal Separator 

Velocity is the primary factor in the performance 

of a centrifugal separator. For a given size 

centrifugal separator, the size of the separated 

particulate is inversely proportional to the square 

root of the gas velocity. Consequently, the 

success of a centrifugal separator is dependent 

on the gas velocity obtained. The minimum 

particle size to be separated is dependent on 

the particulate viscosity, number of turns the 

particulate makes within the separator, and the 

velocity of the gas at the inlet to the separator. 

Impingement Separator 

An impingement separator is in the category of separators that 

provide targets for the particulate to be intercepted. The wire mesh 

separator and vane type separator are two of the most commonly 

used impingement separators. 

Wire Mesh Separator 

The wire mesh separator consists of wire knitted into a pad having 

a number of unaligned isometrical openings. It has 97% to 99% 

free voids and collects the particles primarily by impingement. The 

principal of operation of a wire mesh pad is change of direction. 

The gas flowing through the pad is forced to change direction a 

number of times. Centrifugal action is to a minimum. Impingement, 

therefore, is the primary separation mechanism. A liquid particle 

striking the metal surfaces of a mesh pad, flows downward where 

adjacent wires provide capillary space. At this point, liquid collects 

and continues to flow downward. Surface tension tends to hold 

this drop on the lower base of the pad until they are large enough 

for the downward force of gravity to exceed that of the upward gas 

velocity and surface tension. 

In a mesh pad separator the impingement efficiency falls off rapidly 

at low velocities because the droplets will tend to drift between 

the wires. At high velocities, the element tends to flood. Liquid 

cannot flow downward against the increased upward gas velocity 

force and therefore accumulates. As the voids become full of liquid, 

a portion is re-entrained and discharges with the outlet gas. 

The wire mesh separator is considered primarily a liquid separator; 

dirt, solids, or very viscous liquids of sticky nature will plug the 

voids resulting in poor performance. 

Pocket 

Vane type separators consist of labyrinth 

form of parallel metal sheets with pockets 

to collect separated moisture. Th e gas 

between plates is agitated and ha s to 

change direction a number of times. Some 

degree of centrifugal action is introduced 

as the gas changes direction. The heavier 

particles then are thrown to the outside and 

are caught in the drain pockets. 

VES017011 
Vane type separators fall into two categories - pocket style and hook style. Vane separators 

which incorporate corrugations or hooks protruding into the flow path create pressure drop and 

flow turbulence resulting in potential re-entrainment of separated liquid. Flow within pocket type 

vane separators is sinusoidal and separation is accomplished by both impingement and 

coalescence. Even though the hook style incorporates the same impingement separation 

characteristics, the pocket style vane separator accomplishes the separation function at higher 

velocities without re-entrainment. 

A vane type separator with pockets functions as follows; when liquid 

laden gas vapor approaches the vane plates it is forced to change 

direction, moving the liquid droplets to the plate walls converting the 

liquid to sheet flow in what is called coalescence. This mechanism 

is a function of the path distance and the free stream velocity. Liquid 

sheet flow approaches the vane pocket where it is collected and 

removed from the gas stream and ultimately drains to the liquid^^ y 

holding sump due to gravity. 

Sizing of a vane type separator is dependent on relative density of 

the liquid and gas, relative size of liquid droplets, and relative weight 

ratio of the liquid to gas stream. In most instances the maximum 

liquid to gas weight ratio of van e type separators is 10%. 

In any case, impingement type separators that employ wire mesh 

or vane type elements are limited to liquid particle separation, 

ultimately dirt or solid particulate, and viscous liquids will plug the 

voids in mesh pads or pockets in vane separators resulting in poor 

performance. 

Vane Type Separator 

Vane Pack 

Filter Separator 

Filter Separator 

Filter separators are designed to provide optimum 

performance in mechanical separation, and are used 

in separatin g aerosol s and soli d particles . 

The removal of solids is accomplished by interception 

ofthe particles and its efficiency is dependent on the 

depth of the filter media density and size of the fiber. 

The primary function of a filter separator is the removal 

of small liquid particles that will have a detrimental 

effect on downstream equipment. 

Mechanical separators such as vanes, 

centrifugal or wire mesh, are effective 

for removal of particles above 6 

microns, where the liquid to gas 

weight ratio is greater than 1% by 

weight, light liquid loads (less than1%) 

characteristically have particle size 

distribution of less than 6 microns and 

therefore, mechanical separators with 

vane, centrifugal, or wire mesh 

elements are not effective. 

To collect liquid particles less than 6 

micron require particle conditioning. 

One of the most practical methods of 

particle conditioning is coalescence. 

Solids 

Hollow Filter Cor e 

Hii'i'i i HiiiiMUHin ti'iim i iimmHJuiiJiHiiiiijiHiiuum iliiiiillf^lll 

F/7fer Separator Cartridge Filter 

VES017012 
Coalescence is the mechanism where small droplets are agglomerated on a fiber mat or surface, 

and forms a continuous liquid film which periodically shears and releases large droplets back 

into the gas stream. The size of these droplets depend on the surface tension and viscosity of 

the liquid and the velocity of the gas relative to the liquid. 

The filter separator uses cylindrical coalescing elements for particle conditioning. The design 

of these elements is the most important component in the design of a filter separator. The first 

step in the design of the elements is an analysis of the liquid particle size and concentration in 

the gas stream. 

There are three mechanisms of particle collection to be considered in the design of filter elements 

for mist elimination. These are inertial impaction, diffusion, and direct interception. 

Inertial impaction (impingement) is the 

aerodynamic behavior of a particle within the 

fibrous media of the filter. If the particle has 

sufficient inertia and low enough surface drag 

characteristics, it will be attracted and held to 

fiber in the media due to Van Der Waals Force. 

DIFFUSION 

A third mechanism is that of direct interception 

(straining). A particle of large enough size, 

relative to the size ofthe fiber will be collected 

even if it has no mass and can follow gas 

flow path. For this reason fiber diameter and 

density are a major consideration in the design 

of filter elements. 

IMPINGEMENT 

Diffusion is the random movement of particles 

called Brownian Motion. Brownian Motion will 

manifest itself as a wobbling, erratic motion 

of the particles about the steady aerodynamic 

gas path. This motion would not of itself result 

in additional collection of particles by fibers if 

the particle concentration were everywhere 

identical. But since the motion is totally random, 

the transfer of particles across any surface 

near a fiber will be proportional to the 

concentration on opposite sides of that surface. 

It can be shown that there is a net transfer of 

particles across a surface and that this transfer 

is in the direction of the fiber surface itself. 

The rate at which such transfer takes place is 

proportional to a diffusion coefficient. 

STRAINING 

VES017013 
The combined properties of inertial impaction 

(impingement), diffusion and direct interception 

(straining) enable us to state qualitatively, at least, 

how aerosol (suspended particles in gases) filtration 

will be effected. The graph depicts the effect which, 

would result if aerodynamic collection alone were 

operative and if Brownian diffusion alone were 

operative. The top curve depicts the sum of these 

two effects. 

It is apparent that the Brownian diffusion curve falls 

toward zero for particles approaching molecular size 

(an aerosol filter does not filter out gases). In addition, 

it is not at all unusual for filter efficiency to drop 

somewhat toward zero for large particles because 

of their poor retention after capture by some filter 

media. The mere transport of particles to the fiber 

surface is not sufficient of itself to ensure their 

collection. They also have to remain on the fiber 

surface. 

t \ J \ 

v A 

\ 

If* 

PARTICLE SIZE OR VELOCITY 1 

0 -DIFFUSION EFFICIENCY 

-IMPACTION EFFICIENCY 

r -TOTAL EFFICIENCY 

-POINT OF MINIMUM EFFICIENCY 

Blow-off is the term referring to the process of re-entrainment ofthe liquid collected by the fibrous 

pad in the gas flowing from the pad. It is effected by several factors - inlet aerosol concentration, 

gas flow velocity, viscosities and densities of the gas and liquid, pad surface area and porosity 

and interfacial and surface tension forces. 

Blow-off efficiency is a function of the 

velocity of the gas and the velocity of 

the liquid through the fiber. At low 

velocities, less than 6 feet per minute, 

the efficiency of collection of the filter 

decreases until the effect of the diffusion 

mechanism becomes apparent whereby 

the smaller particles, by their random 

motion, will again be collected. At the 

blow-off velocity large droplets are 

shearing away from the film of collected 

liquids on the filter fibers. The small 

droplets appear again as a secondary 

product of the larger droplet formation. 

This then, is a velocity region of low 

filtration efficiency. If the filter is to be 

used as a mist eliminator it should be 

designed so that the aerosol velocity 

through the filter is less than the critical blow-off velocity. If however, the filter is to be used as 

an agglomerator element the velocity must be greater than the critical blow-off velocity. 

The ideal operating conditions of an agglomerator filter, is at a velocity just beyond the initial 

blow-off velocity. At this condition more of the smaller droplets entering the pad would be impacted 

because of the higher velocity and the droplets leaving the pad are at its larger size. At higher 

velocities more droplets will be impacted but the blow-off droplets will be smaller in size. 

A very important use of the blow-off characteristics of fibrous mat type eliminator element is that 

of particle agglomeration. Fiberglass or similar materials with fine fibers can be used to collect 

the very small aerosol particles at aerosol velocity which will produce blow-off drainage of the 

collected liquid from the pad. The larger droplets can then be collected by another mechanical 

separator such as vane, wire mesh or a centrifugal unit. 

VES017014 
Hydrocarboi 

0.O04 

O.O03 

sp. gr. 

1.0 

0.9 

0.8 

0.7 

0.6 

-400 

.55-

-300 -200 -100 

i i TKi. 

100 200 300 400 

Temperature °F 

-L..55 -

500 600 700 800 900 1000 

From Gas Processors Suppiers Association, Tulsa 

VES017015 
PROPERTY 

Properties at 14.696 PSIA arti 60 F 

MOLECULAR! 

WEIGHT I 

DENSITY | VISCOSITY (Centipoises) 

COMPOUN D SYMBO L j MOLECULAR! 

WEIGHT I LIQUID LIQUID TEMP 

(1 ht/Ft ^1 'COEFFICIENT 

1

 ' OF DENSITY 

G A S 

(Lbs/Ft3) 

GASSPECFIC 

GRAVITY 

AIR=1.0 

I 

LIQUID G A S 

AIR 28.966 - 0.07640 1.0000 0.172 at-314°F 0.01817 

WATER 18.016 62.365 - - -- 1.1404 0.01255 at212T 

AMMONIA NH3 17.032 38.502 at 106.3 psia - 0.04492 0.5880 0.276 at -40°F 0.00968 

ARGON A 39.944 - - 0.10530 1.3783 — 0.02190 

CARBON MONOXIDE CO 28.010 - - 0.07388 0.9670 „ 0.01723 

CARBON DIOXIDE c o 2 44.010 50.882 at 741 psia - 0.11608 1.5194 0.077 at 107.6 psia 0.01460 

CHLORINE C12 70.914 88.719 at 85.9 psia 018704 2.4482 0.385 at 32°F 0.01309 

HELIUM He 4.003 - - 0.01055 0.1381 „ 0.01923 

HYDROGEN H 2 2.016 - 0.00532 0.0696 0.011 at-434°F 0.00866 

HYDROGEN CHLORIDE HC1 36.465 53.311 at 427 psia - 0.09688 1.268 0.01402 

HYDROGEN SULFIDE H 2 S 34.076 49.222 at 235 psia — 0.08988 1.1764 __ 0.01234 

METHYLCHLORIDE C H 3

C

1 50.49 61.9 al-13"F - 0.13292 1.7398 — 0.01043 

NITROGEN N2 28.016 - - 0.07389 0-9672 - 0.01728 

OXYGEN °2 32.000 - - 0.0844 1.1047 - 0.01971 

SULFUR DIOXIDE so2 64.060 86.923 at 40.6 psia - 0.16897 2.2116 0.3936 at 32"F 0.01230 

METHANE CH4 16.042 - - 0.04240 0.555 — 0.01073 

ETHANE C 2 H 6 30.068 - - 0.07991 1.046 _ 0.00896 

PROPANE C

3

H

8 44.094 31.598atSat.Pr. 0.00171 0.11819 1.547 0 108 0.0078 

n- BUTANE C

4

H

10 58.120 36.378 at Sat.Pr. 0.00111 0.15822 2.071 0.180 0.0073 

n - PENTANE C

5

H

12 72.146 39.29 0.00086 0.19028 2.4906 0.251 0.00656 

n-HEXANE C

6

H

14 86.172 41.345 0.00077 0.22728 2.9749 0.337 -

n - HEPTANE C

7

H

16 100.198 42.851 0.00069 0.26428 3.4591 0-425 -

n-OCTANE C

8

H

18 114.224 44.010 0.00063 0.30126 3.9432 0.555 0.00675 at 213°F 

n - NONANE C G H 2 0 128.250 44.942 0.00059 0.33826 4.4275 0-676 O.00675 at212°F 

n-DECANE C 1F) H 2 2 142.276 45.736 0.00055 0.37526 4.9118 0.92 at 68" F -

CYCLOPENTANE C

5

H

10 70.130 46.737 0.000730 0.18497 2-4211 0.493 at 56°F -

METHYLCYCLOPENTANE C

6

H

12 84.156 46.927 0.000693 0.22196 2.9053 — -

ETHENE (ETHYLENE) C 2 H 4 28.052 21.25 al 32° F - 0.07399 0.9684 0-07 at 32° F 0.0098 

PROPENE c3

H

fi 42.078 32.465 at Sat. Pr. 0.00177 0.11098 1.4526 - -

1-BUTENE C

4

H R 56.104 37.410 atSal.Pr. 0.00113 0.14797 1.9368 0.16 0.0074 

1 - PENTENE c S

H m 70.130 40.223 0.00087 0.18496 2.4210 — 

1,2- BUTADIENE C

4

H

6 54.088 40.963 at Sat.Pr. 0.00114 0.14266 1.8673 - -

ETHYNE (ACETYLENE) ^2^2 26.036 38.285 at 578 psia - 0.06867 0.8988 - O.00935 at 32°F 

BENZENE C

6

H

6 78.108 55.101 0.000686 0.20601 2.6965 0-700 0.00738 at 57.6°F 

TOLUENE C

7

H

fi 92.134 54.310 0.000592 0.24301 3.1808 0.624 -

STYRENE C R

H

8 104.144 56.756 0.000584 3.27469 3.5954 -

ISOPROPL BENZENE C A

H

1? 120.186 53.967 0.000550 3.31700 4.1492 - -

ACETONE C^H r O 58.08 49.4 - - - 0.336 O.00931 at212°F 

ETHYL ACETATE C4

H R°? 88.10 56.6 - - - 0.471 O.00684 at 32°F 

ETHYL ALCOHOL C,HK 0 46.069 49.513 - 0.12151 1.5905 1.220 O.0108 at212°F 

METHYL ALCOHOL CH40 32.042 49.648 - 0.84514 1.1062 0.620 0.0135at152.2°F 

n-PROPL ALCOHOL CaHfl0 60.09 50.2 at68°F - - - 2.494 O.0093 at212°F 

FURFURAL C,H4 02 96.08 74.7 - - -- 1.49 at 77° F -

MONOETHANOLAMINE (MEA) C 2 H 7 N 0 61.08 63.5 -- - - 37 

ETHYLENE GLYCOL C

2

H

6°2 62.07 69,6 0.00039 — 23 -

DIETHYLENE GLYCOL C

4

H

10 U3 106.12 70.0 0.00041 47 -

TRIETHYLENE GLYCOL C

6

H

14°4 150.17 70.4 0.00044 — — 51 -

TETRAETHYLENE GLYCOL C

8

H

18°5 1942 70.5 0.00043 — 60 -

PROPYLENE GLYCOL C

3

H

8°2 76.09 64.9 0.000425 — — 77 --

DIPROPYLENE GLYCOL C

6

H

14°3 134.17 64.2 0.0004 — — 150 -

TRIPROPYLENE GLYCOL C

9

H

20°4 192.3 64.1 0.00048 - 100 -

TYPICAL CYLINDER -- - 56.9 -- — — 1735 -

LUBRICATING OIL. (DTE-105) 

VES017016 
1ABLE 

unless Otherwise Noted 

CRITICAL CONSTANT S REDUCED 

CONDITIONS Freezing 

Point 

at 14.696 

PSIA ("F) 

Boiling 

Surface Tension i 

(Dynes/Cm.) RATIO OF 

TEMP -

ERATURE 

T C CF) 

PRESSURE 

P c (PSIA) 

COMPRESSBUTY 

FACTOR 

(CRITICAL) 

T

5

 (60°F) PS (14.596) 

P C 

Freezing 

Point 

at 14.696 

PSIA ("F) 

Point 

at 14.696 

PSIA ("F) 

LIQUID TO 

VAPOR 

LIQUID TO 

AIR 

SPECIFIC 

HEATS 

FOR GAS 

-221.3 547 0.29 2.180 0.027 - -317.7 1.40 

705.4 3206 0.232 0.446 0 Q05 32 212 - 73.42 -

271.4 1657 0.25 0.711 0.009 -107.9 -28.1 23 at 52° F 1.31 

-187,7 705 0,29 1.911 0.021 -308.6 -302,3 -- 1.67 

-218 510 0.29 2.150 0.029 -340.6 -313.6 9,8at-315.4°F 1.40 

88.0 1073 0.29 0.949 0.014 - -109.3 1.2 at68°F 1.30 

291 1120 0.29 0692 0,013 -150.9 30,3 18.at.68°F....... 1.37 

-450.2 33.2 . . . 54.705 0.443 -458 at 382 psia -452 _. 1.66 

-399.8 188 0.29 8.676 0.078 -434.4 -422.9 1.91 at-423°F 1.41 

124.5 1199 0.29 0.890 0.012 -173.6 -121 __ 

. .66at 68.°.F. 

1.41 

212.7 1306 JL29_ 0.773 0.011 -121.9 -76.5 __ 1.32 

289.6 968 0.27 0.694 0.015 -126.1 -10.3 - 16.2 at68°F 1.20 

-232.8 492 0.29 2.290 0.030 -345.6 -320.4 10.53 at-333.4°F 1.40 

-181.8 730 0.29 1.870 0.020 -361.1 -297.4 13.2 at-297.4°F 1.40 

315 1142 0.29 0.671 0.013 -98.9 14 - 1.25 

-116.5 673.1 0.29 1.514 0.022 -296.5 258.7 2.8 at -150°F 1.31 

90.1 708.3 0.288 0.945 0.021 -297.9 -127.5 1.6 1.19 

206.3 617.4 0.278 0.780 0.024 -305.8 -43.7 7,8 1.14 

305.6 550.7 0.274 0.679 0.027 -217.0 31.1 13.1 1.09 

385.9 489.5 0.286 0.615 0.030 -201.6 96.9 16.6 -

454.5 439.7 0.264 0.568 0.033 -139.6 155.7 8,9 18.43 al 68°F -

512.6 396.9 0.260 0.535 0.037 -131.1 209.2 20.8 -

565.2 362.1 0.256 0.507 0.041 -70.2 258.2 21.8 ai 68°F -

613 345 0.250 0.484 0.043 -64.4 303.4 - -

655 320 0.246 0.466 0.046 -21.5 345.2 — -

461.5 654.7 3 276 0.564 0.022 -137.0 120.7 -_ 

499.3 549.1 0.273 0.542 0.027 -224.4 161.3 -

49.8 742.1 0.269 1.020 0.020 -272.5 -154.7 •,: a!32°F 1.24 

197.4 667 0.274 0.791 0.022 -301.5 -53.9 1.15 

295.6 583 0.277 0.688 0.025.... 

-301.6 20.7 13.5 1.11 

394 586 0.27 0.609 0.025 -265.4 86.0 -

339 653 0.27 0.651 0.023 -213.3 50.5 - 1.12 

97.4 905 0.274 0.933 0.016 -114 -119 - 1.24 

553 714 0.274 0.513 0.021 42.0 176.2 28.89 at 68°F 28.85 at 68°F -

609.5 611 0.260 0.486 0.024 -139.0 231.1 28.5 at 68°F --

706 580 0.27 0.446 0.025 -23.1 293.4 _ 32.14 at 66"F -

685 473 0.27 0.454 0.031 -140.9 306.3 - -

455 690 0.25 0.568 0.021 -139 -133 23.7 at 68"F 23.7 al 68°F -

482.2 555 0.25 0.552 0.026 -118.4 171 „ 23,9 at 68°F -

469.6 927 0.25 0.559 0.016 -179.1 173.3 22.75 at 68"F 22 at 77°F -

464 1157 0.25 0.563 0.013 -144 148.1 20.14 at 122"F 22.61 at 68°F -

506.7 734 0,25 0.538 0.020 -196.6 207 23.78 al 68°F -

- - - — -33.7 323.3 43.5 at 68°F 43.5 at 68°F -

- - - - 50.9 342 - - -

- - - - 8 387.1 47 at77°F 47.7 at 68°F 

-- - - - ~ 

17 472.6 44 at77"F -

- -- - - - 19 545.9 45at77°F -

— 

- - - - - 22 597.2 45 at 77°F 

-

— 

- - -- - -- — 369 36 at 77°F -

~ - - - - - 447.8 33 at77°F -

- - - - - ~ 514.4 34 at 77°F -

- - - -

-- - ~ 

VES017017 
References : 

"Chemical Engineers Handbook", Perry, R. H., Don W. Green, 7th. ed., © 1997, McGraw Hill Company, Inc. 

"Engineering Thermodynamics" Francis F. Huang, ©1976, Macmillan Publishing Co. Inc. 

"Handbook of Separation Techniques for Chemical Engineers", Philip A Schweitzer, 3rd ed, ©1997 McGraw Hill 

Company, Inc. 

"Fundamentals of Fluid Mechanics", Bruce R. IVIunson, Donald F. Young, Theodore H. Okiishi, 4th ed, ©2002 John 

Wiley & Sons, Inc. 

"Gas Engineers Handbook", 1st ed, ©1965 Industrial Press Inc. 

"Petroleum Fluid Flow Systems", O.W. Boyd, 1sted., ©1983, Campbell Petroleum Series 

"Separation Handbook", E. J. Halter, 1st ed., ©1966, Burgess Manning Company 

"Applied Process Design", Ernest E. Ludwig, Volume 1, 3rd ed., Gulf Publishing Company 

"Engineering Data Book", Gas Processors Suppliers Association, Volumes 1 and 2, 11th ed., ©1998, Gas Processors 

Suppliers Association. 

"Flow of Fluids", Engineering Department, Crane Valves, ©1988, Crane Co. 

Muelle r Environmenta l Designs , Inc. 

7607 Wright Road 

Houston, Texas 77041 

713-465-0995 

Fax 713-465-0997 

E-Mail: MUELLER-ENVIRON@worldnet.att.net 

www.muellerenvironmental.com 200-059 

VES017018 
VES017019-017201.pdf 
An Investigation of the Particle Dynamics of 

a Multi-component Solid Phase in a Dilute 

Phase Pneumatic Conveying System 

Yon g L u 

Institute for Materials and Processes 

School of Engineering and Electronics 

Thesis submitted for the degree of Doctor of Philosophy 

The University of Edinburgh 

March 09 
Abstract 

In order to mitigate the risk of global warming by reducing C0 2 emissions, the co-firing 

technique, burning pulverized coal and granular biomass together in conventional pulverised fuel 

power station boilers, has been advocated to generate "greener" electricity to satisfy energy 

demand while continuing to utilize existing rich coal resources. A major problem is controllably 

distributing fuel mixtures of pulverized coal and granular biomass in a common pipeline, thus 

saving much investment. This is still under development in many co-firing studies. This research 

into particle dynamics in pipe flow was undertaken in order to address the problem of 

controllable distribution in co-firing techniques and gain an improved understanding of 

pneumatic conveying mechanisms. 

The objectives of this research were, firstly, to numerically evaluate the influence of various 

factors on the behaviour of particles of the different materials in a horizontal pipe gas-solid flow, 

secondly, to develop an extended technique of Laser Doppler Anemometry in order to detennine 

cross-sectional characteristics of the solid phase flow in the horizontal and vertical legs of a 

pneumatic conveying system, and, thirdly, to develop a novel imaging system for visualizing 

particle trajectories by using a high definition camcorder on a cross-section illuminated by a 

white halogen light sheet. Finally, a comparison was made of cross-sectional flow characteristics 

established by experiments and those simulated by using a commercial Computational Fluid 

Dynamics code (Fluent) and the coupling calculations of Fluent & EDE M (a commercial code of 

Discrete Element Method) respectively. 

Particle dynamic behaviour of the solid phase in a dilute horizontal pipe flow was investigated 

numerically by using the Discrete Phase Model (DPM) in Fluent 6.2.16. The numerical results 

indicate that the Saffinan force plays an important role in re-suspending particles at the lower 

pipe boundary and that three critical parameters ofthe critical air: conveying velocity, the critical 

particle size and the critical pipe roughness, exist in pneumatic conveying systems. The Stokes 

number can be used as a similarity criterion to classify the dimensionless mean particle velocity 

of the different materials in the fully developed region. 

An extended Laser Doppler Anemometry (LDA) technique has been developed to measure the 

distributions of particle velocities and particle number over a whole pipe cross section in a dilute 

pneumatic conveying system. The first extension concentrates on a transform matrix for 

predicting the refracted laser beams' crossing point in a pipe according to the shift coordinate of 

the 3D computer-controlled traverse system on which the probes of the LD A system were 

VES017020 
mounted. Another part focussed on the proper sampling rate of LD A for measurements on the 

gas-solid pipe flow with polydispersing particles. A suitable LD A sampling rate should ensure 

that enough data is recorded in the measurement interval to precisely calculate the particle mean 

velocity or other statistical values at every sample point. The present study explores the 

methodology as well as fundamentals of measurements of the local instantaneous density of 

particles as a primary standard using a laser facility. 

The extended LD A technique has also been applied to quantitatively investigate particle dynamic 

behaviour in the horizontal and vertical pipes of a dilute pneumatic conveying system. Three 

kinds of glass beads were selected to simulate the pulverized coal and biomass pellets transported 

in a dilute pneumatic conveying system. Detailed information on the cross-sectional spatial 

distributions of the axial particle velocity and particle number rate is reported. In the horizontal 

pipe section, experimental data on a series of cross-sections clearly illustrate two uniform fluid 

patterns of solid phase: an annular structure describing the cross-sectional distribution of the axial 

particle velocity and a stratified configuration describing particle number rate. In the vertical pipe 

downstream of an elbow R/D=1.3, a horseshoe-shaped feature, which shows that the axial 

particle velocity is highest in wall regions of the pipe on the outside ofthe bend for all three types 

of glass beads on the section OD close to the elbow outlet. The developments of cross-sectional 

distributions of particle number rate indicate that the horseshoe-shaped feature of particle flow 

pattern is rapidly dispersed for particles with high inertia. 

A video & image processing system has been built using a high definition camcorder and a light 

sheet from a source consisting of a halogen lamp. A set of video and image processing algorithms 

has been developed to extract particle information from each frame in a video. The experimental 

results suggest that the gas-solid flow in a dilute pneumatic conveying system is always 

heterogeneous and unsteady. The parameter of particle mass mean size is superior to particle 

number mean size for statistically describing the unsteady properties of gas-solid pipe flow. It is 

also demonstrated that the local data of particle number rate or concentration are represented by a 

stratified structure of the flow pattern over a horizontal pipe cross-section. 

Finally, comparisons of numerically predicated flow patterns and experimental ones show that 

there is reasonable agreement at pipe cross-sections located at horizontal positions less than half 

the product of particle mean velocity and mean free fall time in the pipe from the particle inlet. 

Further away from the inlet, the numerical results show flow patterns which are increasingly 

divergent from the experimental results along the pipe in the direction of flow. This discrepancy 

indicates that particles' spatial distribution in the pipe is not accurately predicted by the Discrete 

Phase Model or Fluent coupled with EDEM. 

ii 

VES017021 
Declaration 

I declare that this thesis describes work carried out in the Institute for Materials and 

Processes, School of Engineering and Electronics, the University of Edinburgh. It is 

the original work of the author except where otherwise stated. This thesis has been 

composed by myself and was not submitted in any form at another university. 

Signed 

Yong Lu 

Edinburgh, 17 March 2009 

iii 

VES017022 
Acknowledgements 

I would like to express my deepest gratitude to my supervisors, Dr. Donald H. Glass and 

Professor William J. Easson. Their dedication and enthusiasm to scientific research, 

their brilliant idea and encouragement were essential to built up this thesis. Without their 

precious help, I would have never come to Edinburgh and started my Ph.D. 

I also want to thank Dr. Martin Crapper, who has advised and guided me for numerical 

simulations by using EDEM. Here I am grateful to DEM Solutions Limited, Edinburgh for 

their assistance with this numerical work. 

Many thanks to Mr. Bobby Hogg, Mr. Neil Wood and Mr. Roby, who gave much helps of 

providing technical support for experiments. And also many thanks to my friends near 

and far for their friendship and the happy moments we shared together, especially to Dr. 

Hao Wang with whom I shared the flat and encouraged each other in past two years. 

I am extremely grateful to the committee of 2005 UK/China Youth Scholarships for 

Excellence, University of Edinburgh and Southeast University for the financial support. 

These supports have made my study at Edinburgh possible. 

Finally, I dedicate this thesis to my parents, [Minlong Lu| and Lichun Wang, my parents 

in-law, Jitang Fan and Fenghua Shen, my wife, Shisen Fan and my lovely daughter, 

Zixin Lu, who have always been there to support me and give me strength and 

motivation to accomplish this work. 

iv 

VES017023 
Dissemination 

This work has been presented in several conferences and meetings, and has written in 

papers that have been published and are in the process of being published, given as 

follows. 

Publication s 

Lu Y. , Glass H. D., Easson J. W., (2009). An investigation of particle behaviour in gas-solid 

horizontal pipe flow by an extended LD A Technique. Fuel. Accepted (JFUE-D-08-00741). 

Lu Y. , Glass D. H., Easson W. J., (2008a). Investigating particles behavior in gas-solid horizontal 

pipe flow by an extended Laser Doppler Anemometry Technique. In: Proceedings of 7th 

European Conference on Coal Research and its Applications, September 2n d - 5t h, Cardiff, 

U K. 

Lu Y-, Glass D. H., Easson W. J., Crapper M. , (2008b). Investigation of the Flow Patterns of 

Gas-solid granular flow in a horizontal pipe by FLUENT & EDEM. In: Proceedings of 

Particle Systems Analysis 2008, September 1st - 3r d, Stratford, UK. 

Lu Y. , Glass D. H., Easson W. J., (2007). Investigating Flow Properties of Coal Biomass Blends 

in Pneumatic Conveying System by Computational Fluid Dynamics. In: Proceedings of 

2007 international conference on Coal Science and Technology, August 28*- 31t h, 

Nottingham, UK. 

Presentation s 

Lu Y. , Glass D. II., Easson W. J., (2008a). Investigating particles behavior in gas-solid horizontal 

pipe flow by an extended Laser Doppler Anemometry Technique. 7th European 

Conference on Coal Research and its Applications, September 2ai - 5t h, Cardiff, UK. (Oral 

presentation). 

Lu Y , Glass D. H., Easson W. J., Crapper M. , (2008b). Investigation of the Flow Patterns of 

Gas-solid granular flow in a horizontal pipe by FLUENT & EDEM. In: Proceedings of 

Particle Systems Analysis 2008, September 1st - 3r d, Stratford, UK. (Oral presentation). 

Lu Y. , Glass D.H., Easson W. J., (2008c). Visualizing particle characteristics in a horizontal 

pneumatic conveying system, In: 21st Annual Scottish Fluid Mechanics Meeting, May 22n d, 

Edinburgh, UK. (Poster) 

Lu Y. , Glass D. H., Easson W. J., Crapper M. , (2008d). A comparison of the Numerical Results 

Investigated by EDEM & Fluent and the Experimental Data in a Gas-solid Horizontal Pipe 

Flow. In: Proceedings of EDE M User Group Meeting 2008, April 28th - 30th, Edinburgh, 

U K. (Poster) 

Lu Y. , Glass D. H., Easson W. J., (2007). Investigating Flow Properties of Coal Biomass Blends 

in Pneumatic Conveying System by Computational Fluid Dynamics. 2007 international 

conference on Coal Science and Technology, August 28*- 31t h, Nottingham, UK . (Oral 

presentation). 

v 

VES017024 
Table of contents 

Abstract i 

Declaration iii 

Acknowledgements iv 

Dissemination v 

Table of contents vi 

Table of figures ix 

Table of tables xiii 

Nomenclature xiv 

Chapter 1 1 

1.1 GLOBAL WARMING AND CO-FHUNG TECHNIQUES 1 

1.2 CO-FIRING ACTIVITIES AND THE TECHNICAL PROBLEMS 

OF HANDLING THE FUEL MIXTURE 3 

1.3 PNEUMATIC CONVEYING FUNDAMENTALS FOR CO-FIRING 

APPLICATIONS 5 

1.4 OBJECTIVES AND CONTRIBUTIONS IN THIS THESIS 7 

Chapter 2 9 

2.1 LITERATURE REVIEW ON PNEUMATIC CONVEYING IN COFIRING APPLICATIONS 9 

2.2 NUMERICAL MODEL AND FLUID CONDITIONS 11 

2.2.1 Discrete Phase Model for gas-solid simulations 11 

2.2.2 The k-e turbulence model 12 

2.2.3 Numerical convergence criteria 13 

2.2.4 Model geometry and mesh Grids 13 

2.2.3 Flow Conditions and Selecting Numerical Parameters 14 

2.3 RESULTS AND DISCUSSION 16 

2.3.1 Particle trajectories in the pipe 17 

2.3.2 The effect of air conveying velocity 20 

2.3.3 The effect of pipe roughness 21 

2.3.4 The effect of particle size 21 

2.3.5 The characteristics of particle mean velocity in terms of particle 

Stokes Number 22 

2.4 CONCLUSIONS 25 

Chapter 3 27 

3.1 INTRODUCTION 27 

3.1.1 The briefprinciple of LDA 27 

vi 

VES017025 
3.1.2 Literature view of LDA applications in gas-solidflow 29 

3.2. EXPERIMENTAL APPARATUS 39 

3.3 DEVELOPING THE SPATIAL TRANSFORM MATRIX FOR LDA 

PIPE FLOW MEASUREMENT 40 

3.3.1 Parameters of the LDA system 40 

3.3.2 Optic principle of a ray crossing a pipe wall 41 

3.3.3 Optic paths of different colour beams through a pipe 45 

3.3.4 Developing the coordinate transform matrix for LDA pipe flow 

measurement 46 

3.4 SAMPLE RATE OF LDA FOR GAS-SOLID FLOW 

MEASUREMENT 50 

3.5 SUMMARY 52 

Chapter 4 53 

4.1 INTRODUCTION 53 

4.2 MEASURING THE CROSS-SECTIONAL CHARACTERISTICS 

OF THE AXIAL AIR VELOCITY (U M ) IN A HORIZONTAL PIPE 54 

4.3 AIR VELOCITY IN THE RIG VERTICAL PIPE AFTER A 90° 

BEND 57 

4.3 CONCLUSIONS 66 

Chapter 5 68 

5.1 FLUID CONDITIONS AND MEASUREMENT PARAMETERS 68 

5.2 DETERMINING THE PROPER SAMPLE RATE OF LDA 70 

5.3 EXPERIMENTAL RESULTS ON PARTICLE DYNAMICS IN A 

HORIZONTAL PIPE 72 

5.3.1 Features of the measured parameters at a typical point 73 

5.3.2 Characterizing the experimental data on a pipe cross-section 74 

5.3.3 The development of cross-sectional fluid patterns in a horizontal pipe 

75 

5.3.3.1 Glass beads less than 50/im (the first type) 79 

5.3.3.2 Glass beads within the range 70 to 110 /um (the second type) 82 

5.3.3.3 Glass beads within the size range 180~300fim (the third type) 85 

5.4 PARTICLE DYNAMIC BEHAVIOUR IN A VERTICAL PIPE 

DOWNSTREAM OF A 90° ELBOW 88 

5.4.1 Glass beads less than 50ym (the first type) 89 

5.4.2 Glass beads within the size range 70~110jum (the second type) 94 

5.4.3 Glass beads within the size range 180~300(tm (the third type) 99 

5.5 DISCUSSION AND COMPARISON 104 

5.5.1 The discrepancy between air velocity and particle velocity in the 

horizontal pipe 104 

5.5.2 Particle dynamics in the vertical pipe downstream of a right angle 

elbow 108 

5.6 SUMMARY AND CONCLUSIONS 110 

v i i 
Chapter 6 112 

6.1 INTRODUCTION 112 

6.2 EXPERIMENTAL SETUP AND FLOW CONDITIONS 116 

6.3 VIDEO & IMAGE PROCESSING SYSTEM 117 

6.3.1 Principle of decoding a MPEG-2 video 119 

6.3.2 Strategy for processing large volume of uncompressed frames (AVI 

files) 121 

6.3.3 Particle diameter and spatial position measurement 122 

6.4 RESULTS & DISCUSSION 128 

6.5 SUMMARY AND CONCLUSIONS 133 

Chapter 7 135 

7.1 INTRODUCTION 135 

7.2 NUMERICAL MODELS AND THEIR PARAMETERS 137 

7.2.1 Mathematical models 137 

7.2.2 Experimental rig and numerical geometry 140 

7.2.3 Fluid conditions at inlet 140 

7.2.4 Experimental cross-sectional flow patterns of solid phase 142 

7.3 RESULTS AND COMPARISONS 143 

7.3.1 Flow patterns of solid phase investigated byDPM 143 

7.3.2 Flow patterns of solid phase investigated by Discrete Element 

Method 146 

7.4 DISCUSSION 147 

7.5 CONCLUSIONS 149 

Chapter 8 151 

8.1 SUMMARY OF RESULTS 151 

8.2 RECOMMENDATIONS FOR FUTURE WORK 156 

8.3 PUBLICATIONS ARISING FROM THE PROJECT 157 

References 158 

viii 

VES017027 
Table of figures 

Figure 1 The sketch of CO2 cycle in co-firing applications 2 

Figure 2.1 Experimental rig (a) and the mesh grids of the numerical model (b, c) 14 

Figure 2.2 Geldart's classification of fluidization behaviour for fluidization 15 

Figure 2.3 The convergence behaviour of numerical simulations, (a) the evolution of streamwise air velocity profile across the pipe at x-2.5m (b) the development ofthe 

relative error of the velocity to its average value versus numerical iteration on 

nine reference points 17 

Figure 2.4 Particles trajectories (running direction from right to left) 18 

Figure 2.5 Trajectories and the variations of (U p x , U p y ) for two selected particles 19 

Figure 2.6 Particle trajectories with different conveying velocities 20 

Figure 2.7 Particles trajectories with different pipe roughness heights 21 

Figure 2.8 Trajectories with different particle sizes 21 

Figure 2.9 The evolution of the mean particulate velocities U p x and U p y for pulverised coal23 

Figure 2.10 Evolution ofthe mean particulate velocities U p x and U p y for wood particles.... 24 

Figure 3.1 The schematic diagram of a laser-Doppler anemometry (DANTEC, 2008) 28 

Figure 3.2 Sketch ofthe experimental rig... 39 

Figure 3.3 The geometric sketch o f a beam refraction on a curve 41 

Figure 3.4 The geometric sketch of a beam translating a distance L 42 

Figure 3.5 The schematic of a ray crossing a pipe 43 

Figure 3.6 Flow chart of determining the optical path of a ray crossing a pipe wall 44 

Figure 3.7 Crossing point of blue beams shifting towards the pipe center line (Units: mm)46 

Figure 3.8 Cross point of green beams moving to the pipe center line (Units: mm) 46 

Figure 3.9 The experimental coordinate (Oxzy), the probe coordinate (O'x'yV ) and the 

initial arrangement of the probe 47 

Figure 3.10 Variations ofthe crossing point of blue beams against probe angle 48 

Figure 3.11 Variations ofthe crossing point of green beams against probe angle 49 

Figure 3.12 In the horizontal pipe at the -5° probe angle 50 

Figure 3.13 In the vertical pipe at the -5° probe angle 50 

Figure 3.14 The sketch of particle signals recorded by LD A 51 

Figure 4.1 2D schematic diagram of two selected cross sections in the rig horizontal pipe.. 54 

Figure 4.2 Velocity profiles of the axial air velocity (Ux ) at x=-200, -300 mm along 56 

Figure 4.3 Cross-sectional distribution ofthe axial air velocity distribution (Ux ) at two 

horizontal cross-sections of x=-200 and -300mm from the bend inlet respectively, 

under air conveying velocity 12m/s) 57 

Figure 4.4 2D schematic diagram of cross sections along the vertical pipe behind a 90° bend 

(R/D=1.3, R is the bend radius 100mm and D is pipe diameter 75mm; 0D is 

15mm away from the bend outlet) 57 

Figure 4.5 Experimental cross-sectional distributions of the axial air velocity (U a y , m/s) 

along the vertical pipe behind a 90° bend (R/D=l .3, R is the bend radius 100mm 

and D is pipe diameter 75mm; 0D is 15mm away from the bend outlet) 59 

ix 

VES017028 
Figure 4.6 Numerical cross-sectional distributions ofthe axial air velocity (U a y , m/s) along 

the vertical pipe behind a 90° bend (R/D=l .3, R is the bend radius 100mm and D 

is pipe diameter 75mm; OD is 15mm away from the bend outlet) 60 

Figure 4.7 Experimental cross-sectional distributions of the horizontal air velocity (Uax, m/s) 

along the vertical pipe along the vertical pipe behind a 90° bend (R/D=1.3, R is 

the bend radius 100mm and D is pipe diameter 75mm; 0D is 10mm away from 

the bend outlet) 61 

Figure 4.8 Numerical cross-sectional distributions ofthe horizontal air velocity (Uax) along 

the vertical pipe behind a 90° bend (R/D=1.3, R is the bend radius 100mm and D 

is pipe diameter 75mm; 0D is 10mm away from the bend outlet) 63 

Figure 4.9 Profiles of Uax and U a y along the x axis at four selected vertical cross-sections. 64 

Figure 4.10 Profiles of Uax and U a y along the z axis at four selected vertical cross-sections 66 

Figure 5.1 Particle distribution for the second type of glass beads 69 

Figure 5.2 Particle number rate against the sample rate of LD A on the section 300mm away 

from the elbow inlet air velocity 12m/s, solid loading ratio about 1.6% 71 

Figure 5.3 The measured particle velocity at a sample point in three axes 73 

Figure 5.4 Cross-sectional flow patterns described by the distributions of particle mean 

velocity and particle number rate respectively (Ua=12m/s; particles within the 

size range 70~1 lOum) 74 

Figure 5.5 The sketch of eight measurement cross-sections along the horizontal pipe 76 

Figure 5.6 Cross-sectional distributions ofthe mean values of {Up x } and the granular 

temperature {©} as well as particle number {N} at an air conveying velocity 

I2m/s 78 

Figure 5.7 Development of cross-sectional distributions of the dimensionless axial particle 

velocity {Up x / Ua } ofthe first type of glass beads (<50um) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 0.19% 80 

Figure 5.8 Development of cross-sectional distributions ofthe dimensionless particle number 

rate {ln(N/ 2(N))} ofthe first type of glass beads (<50um) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 0.19% 81 

Figure 5.9 Development of cross-sectional distributions ofthe dimensionless axial particle 

velocity {Up x / Ua } ofthe second type of glass beads (70~1 lOum) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 1.9% 83 

Figure 5.10 Development of cross-sectional distributions of the dimensionless particle 

number rate {ln(N/ S(N))} ofthe second type of glass beads (70~1 lOum) under 

an air conveying velocity Ua=10m/s and the solid mass loading ratio 1.9% 84 

Figure 5.11 Development of cross-sectional distributions of the dimensionless axial particle 

velocity {Up x / Ua } of the third type of glass beads (180~300um) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 1.9% 86 

Figure 5.12 Development of cross-sectional distributions ofthe dimensionless particle 

number rate {ln(N/ S(N))} of the third type of glass beads (180~300um) under 

an air conveying velocity Ua=10m/s and the solid mass loading ratio 1.9% 87 

Figure 5.13 The arrangement of measurement cross-sections in the vertical pipe 89 

Figure 5.14 Development of cross-sectional distributions ofthe dimensionless axial particle 

velocity {Up y / Ua } ofthe first type of glass beads (<50um) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 0.19% in the 

vertical pipe behind an elbow (R/D=1.3) 91 

Figure 5.15 Development of cross-sectional distributions ofthe dimensionless horizontal 

particle velocity {Up x /max(abs(Up x ))} ofthe first type of glass beads (<50fim) 

under an air conveying velocity Ua=10m/s and the solid mass loading ratio 1.9% 

in the vertical pipe behind an elbow (R/D=1.3) 92 

Figure 5.16 Development of cross-sectional distributions of the dimensionless particle 

x 

VES017029 
93 

Figure 5.17 Development of cross-sectional distributions ofthe dimensionless axial particle 

velocity {Up y / Ua } of the second type of glass beads (70~110(xm) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 0.19% in the 

vertical pipe behind an elbow (R/D=1.3) 96 

Figure 5.18 Development of cross-sectional distributions of the dimensionless horizontal 

particle velocity {Up x /rnax(abs(Up x ))} ofthe second type of glass beads 

(70-1 lOum) under an air conveying velocity Ua=10m/s and the solid mass 

loading ratio 1.9% in the vertical pipe behind an elbow (R/D=l .3) 97 

Figure 5.19 Development of cross-sectional distributions ofthe dimensionless particle 

number rate {N/E(N)} of the second type of glass beads (70-1 lOum) under an 

air conveying velocity U„=10m/s and the solid mass loading ratio 1.9% in the 

vertical pipe behind an elbow (R/D=1.3) 98 

Figure 5.20 Development of cross-sectional distributions of the dimensionless axial particle 

velocity {Up y / Ua } ofthe third type of glass beads (180~300um) under an air 

conveying velocity Ua=10m/s and the solid mass loading ratio 0.19% in the 

vertical pipe behind an elbow (R/D=l .3) 100 

Figure 5.21 Development of cross-sectional distributions ofthe dimensionless horizontal 

particle velocity {Up x /max(abs(Up x ))} ofthe third type of glass beads 

(180~300um) under an air conveying velocity Ua=10m/s and the solid mass 

loading ratio 1.9% in the vertical pipe behind an elbow (R/D=1.3) 101 

Figure 5.22 Development of cross-sectional distributions ofthe dimensionless particle 

number rate {N/£(N)} ofthe third type of glass beads (180-300um) under an 

air conveying velocity Ua=10m/s and the solid mass loading ratio 1.9% in the 

vertical pipe behind an elbow (R/D=1.3) 103 

Figure 5.23 The vertical profile of axial mean velocity for four types of particles at the 

section ID in the horizontal pipe (Ua=10m/s, air convey velocity; First: glass 

beads less than 50[im; Second: glass beads within 70-110 um; Third: glass 

beads within 180-300 am) 104 

Figure 5.24 Experimental data of Huber and Sommerfeld (1994) for flow in a horizontal pipe 

by Phase Doppler Anemometry under air conveying velocity 23m/s for glass 

beads with a mean size 40um 105 

Figure 5.25 The experimental data obtained by Kussin and Sommerfeld (2002) 106 

Figure 5.26 Tsuji's experimental data (1982) by LD A in a horizontal pipe 106 

Figure 5.27 Experimental data of Yilmaz and Levy (2001) by using Optical-Fibre 

velocimetry 107 

Figure 5.28 Profiles of the axial particle velocity {Up y } and particle number rate {N} along 

the x axis on the section 0D in the vertical pipe downstream of the right angled 

elbow. (Ua=10m/s, air convey velocity; First: glass beads of size less than 50um; 

Second: glass beads within the size range 70-110 um; Third: glass beads within 

the size range 180-300 um; R: pipe radius 37.5mm) 108 

Figure 5.29 Profiles ofthe axial particle velocity and particle mass concentration (Huber and 

Sommerfeld, 1994) 109 

Figure 5.30 Profile of the axial particle velocity and particle concentration at the position of 

z/D=l under different fluid conditions (Yilmaz andLevy, 2001) 110 

Figure 6.1 Sketches of experimental rig and optical arrangement for experiments 116 

Figure 6.2 Flow chart of video and image processing 118 

Figure 6.3 Three types of pictures in a video stream (red color arrow means bidirectional) 119 

x i 
Figure 6.4 Four specific frames in an AV I video 121 

Figure 6.5 The region of the pipe cross-section on a frame 123 

Figure 6.6 Particle images on the pipe cross-section 124 

Figure 6.7 Denoising the image (a) and separating the background (b) 125 

Figure 6.8 The image with its background subtracted (a) and its binary image (b) 126 

Figure 6.9 Particle sample chain codes with 8-way connectivity template 127 

Figure 6.10 Particle diameter frequencies by number and by mass on four example frames 

129 

Figure 6.11 The variations of particle number mean size and particle mass mean size over Is 

: 130 

Figure 6.12 Cross-sectional distributions of the dimensionless particle counting number and 

the dimensionless particle concentration (N is the particle counting number; R is 

the pipe radius; {Cp } is the local concentration o f solid phase) 132 

Figure 6.13 The contour of {ln(N/Z(N)} on the section 3D measured by LD A (selected from 

Figure 5.12) 132 

Figure 7.1 Experimental rig and schematic diagram of numerical geometry 140 

Figure 7.2 Air velocity (U x, U y and Uz ) at inlet 141 

Figure 7.3 Cross sectional distributions of Particle velocity and particle number rate at inlet 

over Is 141 

Figure 7.4 The contours of particle velocity Upx and particle count over Is at the section 

x=455mm 142 

Figure 7.5 The contours of particle velocity U p x and particle count over Is at the section 

x=1000mm 142 

Figure 7.6 Contours ofthe dimensionless particle instantaneous velocities {Up x / Ua } and its 

time-averaged value {<Up x /Ua>} over 1000ms on the section x=455rrun 

obtained from simulation using FLUEN T DP M (Ua=12m/s) 144 

Figure 7.7 Contours ofthe time-averaged axial particle velocity {<Upx>} and particle 

number {N} over Is on two sample sections (x=455mm, 1000mm) obtained 

from FLUEN T DP M 145 

Figure 7.8 The flow patterns of particle mean velocity and particle number on two sample 

sections (x=455mm, 1000mm) obtained by using FLUENT & EDE M 146 

Figure 7.9 Comparison ofthe numerical and experimental vertical profiles of {<Upx>/Ua} at 

four different positions along the fluid flow direction (the continuous blue line is 

the profile at inlet) 147 

Figure 7.10 Comparison ofthe numerical and experimental vertical profiles of particle 

number over Is at four different positions along the fluid flow direction 148 

xii 
Table of tables 

Table 2.1 DP M model setup 12 

Table 2.2 Numerical parameters 16 

Table 2.3 Particles Stokes number and terminal velocity 22 

Table 2.4 Particle mean velocity in the fully developed region 25 

Table 3.1 The applications of an individual LD A (Single phase measurement) 32 

Table 3.2 The applications of LD A combined with other instruments for gas-solid flow 

measurements 33 

Table 3.3 Development of LD A multifunction for gas-solid flow measurement 34 

Table 3.4 LD A parameters 41 

Table 5.1 Comparisons ofthe axial velocity distributions in a vertical pipe downstream of a 

bend among three types of glass beads and air flow 104 

Table 7.1 Numerical parameters for DP M simulations 139 

Table 7.2 Numerical parameters for DEM-Fluent calculations 139 

xiii 

VES017032 
Nomenclature 

(a, b, c) The direction vector of a beam 

Cp Particle concentration 

(C m u , C i , C2) k-e model constants 

CFD Computational Fluid Dynamics 

d, dp Particle diameter 

d f Distance between the interference fringes 

D Pipe diameter 

Dp Particle diameter 

DEM Discrete Element Method 

DPM Discrete Phase Model 

ECT Electrical Capacitance Tomography 

Fs Saffman force 

Fr Fraud number 

g Gravitational constant, 9.81 m/s2 

I Chrominance 

k Turbulent kinetic energy 

K The constant in the Saffman force equation, 6.46 

• Length of horizontal pipe 

L • Length of a particle trajectory 

• The running distance of a beam 

LDA Laser Doppler Anemometry 

(nj, nr) Medium refractive index on incident or refracted side 

MT Optical translation matrix 

MR) Optic transform matrix at a pipe outer surface 

MR2 Optical transform matrix at a pipe inner surface 

N Total number of particles 

PDA Phase Doppler Anemometry 

Q Quadratic chrominance 

r The distance between a feeding position at the inlet and pipe centerline 

• Pipe radius 

R • Lens radius 

• Curve radius 

(R, G, B) (Red, Green, Blue) 

Re Reynolds number 

g • Optical system matrix 

• Area of a particle trajectory 

St Stokes number 

t Travel time crossing a stripe of the interference fringes 

xiv 
The minimum temporal interval between two adjacent particles 

tk Time interval between two adjacent particles 

T Measurement interval 

u Fluid local time averaged velocity 

The maximum value of u 

{"pk} Particle velocity sample space 

u„ The mean particle velocity 

U Fluid velocity 

Uo Air conveying velocity 

u„ Air conveying velocity 

Up Particle velocity 

<U'p2>"2 Granular temperature 

Up, Particle horizontal velocity 

Upy Particle vertical velocity 

u, Particle terminal velocity 

UDF User Defined Function 

Vi Unit vector of an incident beam 

v, Unit vector of the refracted beam 

v. The unit vector of the nonnal ray at a point 

(xc> yo, z0) Reference point of a beam 

(x.y.z) A point coordinate on a line 

• Beam's height on incident or refracted side 

• Beam's initial height 

yo Beam's translation height 

y, Beam's height after refraction 

Y Luminance 

a0 Angle of an outbound beam 

ott Crossing-angle between refracted beam and its normal. 

• The crossing-angle between incident beam and its normal vector 

• Angle of an inbound beam 

s Rate of dissipation of turbulent energy 

S Pipe roughness parameter 

g • Feeding angle 

• The crossing angle of two beams 

@ Granular temperature 

r» 

• Air dynamic viscosity 

• Mean value of the intensities of a group pixels 

c Variance of Root-mean-square ofthe intensities of a group pixels 

v Air kinematic viscosity 

p Fluid density 

XV 

VES017034 
Particle density 

Pipe roughness height 

Beam wavelength 

xvi 

VES017035 
Chapter 1 Introduction 

Coal is predicted to remain the dominant energy source in world electricity generation in the near 

future. Co-firing in power stations is a promising technique to generate enough electricity for the 

energy demand of a developing economy while simultaneously mitigating the risk of Global Warming 

by reducing CO2 ernissions. This chapter is intended briefly to introduce co-firing activities and the 

current key problems of transporting the blend of granular biomass and pulverised coal in co-firing 

applications. Then some terms used in pneumatic conveying research are discussed so as to provide a 

uniform approach to the work referenced in subsequent chapters. The objectives and the main 

contributions of this research are summarized. 

1.1 GLOBA L WARMING AN D CO-FIRING TECHNIQUES 

Unprecedented catastrophic climatic anomalies, such as blazing hot summers, brutal winters, 

hurricanes, typhoons and cyclones have occurred frequently in recent years. These natural 

disasters have been warning and forcing every nation urgently to tackle the serious problems 

produced by global warming, which has been confirmed by overwhelming scientific evidence. 

This shows that the earth's temperature has increased by 0.4°C over the past century due to the 

greenhouse effect caused by man-made fossil combustion (Baxter, 2005). Six gases, namely 

carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), per-fluorocarbons (PFCs) 

and sulphur hexafluoride, are considered as greenhouse gases which can trap the heat from the 

sun or the heat reflected by the earth in the atmosphere and make the earth much wanner. Thus, 

more than 160 nations ratified the 1997 Kyoto Protocol and committed to make legally binding 

reductions in their greenhouse gas emissions (Matsuo, 1998). A new global agreement to fight 

global warming in the post Kyoto Protocol era is expected to be concluded in 2009 at the U N 

Climate Change Conference in Copenhagen (Harris, 2008). 

In 2007, the energy target of the European Union was set to achieve 20% renewable energy by 

2020 and the UK's share was supposed to be 15% of the UK's energy from renewables (BERR, 

2008). To reach this aim, three legal measures, the Climate Change Levy in 2001, the 

Renewables Obligation (RO) in 2002 and the European Emissions Trading Scheme (EU ETS) in 

2005, have been enacted to move the UK's economy from a high-carbon model to a low-carbon 
An investigation ol particle dynamics in a dilute phase pneumatic conveying system 

or carbon-neutral one through providing financial benefits for using renewable sources such as 

solar power, wind power, hydro-electric power, geothermal energy, tidal power, wave energy and 

biofuels. As a result, the level of RO-eligible renewable generation in UK has been increased 

from less than 2% in 2001 to around 4.4% in 2006. Especially for power plants, the licensed 

electricity suppliers will be required to increase annually the fraction of renewable sources in 

(heir supplied electricity to 10% by 2010 (AEA, 2006). 

Among the eligible renewable sources, utilization of biomass as an alternative energy source 

instead of fossil fuel has been advocated and is under intense study across the world. One line of 

enquiry focuses on exploring the technique of generating electricity by using a fuel blend of 

pulverised coal and biomass in power stations. This combustion of fossil fuel mixed with 

biomass is named co-firing or co-combustion. In general, co-firing biomass resources can be 

classified into three categories (IPA, 2006), such as wastes (agricultural production wastes, crop 

residues, mill wood wastes and urban wastes), forest products (wood, logging residues, trees, 

shrubs, sawdust and bark) and energy crops (short rotation woody crops, herbaceous woody 

crops, grasses, starch crops, sugar crops, oilseed crops). 

Coal powder 

Figure 1. Sketch of C0 2 cycle in co-firing applications 

In co-firing applications, not only is biomass utilized as a sustainable alternative energy source 

which saves the usage of fossil fuel, but also CO? emission can be reduced through keeping the 

net amount of CO2 from the biomass neutral or zero-emission in the atmosphere. CO? emission 

in a co-firing power plant is shown in Figure 1. Obviously, biomass absorbs CO? from the 

atmosphere in its growing period (a) and releases oxygen back through photosynthesis (b) at the 

same time. When biomass residuals are co-fired with pulverised coal in power stations, the 

carbon stored in them will combine with oxygen to produce carbon dioxide, which is emitted 

2 

VES017037 
Chapter 1 - Introduction 

back into the atmosphere as an exhaust gas. During the cycle of (a) to (c), the total amount of 

CO2 in the atmosphere can be kept constant through continued planting and growth of the 

biomass. 

According to the data of World Energy Outlook {TEA, 2006), fossil fuel provides over three 

quarters of the UK's electricity generation nowadays and is predicated to remain the dominant 

energy source utilized for power generation until 2030. The global emission of CO2 is estimated 

to be 40 gigatonnes (Gt) in 2030, which is 14 Gt more than the 2004 level. A half of the C0 2 

emission increment will be produced by power generation over the projection period. On the 

other hand, energy demand is substantially increasing under the pressure of rapid urbanization 

and population explosion. As a result, sustainable energy policy must both tackle global warming 

and ensure security of the energy supply. Developing co-firing techniques follow this policy, 

since it is a promising clean way of utilizing abundant coal resources to provide a low-carbon 

energy supply for economic development. 

1.2 CO-FIRING ACTIVITIES AN D TH E TECHNICAL PROBLEM S OF 

HANDLING TH E FUEL MIXTURE 

Basically, biomass co-firing can be categorized as direct co-firing, indirect co-firing or parallel 

co-firing (DTI, 2006). Direct co-firing, which means burning coal with biofuels in the same 

boiler using existing or separate mills and transportation systems, is most commonly and widely 

applied to handle biomass. Conversely, indirect co-firing implies that an on-site gasification or 

pyrolysis unit can be used to convert biomass materials into a clean fuel gas, which can be fired 

alone or combusted with coal or natural gas. In parallel co-firing, fossil fuel and biomass are 

combusted separately and the steam produced by burning biomass is fed to a coal-fired power 

station where it is upgraded to the higher temperature and higher pressure conditions of the large 

coal plant. 

In the UK, biomass facilities have been developed from two plants in 2002 to sixteen units and 

the volume of electricity produced by co-fired biomass has increased significantly to an estimated 

2.5TWh over 2005/06. So far, most types of boilers, such as cyclone boilers, grate boilers, wallfired or tangentially fired pulverized-fuel (PF) boilers and circulating fluidized bed (CFB) boilers, 

have been converted to co-firing biomass with coal. These practical experiences have confirmed 

that burning biomass with fossil fuels has a positive impact both on the environment and the 

economics of power generation (DTI, 2007), 

3 

VES017038 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

Sami et al. (2006) summarized previous co-firing studies in four conclusions: 

(1) Co-firing can be performed at moderate and high percentages in cyclone boilers; 

(2) Co-firing can be performed at low percentages in PC boilers. The co-firing ratio depends on 

the pulverizer performance which in turn depends on the type of biomass fuel used. Most 

biomass fuels have a fibrous structure (e.g. wood, switchgrass) and are difficult to grind to the 

same sizes as coal used in coal pulverizers. Grinding costs determine the extent to which 

biomass can be economically pulverized; 

(3) Co-firing in PC boilers may require separate fuel feed lines depending upon the capacity of 

existing pulverizers, type and condition of biomass fuels and the type of pulverizers used; 

(4) The potential for successful application of co-firing is site-specific. It depends upon the 

characteristics of the power plant being considered, the availability and price of biofiiels 

within 50-100 miles of the plant and the economic value of environmental benefits. 

However, applying current co-firing technology is also challenged by some technical problems 

(Biagini, 2005; Baxter, 2005). The first issue is related to combustor fouling and corrosion due to 

the alkaline nature of the biomass ash. Deposits of ash reduce the heat transfer and may also 

cause severe corrosion at high temperatures. Secondly, additional studies are needed about the 

feeding mechanism transporting the fuel mixture to control the co-firing state in a given furnace. 

This issue is a combination of economics and combustion characteristics. More work is required 

to be done in this area. Third, practical pulverizer performance needs to be examined. 

In current co-firing applications, existing fuel transportation systems can be classified as three 

types (Sami et al., 2001; Ireland et al., 2004). The first is using separate feed lines and separate 

burners for dealing with coal and biomass fuels respectively. It has the advantage of better 

control over fuel flow rates. Thermal output similar to coal-only firing requires higher biomass 

feed rates. It also increases capital and maintenance costs. Firing low heating value biomass 

independently of coal also has a risk of poor combustion efficiency. The second is using separate 

feed lines and a common burner. This method can also be divided into dual inlets and triple inlets 

on furnaces. In dual inlets, coal is injected in the primary air inlet and biomass is injected in the 

swirling secondary air inlet (or vice versa). In triple inlets, two primary air inlets are used to feed 

coal and one swirling secondary air inlet to inject biomass. The third method is that premixed 

coal/biomass blends are transported in common feed lines to a common burner. Since the fuel 

blend shares the same pipelines, this method saves much investment in building a new feed 

system, and is therefore economically advantageous. 

Since dilute pneumatic conveying systems are commonly applied to transport pulverised fuel in 

4 

VES017039 
Chapter 1 - Introduction 

most coal-fired power plants, our research will focus on investigating the characteristics of 

transporting biomass/coal blends in a common dilute pneumatic conveying system. 

1.3 PNEUMATIC CONVEYING FUNDAMENTALS FOR CO-FIRING 

APPLICATIONS 

Co-firing biomass with coal in the same installation has been proved to be an important 

renewables technology, and it has the great economic advantage of re-using the existing 

infrastructure of power stations. Pneumatic conveying, which uses air to convey granular 

materials through a pipeline, has been used commercially to transport the pulverised coal for 

many years in coal-fired power stations. The current existing pneumatic conveying systems 

should be retrofitted to be suitable for transporting the fuel blend. These modifications include 

fuel handling, storage and feed systems. Since biomass granules have significantly different 

characteristics from pulverised coal (for instance, biomass has less heating value than that of 

coals) and are also more fibrous and softer (biomass density varies from 100 to 500 kg/m3 for 

forest wood and coal density ranges from 1100 for low-rank coal to 2330 kg/m3 for high density 

pyrolytic graphite), these differences in properties mean that there are potential problems in 

storage, bulk handling, feeding, combustion, slagging, corrosion and gaseous emissions. 

According to current technical reports on co-firing in large utility boilers (DTI, 2007), the 

handling of the fuel mixture is poor because of particle size variation, high fibre and oversize 

particle content. Additionally, biomass particles may be adherent, corrosive and even abrasive. 

Moreover, downstream parameters in furnaces, such as combustion efficiency, combustion 

behaviour and ash products or pollutants, are also related to the conditions of feeding the fuel 

blend. In view of these factors, it is critical to investigate how to prepare and handle the fuel 

mixture to ensure the optimum operational perfonnance of a co-firing plant. Therefore, the 

proposal for 2007-2009 key topics-2 of TASK 32 (Livingston et al., 2007) outlined the optimized 

biomass technology combinations through a workshop on fuel storage, handling and preparation, 

as well as systems analysis. 

Although pneumatic conveying systems have been widely applied commercially in many 

industrial fields and there are also numerous experimental and numerical published papers 

investigating phenomena like gravity setting, roping, erosion, attrition, pressure-wave 

propagation and gas-solid turbulence modulation in pipes, understanding of fhe mechanism of 

pneumatic conveying is still under development due to its complexity. For example, it is still 

difficult to measure and control the mass flow rate of a solid phase in a pipe and to distribute gas5 

VES017040 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

solid flow from a main pipe to several sub-pipes at an equal mass rate or a specific ratio (DTI, 

2003). 

In research on co-firing applications, most previous studies focused on combustion issues such as 

combustion efficiency, pollutant formation, carbon conversion, ash management, corrosion, fuel 

chemistries associated with blending and other downstream impacts. The relevant literature 

which focuses on transporting the pulverized biomass and coal fuel supply is particularly sparse. 

Before introducing this PhD work in detail, some easily confused definitions in pneumatic 

conveying research are given here for reference in order to provide a uniform approach to the 

work in the next subsequent chapters. These terms are cited in the two references (Mills, 2004; 

Seville et al., 1997). 

Air superficial velocity /superficial fluid velocity. This is the velocity of the air disregarding 

the presence of the solid particles or porous media. In a pipeline, it is the air velocity based 

upon the empty cross-sectional area and neglecting the space occupied by the conveyed 

material. For example, under the condition of fluid flow at volumetric rate Qf along a pipe of 

cross-sectional area A , air superficial velocity is defined to be U(=Qf/A. 

Air phase velocity. It is the same as air superficial velocity. 

"v- Air retention. The ability of a bulk material to retain air in the interstitial spaces between 

particles for a period of time. 

-v- Air conveying velocity at pipeline inlet. This is the superficial air velocity at the point where 

the material is fed into the pipeline. In a single bore pipeline this will be the lowest air 

velocity in the conveying line and so it must be greater than the minimum conveying air 

velocity required to ensure successful conveying of the material. In a vacuum conveying 

system it is approximately equal to the free air velocity. 

Air convevine velocity at pipeline exit. This is the superficial air velocity at the end of a 

conveying line where fhe material is discharged into the receiving vessel. In a single bore 

pipeline this will be the highest air velocity in the conveying line. In a positive pressure 

conveying system it is approximately equal to the free air velocity. 

-v* Cross-sectional flow patterns. These are defined as geometric contours on a pipe crosssectional characteristic of some parameters like particle velocity, particle number rate and 

solid phase concentration in a pneumatic conveying system. 

A Delivered volume concentration. This is defined as the ratio of solid phase volumetric flow 

rate to the mixture volumetric flow rate, which is expressed as Cv<j=Qs/(Qf+Qs), where Q s 

and Qf are fhe solid phase volumetric flow rate and air phase volumetric flow rate 

respectively. 

6 

VES017041 
Chapter 1 - Introduction 

"Y- Flow reeimes. These terms are used to describe the state of gas-solid flow by the evidence of 

its macroscopic appearance along the pipe axial direction. Basically, three major flow 

regimes: suspension flow (dilute phase), moving bed type flow (dense phase) and slug or 

plug type flow (dense phase) have been recognized to exist in pneumatic conveying. 

•s> Free air condition. This is specified as the condition at the standard atmospheric pressure 

(p=l .013xl05 Pa) and the standard atmospheric temperature (t=15°C). 

Free air velocity. This is the superficial velocity of the air when evaluated at free air 

condition. 

•v- Particle velocity / particle actual velocity. This is defined as the velocity of a particle in a 

pipe. Here the solid phase is considered as a group of discrete particles immersed in the fluid. 

-0- Slip velocity. This is defmed as the absolute difference between air phase velocity and solid 

phase velocity. 

Slip ratio. This is defined in terms of the solid phase velocity divided by air superficial 

velocity. 

•y- Solid phase velocity / superficial particle velocity. When solid phase is also considered as a 

continuous fluid like air and has a volumetric rate Q s along a pipe of cross-sectional area A, 

the solid phase velocity U s is then defined as US=QS/A. 

•v- Solid velocity. This is the same as solid phase velocity. 

-$* Void fraction/volume fraction /voidase. It means the volume ratio occupied by the fluid. 

1.4 OBJECTIVES AND CONTRIBUTIONS IN THIS THESIS 

Transporting pulverised coal with biomass in a pneumatic conveying system can be essentially 

considered as a special case of multi-component gas-solid pipe flow, in which the sohd phase 

consists of two or more particulate materials such as coal powder and sawdust These particles 

are distinguished by their density and geometric characteristics. The objectives of this thesis are 

to evaluate the influence of various factors on the particle behaviour of the different materials in a 

pipe and to investigate local information on the solid phase such as cross-sectional velocity 

distribution and particle number distribution by numerical simulation and experiments. The 

results will be of fundamental use in the design of fuel blend transport system. 

These aims were met through the following activities: 

-v- Comparing the similarity of the pulverised coal and biomass particles in a pneumatic 

conveying system by Computational Fluid Dynamics (CFD) and the Discrete Element 

Method (DEM) (Chapter 2; Chapter 7). 

"v* Using Laser Doppler Anemometry (LDA) to investigate the local information on the solid 

7 

VES017042 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

phase at several pipe cross-sections along the fluid direction (Chapter 3; Chapter 4 and 

Chapter 5). 

Using a high definition digital camcorder to visualize and measure the cross-sectional 

characteristics ofthe solid phase (Chapter 6). 

Comparing the experimental data with the numerical results to study the reasons for any 

discrepancies (Chapter 7). 

Specific contributions outlined in this work are: 

•v- Numerical results indicate that there exists a critical air conveying velocity, a critical particle 

size and a critical pipe relative roughness in a pneumatic conveying system. Stokes Number 

can be used to classify the similarity of the mean particle velocity of different materials in 

axial and gravity directions (Lu et al., 2007). 

<• An algorithm to calculate the spatial optical transformation matrix has been developed to 

calibrate the position of the laser beams' crossing point in a circular pipe for LD A 

measurements (Lu et al., 2008a; Lu et al., 2009). 

A video and image processing system has been developed to visualize and measure the 

spatial distribution of particle size and concentration over a pipe cross-section (Lu et al., 

2008c). 

Experimental data shows two cross-sectional flow patterns which can be classified as an 

annular structure of particle velocity contour and a stratified feature describing particle 

number rate contours (Lu et al., 2008a; Lu et al., 2009). 

- Y- Comparison of the numerical results and the experimental data indicate that using fhe current 

Discrete Phase Model (DPM) can reasonably estimate the cross-sectional flow patterns 

within a certain distance from the inlet. This length is approximately equal to the product of 

particle mean velocity and its half free-falling time in pipe. In addition, considering the effect 

of particle-particle interactions will improve the accuracy of the particles' spatial 

distributions in the pipe predicted by numerical simulations (Lu et al., 2008b; Lu et al., 

2008d). 

VES017043 
Chapter 2 

Classifying the Characteristics of Pulverized Coal and 

Granular Biomass in a Dilute Horizontal Pipe Flow 

by Computational Fluid Dynamics 

Particle dynamic behaviour of the solid phase in a dilute horizontal pipe flow was investigated at the 

start of the project by using the Discrete Phase Model (DPM) in Fluent 6.2.16. The numerical results 

indicate that the Saffman force plays an important role in suspending particles at the boundary and 

that three critical parameters of the air: critical conveying velocity, the critical particle size and the 

critical pipe roughness, exist in pneumatic conveying systems. Moreover, the Stokes number can be 

used as a similarity criterion to classify the mean particle dimensionless velocity of the different 

materials in the fully developed region. Later numerical work appears in Chapter 7. 

2.1 LITERATURE REVIEW O N PNEUMATIC CONVEYING IN CO-FIRING 

APPLICATIONS 

In Chapter 1, Co-firing was introduced as a promising technique to mitigate the risk of Global 

Warming by reducing CO2 emission, because biomass co-fired with coal can not only utilize the 

advantages of coal such as abundant resources and high thermal value but also has the great 

environmental benefit of CO2 emission neutrality in the biomass life cycle. Biomass particles and 

pulverized coal can be conveyed into furnaces of a power plant through a separate or common 

pipeline. Using a common conveying system is much the least expensive option since no separate 

feed lines and burners are required and existing installations can be used with minor adaptations 

or retrofitting (Sami M . et al, 2001). But the solution of some technical issues like storing, 

feeding and pulverizing the mixed fuels may still be required before the widespread adoption of 

co-firing in power stations. 

Despite numerous pneumatic conveyor systems having been built to handle enormous amounts of 

granular materials in many industrial fields, understanding of the underlying physical process is 

very limited and there still exist considerable concerns and technical problems due to the 

complexity of gas-solid flow in pneumatic conveying systems (DTI, 2003). The main challenges 

lie in the control and measurement of granular pipe flow on-line. Especially, this is a current 

technical problem (DTI, 2007; Livingston et al, 2007) with different coals and even more so with 

biomass, since there exist significant differences in density, size, geometric shape and moisture 

between granular biomass and pulverized coal. Al l these differences have great influence on the 

VES017044 
Chapter 2 - Classifying the characteristics of granular biomass and pulverised coal by CFD 

state or properties of the mixture flow. For example, pulverized coal is normally conveyed to the 

furnace by a distribution pipe network, which is designed to distribute equally or split the 

pulverized fuel through bifurcations or trifurcations. The evenness of distribution will affect 

combustion efficiency, pollutant production and flame stability. The majority of pneumatic 

conveying systems have been designed and calibrated to handle a specific type of granular 

material. When these conveying systems are retrofitted to convey biomass or fuel blends, the 

default or designed conveying parameters like minimum pick-up velocity, distribution ratio of a 

splitter and solid loading ratio . should be re-calibrated and validated. 

So far, most co-firing researches in power generation have focused on combustion-related 

activities such as measuring and modeling the combustion flame, combustion efficiency and NO x 

reduction, pollutant formation, ash management, the physical properties of coal-biomass blends 

and other issues related to combustion. Only a handful of papers have begun to investigate the 

fundamentals of pneumatic transport of the fuel blends of pulverized coal and granular biomass. 

Granada et al. (2006) specially designed a hopper for steadily injecting pellets of three forest 

residues (Eucalyptus, pine and pine bark). His experimental results indicate that pine bark is the 

most suitable product for co-firing as pellets and that a screw feeder causes higher levels of 

segregation of fuel mixtures. Wall shear and direct shear experiments were carried out by 

Zulfiqar et al. (2006) to study shear between the mixture fuels (typical Australian coal and wood 

chips) and the shear between fuel blends and the wall. The biomass was pine sawdust with 

equivalent diameter 100 um or woodchips with average dimensions of 2*3 cm2 (width by length). 

Their experimental data indicate that coal-woodchip blends will significantly increase the risk of 

blockage and flow stoppage. Melander et al. (2004) used the Particle Image Velocimetry (PIV) 

technique to measure the concentration and velocity of wood fibres in a vertical pneumatic 

conveying system. Their data confirmed that PIV is a useful tool for measuring the properties of 

the dilute fibre-gas flows. The measured profiles of air velocity show that they are strongly 

affected by fhe volume fraction of wood fibres. Joppich and Salman (1999) studied the steadiness 

of a screw feeder used to feed wood powder. By using a vibrator feeder mounted at the 

downstream end of the screw conveyor, the total feeding performance is significantly improved. 

In research into dilute particle-laden pipe flow of a single material, Huber et al. (1998) and 

Sommerfeld (2005) summarized seven important phenomena: particle transport due to turbulence, 

gravitational settling, wall roughness effect, particle-wall collisions, inter-particle collisions and 

inertial behaviour in pipe bends. Mills (2004) published a practical book introducing the 

principles of design of a pneumatic conveying system. In research into the pneumatic conveying 

mechanism, Fan et al. (1998) addressed the basic principles and fundamental phenomena 

10 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

associated with gas-solid flows, as well as characteristics of selected gas-solid flow systems 

including dilute transport or pipe flow of gas-solid suspensions. Similarly, Seville et al. (1997) 

also summarized particle characterization, particle mechanics and solid assembly mechanics in 

gas-solid flow. With regard to the pneumatic conveying of granular biomass, Cui et al. (2006) 

reviewed studies into transporting agricultural grains and pulp fibres as well as related flow 

measurement techniques and modeling. 

Although there have been numerous experimental and theoretical studies into pneumatically 

conveying a single granular material, the corresponding experience and results are not adequate 

to be used to solve fhe previously discussed technical issues such as equal distribution of the 

mixture fuels from a main to several sub-pipes in a pneumatic conveying system. Thus, it is 

necessary to study local information on the solid phase, such as particle spatial distribution and 

solid concentration distribution and the microscopic mechanism of pneumatic conveying. 

Moreover, conveying pulverized coal with biomass can be considered as a special gas-solid flow 

whose solid phase consists of two or more types of solid particles. The objective of this chapter is 

to study the different influences on the particle dynamic properties of pulverised coal and 

granular biomass in a dilute horizontal gas-solid pipe flow and to classify their similarity by 

analysing fhe characteristics ofthe dimensionless parameters. 

2.2 NUMERICAL MODEL AN D FLUID CONDITIONS 

2.2.1 Discrete Phase Model for gas-solid simulations 

Recent rapid progress in the theoretical and experimental study of turbulence modulation has led 

to great progress in understanding some basic mechanisms of gas-solid pipe flow, which can be 

normally modelled by the Volume of Fluid (VOF) model, Eulerian models and Discrete Phase 

Models (DPM). There is also a model called CFD-DEM, or the Combined Continuum and 

Discrete Method (CCDM), in which the motion of the solid phase is described by the Discrete 

Element Method (DEM). Every particle of the solid phase is tracked by applying Newton's laws 

of motion. Correspondingly, the air phase is considered as a continuum fluid by using 

Navier-Stokes equations in the traditional Computational Fluid Dynamics (CFD) manner. (Tsuji 

Y., 1993; Crapper M. , 2006; Chu K.W. and Yu A.B., 2007). 

Since the solid volume fraction in dilute pneumatic conveying is very small (less than 10%), the 

effect caused by inter-particle collisions is usually ignored in the numerical simulations. 

Compared with the Eulerian-Eulerian Model, the DPM model considers a solid phase consisting 

of particles randomly dispersed in an isotropic fluid. Their trajectories are computed by solving 

the equations governing movement of the solid phase in a Lagrangian frame of reference. In 

contrast, the conveying air is regarded as a continuous phase whose transport equations are 

11 
Chapter 2 - Classifying the characteristics of granular biomass and pulverised coal by CFD 

solved in an Eulerian frame of reference. This model is more appropriate for tracking every 

particle trajectory in the fluid. To deal with phase interactions, three regime types comprising 

one-way, two-way and four-way coupling calculations between fhe solid phase and gas phase can 

be classified by solid volume fraction less than 10"6, larger than 10"6 and beyond 0.1% 

respectively (Elghobashi, 1994). In this work, DPM simulations are used to select the two-way 

coupling for dilute pneumatic transportation, and two additional forces beside the drag force are 

to be considered. They are the virtual mass force required to accelerate the fluid surrounding 

the particle and the Saffman lift force due to shear. The detailed transportation equations of DP M 

can be found in Fluent User Guide (Fluent Inc., 2006). 

2.2.2 The k-e turbulence model 

For single phase flow, no single turbulence model is universally accepted as being superior for all 

classes of problem. The k-e model has been widely and successfully applied for single phase 

turbulent flow (gas or liquid), although there are also other methods like direct numerical 

simulation (DNS), large-eddy simulation (LES), and discrete-vortex simulation (DVS) under 

development. 

Table 2.1 DP M model setup 

Type Choices 

Solver 

-v* Segregated solver 

Space: 2D or 3D 

<• Steady 

Turbulence 

modelfk-s 

Model) 

Model Constants: C m u =0.0845 

C, =1.42C2=1.68 

RNG options: Differential viscosity model 

Near-wall treatment: standard wall 

functions 

<- Full buoyancy effects 

Discrete phase 

option 

Interaction with continuous phase 

•fr Update DPM sources every 15 flow 

iterations 

-v- Saffman lift force 

Two-way turbulence coupling 

Gravity 

-y> the negative direction of y axis 

* Value: 9.81 m/s2 

Wall collision 

model 

-4- Reflection 

•v- Reflection coefficients of nonnal and 

tangential directions are 1 

• Roughness constant: 0.5 

"v- Roughness height ranges from 0 to 2.32 

mm 

The modification of the gas turbulence in particle-laden flows is not well understood until now. 

Many factors, such as particle size, density, and volume fraction, have been identified as having 

much influence on the turbulence intensity. For example, experimental data (Tsuji et al., 1984; 

Gore et al., 1989) show that larger particles tend to enhance the turbulence and smaller particles 

attenuate it. In general, the k-s turbulence model has been widely used to describe the air phase in 

gas-solid flow due to this model having been successfully applied for single phase flow (air or 

12 

VES017047 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

liquid). 

The k-e model assumes that the flow is fully turbulent and isotropic, and that the effects of 

molecular viscosity are negligible. Nowadays the k-e model has been developed into the Standard 

k-e Model, the Realizable k-e model and the Renormalization Group (RNG) k-e Model. Al l these 

three models have a similar approach, including transport equations for the turbulent kinetic 

energy (k) and its rate of dissipation (s). The major differences lie in the equations used for 

calculating turbulent viscosity and the expression of the generation and destruction terms in the g 

equation. Compared with the other two models, the Standard k-s Model is relatively simple to 

implement, leads to stable calculations that converge relatively easily, and gives a reasonable 

prediction for many flows. Its disadvantages are that it gives poor predictions for swirling, 

rotating and strongly separating flows. Since our studies focus on the properties of dilute phase 

granular materials in a horizontal pipe, the k-e model is selected for describing air phase in our 

numerical simulations. The parameters of DPM setup in Fluent 6.2.6 are briefly listed in Table 

2.1. 

2.2.3 Numerical convergence criteria 

A solution is normally assumed to be convergent when the sum of its normalized residuals (for 

example, mass, velocity components, turbulent kinetic energies, turbulent and dissipation 

energies) is less than a specified small tolerance. This tolerance is usually set to 1/1000 by default. 

In some cases, Kohen et al. (1994) and Huber et al. (1998) found that residuals were not suitable 

for use as a criterion due to their oscillating at each coupled calculation. For this reason, they 

advised that the evolution of the stream-wise gas velocity profile at several reference positions 

should be used as the convergence criterion. In this work, most numerical solutions are used with 

the standard residuals criterion. In cases where the residuals jump or oscillate, the convergence is 

determined by the evolution of the stream-wise gas velocity profile at seven specific points. 

2.2.4 Model geometry and mesh Grids 

In order to provide future comparisons between the numerical results and the experimental data, 

the domain of our numerical model is similar to the dimensions of the horizontal part of the lab 

pilot rig, which will be discussed in detail in section 3.2. Figure 2.1 shows both the sketch of the 

rig and the mesh grids of the numerical model. The rig in Figure 2.1(a) is a pneumatic conveying 

system with positive pressure including a fan, a cyclone and several glass pipes. Figures 2.1(b) 

and (c) show the total length of the numerical model and the cross-sectional mesh grid at the 

model inlet, which are meshed by employing adjustable boundary-fitted coordinate grids 

respectively. The distance of the first cell adjacent to the wall is set to 0.5mm in order to 

13 

VES017048 
Chapter 2 Classifying the characteristics of granular biomass and pulverised coal by CFD 

investigate local fluid behaviour in the pipe boundary region. The whole domain is meshed into 

0.6 million cells. The origin point of the reference coordinate is located at the centre of the pipe 

inlet where y positive axis is opposite to gravity and z axis is along pipe radial direction. The x 

positive axis is along fluid How direction, 

(b) I.=3000mm. in x axis

 x 

i 

(a) The lab rig (c) pipe inlet (t)>B()mm) in the plane of Oyz 

Figure 2.1 Experimental rig (a) and the mesh grids of the numerical model (b, c). 

(Please note that the scale factor of every diagram is different for clarity and the system coordinate is 

located at the pipe inlet centre.) 

2.2.3 Flow Conditions and Selecting Numerical Parameters 

In order to be able to extend the experimental results used to meaningful engineering applications, 

some dimensionless numbers such as Reynolds number, Froude number and Stokes number 

have been used for characterizing the dynamic similarity between the pilot rig and the large-sale 

pneumatic conveying system in a power plant. These three essential parameters are individually 

expressed as 

pUD 

Re 

Fr = 

u 

(2.1) 

st = ?>D>

v 

l&pD 

where p, u and U are density, dynamic viscosity and velocity for air phase separately. D is pipe 

diameter. L is a characteristic length descriptive of the flow field and is usually defined as being 

equal to pipe diameter for pipe flow research. D p and p p are particle diameter and particle density 

respectively. 

14 

VES017049 
All investigation of particle dynamics in a dilute phase pneumatic conveying system 

(iiddings and Aroussi (2004) indicated that keeping the Froude number (based on the pipe inner 

diameter as the reference length) constant could make the pipe friction behaviour of the 

experimental rig similar to that of a full-scale power plant pipe. For example, a 25m/s conveying 

velocity and (|)330mm (inner diameter) pipe are two normal operating parameters used in the 

dilute pneumatic conveying systems of a pulverized coal-fired power plant. Our attainable 12m/s 

air conveying velocity can attain dynamic similarity with the experimental rig with 80mm ID 

pipes. In addition, a relatively high conveying air velocity must be maintained for dilute phase 

conveying. The typical order of air conveying velocity is about 10-12 m/s for fine powders like 

pulverized fuel and wheat flour and 14-16 m/s for fine granular materials like granular sugar and 

pearlite. If the conveying velocity drops by more than 10 or 20 percent in the pipeline, it is likely 

to cause a blockage problem (Mills, 2004, p429). Thus the air conveying velocity is to be in the 

range of 10~48m/s in our simulations. 

20 50 100 500 1000 5000 

Mean particle size (|xm) 

Figure 2.2 Geldart's classification of fluidization behaviour for fluidization 

with ambient air (Mills, 2004) 

According to their air retention properties, Geldart (1973) classified granular materials into four 

distinct groups A, B, C and D by using density and average mean diameter in his fluidization 

research. The experimental results of Mills (2004) confirmed that the materials in group A have 

very good retention properties and the materials in group C have very good permeability. As such, 

granular materials of group A and C are suitable to be pneumatically transported either in the 

dense or dilute phase. Biomass density varies from 100 kg/m3 for straw to 700 kg/m3 for forest 

15 
Chaplcr 2 Classifying ihe characteristics of granular biomass and pulverised coal by CKD 

hard wood and coal density ranges from 1100 kg/m3 for low-rank coal to 2330 kg/m3 for high 

density pyrolytie graphite. The pulverized coal used in modern pulverized coal-fired power 

stations is normally less than 1 lOum in size. However, the granular biomass consumed in many 

demonstration co-firing power plants is usually filtered to be less than 3mm by a 6 mesh sieve 

before transportation (NETBIOCOF, 2006). Considering the blockage risk due to transporting 

oversize granular biomass mixed with pulverized coal in a common pipe, a more suitable 

granular biomass should be less than 1mm in size (equivalent diameter of a sphere) according to 

Geldart's Chart (Figure 2.2). In addition, glass beads will be used to simulate the gas-solid flow 

of the mixture fuels in the lab experiments. As such, the numerical simulations will investigate 

the dynamical properties of three materials (coal, wood and glass beads) of different sizes in a 

horizontal straight pipe. The detailed numerical parameters are listed in Table 2.2. 

Table 2.2 Numerical parameters 

Type Parameters Value 

air conveying velocity (Ua, m/s ) 10,15,18,20, 24, 30,48 

Pipe 

diameter (D, mm) 80 

Pipe roughness HeightfA, mm) 0,0.08, 0.16,0.32,0.67, 1.31,2.32 

relative roughness(A/D) 0, 0.1%, 0.2%, 0.3%,0.83%, 1.6%, 3.3% 

loading 

ratio 

(kgsVkgs-') 0.01 

particle 

glass 

density (kg/m3) 2500 

particle 

glass diameter (um) 20, 40, 60, 100, 200, 300, 450, 500 

particle 

glass 

shape Spherical 

particle 

coal 

density (kg/mJ) 1550 

particle 

coal diameter (pm) 10, 20, 30, 35, 40, 50, 60, 70, 80, 100, 

200, 300 

particle 

coal 

shape Spherical particle 

Wood 

chips 

density (kg/m3) 185 particle 

Wood 

chips 

equivalent diameter 

(pm) 100,146,204,296,430,581,726,871,1016 

particle 

Wood 

chips 

Coal diameter with the 

same Stokes number 

(um) 

29, 50, 70, 100, 150, 200, 250, 300, 350 

particle 

Wood 

chips 

Shape spherical 

Feeding 

position (m) At the pipe inlet (x=0) 

Feeding 

Initial velocity Zero (static injection) Feeding 

parcels 

3D 4500 Feeding 

parcels 2D 200 

Discrete 

phase 

boundary 

condition 

The elastic model of particle-wall 

collision 

Both the normal coefficient and tangential 

coefficient are equal to 1. 

2.3 RESULTS AND DISCUSSION 

Numerical simulations have been performed for different sets of values of the parameters in 

Table 2.2. All particles are injected homogeneously and statically (zero initial velocities) from the 

pipe inlet. In some fluid conditions, it was found that the residuals of air turbulent kinetic energy 

16 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

Ik) and its rate of dissipation (i;) may not have been convergent to be less than 1/1000, which is 

Ihe default convergent value for each residual, although the other four residuals (continuity and 

three components of air velocity) had met the convergence criteria. For this case, the criteria were 

modi tied to detennine the evolution of air velocity profile at the nine reference points. The 

simulations were regarded as being convergent when the velocity profile remained steady after a 

finite number of iterations. The typical evolution of air velocity profiles at the nine reference 

points is shown in Figure 2.3(a). These nine points are equally distributed at x=2.0m on the 

vertical line along pipe radius in y direction. The air velocity profile is observed to be developed 

steadily after the 60, h two-coupling computation, as shown in Figure 2.3(a). The relative error of 

air velocity relative to its average value at the nine reference points fell close to zero after 800 

iterations in Figure 2.3(b). Thus, the numerical computations are considered as being convergent 

after 800 iterations for the current fluid conditions. 

OS < - r • 

-0.5; - -' ' 1 '• i 

0.5 0.6 0.7 OB 0 9 1 11 1.2 13 1.4 \ 100 200 300 400 5i)0 COO 700 BOO MO 1!>0O 

U,,'U(1 ilwati'Mi 

(a) (b) 

Figure 2.3 The convergence behaviour of numerical simulations, (a) the evolution ofthe stream-wise air 

velocity profile across the pipe at x=2.0m on seven selected iteration numbers of two-coupling 

computations (b) the development of the relative error of the velocity to its average value versus numerical 

iteration on nine reference points. 

2.3.1 Particle trajectories in the pipe 

Numerical simulations were firstly performed to explore particle trajectories in the pipe. Figure 

2.4 only shows trajectories of glass beads (100pm) injected along the y axis in a smooth pipe 

(rj)80mm) under air conveying velocity 24m/s. Here, R means pipe radius and y stands for the 

feed position of a particle. Both figures in Figure 2.4 clearly display the phenomenon of gravity 

settling. It shows that the peak positions of particles gradually decrease along the pipe due to the 

gravity force. When they hit the pipe surface, particles will bounce and move along the pipe by 

saltation. Since particles are injected at different positions on the pipe inlet, the trajectories ofthe 

particles will not have any axial symmetry property at the particle impact point on the wall due to 

the curved surface of the pipe, even though an elastic particle-wall collision model is applied in 

the calculations. In Figure 2.4(b), we can clearly observe that particles injected close to the wall 

have a jumping path from their feed positions distinct from others in the mainstream region. This 

17 
Chapter 2 Classifying Ihe characteristics of granular biomass and pulverised coal by CFD 

indicates that a particle close to the wall must be acted on by an additional force in the y direction, 

and that this force is larger than the gravity force on the particle. In the boundary region, this 

additional force is the Saffman lift force induced by flow velocity gradients in the y direction. It 

can be expressed as (Fan L.S., 2003). 

oy 

(2.2) 

where Fs is the Saffman force, the constant K is 6.46 (kgmV 3) , U a is the air velocity and U p is 

the particle velocity, p is the air absolute viscosity and v is the air kinematic viscosity. The 

influence of the Saffman force on particles will be discussed in the following three sections 

(2.3.2-4) in terms of variations of air conveying velocity, pipe roughness and particle size. 

35 30 25 20 

(a) 3D trajectories (b) Trajectories in y direction 

Figure 2.4 Particles trajectories (running direction from right to left) 

(Glass beads, dp=100um, Ua=24m/s,<|)80mni smooth pipe, solid loading mass ratio 1% kgs'Vkgs"') 

Figure 2.5 shows the trajectories of two particles injected at two points of y/R equal to -0.90 and 

-0.99 respectively, which are close to the pipe bottom. These trajectories were selected from 

those in Figure 2.4 for clearly visualizing particle trajectories in the boundary region. Observing 

two selected trajectories in the y direction (Figure 2.5a), we can see that the particle injected at 

the position of y/R=-0.99 has a distinct jump at the start point, which is due to its Saffman force 

being larger than its gravity force. On the contrary, the particle injected at the position of 

y/R—0.90 does not have such a feature. Behind the first particle-collision point, the Saffman 

force acting on particles should be decreased since particles have been accelerated in the distance, 

which is more than x/D=15. However, the second peak in the y direction is higher than the first 

one on each particle trajectory, which indicates that the reflected U p y (particle velocity in the y 

direction) is larger than the impacted U p y at the collision point. On the other hand, the trajectories 

of two particles in the z direction (Figure 2.5b) displays the fact that their positions in the z 

direction begin to be away from their feeding positions of z=0 behind the first particle-wall 

collision point. The above phenomena can be also explained by the variations of U p x (the axial 

18 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

particle velocity) and U p y (the vertical particle velocity) in Figure 2.5(c) and (d) for two selected 

particles separately. According to the trajectory before the first particle-wall collision point 

Figure 2.5 (c) and (d), the similarity for both particles lies in that they are accelerated to have an 

increasing U p x in Ihe x axis from the static state at the feeding positions, ln contrast, the 

difference between two particles is that the particle injected at the point r/R=-0.99 has obtained a 

positive U p y which makes it suspended in the y direction (as shown in Figure 2.5 (c)). The 

particle injected at the point r/R=-0.9 falls down due to gravity force in the y direction. When a 

particle hits the pipe wall, we can clearly see that not only the sign of U p y is changed from the 

negative to the positive but also the magnitude of U p y in the rebound direction is larger than that 

in the impact direction. Consequently, the second peak in a particle trajectory is higher than the 

first peak in the y direction. In addition, compared with that in the impact direction, the 

magnitude of U p x decreases a little after particle-wall collision. This indicates that some particle 

momentum on the x axis has been transferred to that in the y direction during particle-wall 

collision, since the normal line of the particle-wall elastic collision is along a pipe radius rather 

than the y axis. 

y/R.-O.OO 

y/R=-U90 lj 

45 40 35 30 25 20 '5 ID 5 0 4b 40 35 30 25 21) 15 '0 

(a) Trajectories in y direction (b) Trajectories in 7. direction 

Panicle peeled at y/R=-0.99 

(c) For the particle injected at y/R=-0.99 (d) For the particle injected at y/R=-0.°0 

Figure 2.5 Trajectories and the variations of (Upx, Up)) for two selected particles 

The above discussions show that both Saffman force and particle-wall collision on pipe wall 

exert an important influence on particle behaviour at the pipe boundary. In order to only 

19 
Chapter 2 Classifying the characteristics of granular biomass and pulverised coal by CFD 

investigate the effect ofthe Saffman force, the following three sections from 2.3.2 to 2.3.4 will 

present the numerical simulations in two dimensions for clarity, corresponding to the z=0 section 

in 3D mode for 100pm glass beads. In this case, the normal line of particle-wall collision is along 

the y axis and there is no momentum exchanged among three components of a particle velocity 

during particle-wall collision. 

2.3.2 The effect of air conveying velocity 

-0.82 

-0.84 

-0.86 

-0.88 

S 4.91 

-0.92 

-0.94 

-0.96 

Ua=18nVs 

Ua=20nVs 

Ua=24m/s 

Ua=30m/s 

U=48m/S 

15 20 25 30 35 40 

X/D 

Figure 2.6 Particle trajectories with different conveying velocities 

(lOOum glass beads, (j>80mm smooth pipe, solid loading ratio l%, 2D model) 

In the 2D mode just as at the z=0 section in the 3D model, 200 parcels (a parcel means a group of 

particles with the same diameter injected at the same position) were equally injected over the 

pipe inlet. The three influences of conveying velocity, wall roughness and particle size are 

investigated for exploring particle characteristics in the boundary region. Changing only air 

conveying velocity while keeping other parameters constant, the numerical results of air velocity 

effects on the boundary region are shown in Figure 2.6. It shows the trajectories of a single 

particle injected at the position of r/R=97.5% with different air conveying velocities. Compared 

with other points, an injection point in the boundary region is close to the pipe bottom where the 

Saffman force should be strong due to the strong gradient of air velocity on the y axis. Observing 

the first peak of the trajectories in Figure 2.6, we can see that the particle jumps higher and 

higher when air conveying velocity increases from 16 to 30m/s. However, the particle can jump 

no higher when the air conveying velocity is more than 30m/'s. In this case, it follows the 

previous highest path. In summary, a stronger air conveying velocity generally benefits by lifting 

a particle higher from the boundary region toward the pipe mainstream field, but it can not be 

lifted much higher when air velocity is more than a certain value. This value is defined as the 

critical air conveying velocity. Here the value of 30m/s is the critical velocity for the current fluid 

conditions. Note that the numerical simulations have been carried out from 16 to 48 m/s at 2m/s 

20 

VES017055 
Aii investigation ol' particle dynamics in a dilute phase pneumatic conveying system 

increments and the data of only live typical air velocities are shown in Figure 2.5 for clarity. 

2.3.3 The effect of pipe roughness 

The effect of pipe roughness, keeping other parameters constant, is now considered. The effect is 

illustrated in Figure 2.7, under the conditions of transporting 100pm glass beads by a 24m/s air 

conveying velocity in a (j>80mm pipe. This figure shows the trajectories of a single particle 

injected at the position of r/R=97.5% with the different pipe roughnesses. We observe that a 

particle in the boundary region will be returned to a much higher position for increases in pipe 

roughness. However, when the pipe relative roughness is more than 1.64%, the trajectories ofthe 

particle are the same and the particle can no longer be lifted much higher. We call the value of 

1.64% the critical pipe relative roughness for the current fluid conditions. 

<? > . A=0.20% 

-0.75 :.r " 4=0.40% 

: J=0.S3% 

S -0.85 

-0.9 

-0.95 ."' .. • 

9 

-1

 1 

D 5 10 15 20 25 30 35 40 45 50 

Figure 2.7 Particles trajectories with different pipe roughness heights 

(Ua=24nVs, lOOum glass beads, <ji80mm pipe, solid loading ratio I %. 2D model)) 

2.3.4 The effect of particle size 

-0.84; , r -

dp ,20„ 

-0.86 • d0=40,, 

-0.88 

d =100|I 

<i =300(1 

43.9 d3=450(1 

E -0.92 : •:• 

> 

-0.94 

-0.96.' 

I 

i J , 

-0.98. • ' " •" 

V 5 10 15 20 25 30 35 40 45 50 

x/D 

Figure 2.8 Trajectories with different particle sizes 

(Ua=24m/s, glass beads, <|)80mm pipe, solid loading ratio 1%. 2D model)). 

21 

VES017056 
Chapter 2 Classifying the characteristics of granular biomass and pulverised coal by CFD 

Iii general, the bigger a particle is, the stronger the gravity force acting on it. Also, according to 

equation (2.2), the larger a particle, the stronger the Saffman force produced. As such, the 

suspension possibility of a particle in the boundary region is determined by the relative 

magnitude of the gravity force and the Saffman forces on it. Figure 2.8 shows the effect of 

particle size under the conditions of 24m/s air conveying velocity in the tj>80mm smooth pipe. It 

shows the trajectories of a single particle injected at the position of r/'R~97.5% with the different 

sizes. Particles from 10 to 450pm at an increment of 10 um have been investigated, but the 

particle trajectories of only five specific sizes are shown in Figure 2.8 for clarity. Basically, the 

bigger a particle is. the higher it can be suspended if its size is in the range of 10-100pm. But this 

phenomenon will no longer exist when a particle is larger than 100p.m. As such, the value of 

100pm is defined as critical particle size for the current fluid conditions. 

As indicated in equation (2.2), the Saffman force in the pipe boundary region will be stronger 

with increasing air conveying velocity, particle size and pipe roughness. However, the critical 

values of these three parameters can not be directly deduced from equation (2.2). The three 

critical parameters explored by the numerical results will benefit in determining the minimum air 

conveying velocity in dilute pneumatic conveying systems. 

2.3.5 The characteristics of particle mean velocity in terms of particle Stokes 

Number 

Table 2.3 Particles Stokes number and terminal velocity 

Coal 

(1550kg/m5) 

Woodchip 

(185kg'm') 

Glass 

(2500kg/m') 

d (um) 

Stokes 

Number 

Terminal Velocity 

U M (m/s) 

d (um) d (um) 

10 0.072 0.0046 30 8 

20 0.287 0.0183 59 16 

30 0.645 0.0408 88 24 

40 1.147 0.0718 117 32 

50 1.793 0.1103 146 40 

60 2.581 0.1548 175 48 

70 3.514 0.2035 204 56 

80 4.589 0.2546 233 63 

90 5.808 0.3065 262 71 

100 7.171 0.3587 291 79 

150 16.13 0.6664 436 119 

200 28.68 0.9472 581 158 

250 44.82 1.2830 726 197 

300 64.53 1.6129 871 237 

350 87.84 1.9328 1016 276 

Stokes Number (St) is defined as the ratio of particle inertia to fluid drag. It is known that a 

particle having a small Stokes Number has a good capability of following the fluid. Since 

granular biomass is usually lighter and larger than pulverized coal, the question whether they 

have any similar characteristic in pipes is investigated in this section. We will use the Stokes 

22 

VES017057 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

Number to classify the similarity between pulverized coal and granular biomass characteristics in 

the direction of fluid How and apply the terminal velocity (Up t) for the dimensionless analysis of 

particle velocities in the gravity direction. The detailed equations for calculating the terminal 

velocity of a particle are listed in the book Processing of Particulate Solids (Seville et al., 1997). 

Table 2.3 shows values of St and U p l for typical samples of pulverized coal, wood chips and glass 

beads. 

In order to address the similarity of granular biomass and pulverised coal, numerical simulations 

have been carried out on the transportation of nine kinds of mono-distributed granular woodchips 

(100~10l6pm) and 10 types of mono-distributed pulverised coal (10~150pm) by 10m/s 

conveying air with solid mass loading ratio 1% in a horizontal smooth 4>80mm straight pipe. Each 

kind of particle was released from the pipe inlet with a homogenous distribution across the whole 

inlet area. The detailed boundary conditions and related numerical parameters are listed in Tables 

2.1 and 2.2. Here we have focused our attention on the analysis of the mean particulate velocities 

of the solid phase, since the mean quantities are of more interest in engineering applications. 

| (a) 

=

 07- .. 

30 40 50 60 70 *0 

/JO 

126 lo 20 30 40 60 60* " 70 80 

i/O 

Figure 2.9 The evolution of the mean particulate velocities Up ) and Dp ) for pulverised coal 

(Ua=10m/s; smooth pipe; coal particles size 10~150gm; solid loading ratio 1%, 3D model) 

In general, the mean particulate horizontal velocity (Up x ) on the x axis or fluid direction is 

rendered dimensionless by the air conveying velocity (Ua ) and the mean particulate vertical 

velocity (Up y ) on the y axis or gravity direction is made dimensionless by its terminal velocity. 

The development of these two dimensionless velocities is shown in Figure 2.9 (a) and (b) 

respectively. 

The evolution of the horizontal dimensionless velocity U p x / U a in Figure 2.9(a) shows the 

23 
Chapter 2 Classifying the characteristics of granular biomass and pulverised coal by CFD 

progression of the solid phase from the acceleration region into the fully developed state. This 

process depends on the mean particulate size of the solid phase. The larger the mean particulate 

size, the longer is the acceleration distance.. In a comparison, fine particles like those of 10pm 

diameter have almost no obvious acceleration distance before they attain a velocity equal to that 

ofthe conveying air (Ua ). This agrees with the theoretical prediction of Seville, (1997), that the 

fine particles have a good ability to follow the fluid. On the contrary, particles with a larger mean 

size such as 150 pm usually have a longer acceleration distance before attaining their peak 

velocities in the horizontal direction. Their horizontal mean velocities subsequently begin to 

decease gradually until they reach a steady value whose dimensionless value is equal to 0.6 

OV/u a ) . 

With regard to the variation of the vertical dimensionless velocity U p y / U a in Figure 2.9(b), we 

can see that the mean particulate vertical velocities (Up y ) of fine coal particles less than 20pm in 

diameter rapidly develop to their corresponding terminal velocity. However, the mean particulate 

vertical velocities of larger particles more than 80pm in diameter tend to zero after they enter the 

fully developed region. This indicates that the vertical velocities of these larger particles have 

equivalent possibilities of being in the up or down direction with the same absolute velocity on 

the y axis. 

(to 

Figure 2.10 Evolution of the mean particulate velocities Up v and Up y for wood particles 

(Ua=10m/s; smooth pipe; Wood chips 100~1016|jm; solid loading ratio 1%, 3D model) 

Since particles with a low Stokes Number (less than 1) have a good ability to follow the fluid no 

matter what kinds of materials they are, we only focus on woodchips with equivalent diameter 

more than 100pm, corresponding to particles with a Stokes number >1 (Table 2.3). Figure 2.10 

shows the variation of the dimensionless velocities of the larger wood chips. As with pulverised 

24 

VES017059 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

coal, the larger granular woodchips compared with the smaller ones have a longer acceleration 

distance to before attaining fully developed flow. Subsequently, their horizontal mean velocity 

reaches a steady value whose dimensionless value (Up xAJ a ) is also equal to 0.6. The averaged 

value of the mean particulate vertical velocities for the larger particles also rapidly reaches zero. 

Summarizing the foregoing discussions, we can classify the characteristics of the mean 

particulate velocities in the horizontal and vertical directions into three types by Stokes Number, 

as listed in Table 2.4. This table indicates that the particles of the different materials have similar 

dynamic characteristics, such as particulate mean velocities in the horizontal and vertical 

directions, if their Stokes Numbers are equal. 

Empirically, it is reported that the slip ratio, defined in terms of the mean velocity of the solid 

phase divided by the conveying velocity, is usually around 0.8 (Mills, 2004). This definition is 

similar to the dimensionless mean particulate horizontal velocity in this chapter if the mean 

velocity of the particles is considered as the solid phase velocity. According to Table 2.4, the 

value of 0.8Ua is the median value ofthe mean dimensionless particle velocity in the horizontal 

direction. Since we use mono-dispersed particles in the numerical simulations, the slip ratio of 

the poly-dispersed solid phase will vary in the range of (0.6~l)Ua depending on its mean 

particulate size. In the vertical direction, particles with Stokes Number less than 1 can be easily 

suspended in a fluid since their vertical velocities equal the corresponding terminal velocities. 

However, for particles with Stokes Number more than 3.5, the averaged value of particle velocity 

in the y direction tends to be zero. This implies that these particles bounce or slide along the pipe 

with a high frequency of particle-wall collisions. 

Table 2.4 The mean axial particle velocity in the fully developed region 

Stokes Number <0.6 0.6-3.5 >3.5 

1 0.6-1 0.6 

u„v/u, -1 -1-0 0 

2.4 CONCLUSIONS 

A detailed analysis of three factors (air conveying velocity, pipe roughness and particle size) that 

influence particle trajectories in the boundary region has been performed by carrying out 

numerical simulations using the Discrete Phase Model in Fluent 6.2.16. The numerical results 

demonstrate that the Saffman Force plays an important factor in a dilute pneumatic conveying 

system, especially for determining whether a particle will be re-suspended from the boundary 

region. This analysis shows that there exist three critical parameters: the critical air conveying 

velocity, the critical particle size and the critical pipe relative roughness. Generally, increasing 

any one of air conveying velocity, pipe roughness and particle size will improve the chances of a 

particle being re-entrained from the boundary region, in agreement with equation (2.1) which can 

25 

VES017060 
Chapter 2 - Classifying the characteristics of granular biomass and pulverised coal by CFD 

be used to calculate the Saffman force. In addition, the numerical results also reveal that the 

critical values of each of these three parameters will represents a limit beyond which an increase 

gives no improvement in particle re-entrainment. 

The characteristics of the dimensionless mean particulate velocity in the horizontal and vertical 

directions have also been investigated. The particle mean velocities of the different materials can 

be classified into three types in the horizontal and vertical directions non-dimensionalising as the 

Stokes Number or dividing by terminal velocity respectively. The numerical results indicate that 

particles with Stokes Number less than one have a mean velocity equal to the air conveying 

velocity in the horizontal direction and a mean vertical velocity equal to the terminal velocity in 

the fully developed region. Correspondingly, for larger particles with Stokes Number more than 

3.5, their mean dimensionless velocity U p x / U a tends towards 0.6 in the horizontal direction and 

their mean dimensionless velocity U p y / U a essentially zero in the vertical direction. The mean 

dimensionless velocities of particles with Stokes number between 1 and 3.6 are in the range of 

0.6-1 in the horizontal direction and -1-0 in the vertical direction. 

The effects of non-spherical particles and inter-particle collisions are ignored in these numerical 

simulations. The latter effect will be studied by using CFD-DEM (Discrete Element Method) 

coupling simulations in Chapter 7. 

26 

VES017061 
Chapter 3 

Extending Laser Doppler Anemometry for 

particle flow measurement in pipes 

An extended Laser Doppler Anemometry (LDA) technique has been developed to measure the 

distributions ofparticle velocities and particle number over a whole pipe cross section in a dilute 

pneumatic conveying system. The first extension concentrates on the transform matrix for 

predicting the laser beams' cross point in a pipe according lo the shift coordinate of the 3D 

computer-controlled traverse system on which the probes of the LDA system were mounted. The 

second focuses on the proper LDA sample rate for the measurement of gas-solid pipe flow with 

polydisperse particles. A suitable LDA sample rate should ensure that enough data is recorded in 

the measurement interval to precisely calculate the particle mean velocity or other statistical 

values at every sample point. The present study explores the methodology as well as 

fundamentals of measurements, using a laser facility, of the local instantaneous density of 

particles as a primary standard. 

3.1 INTRODUCTION 

The technique of Laser Doppler Anemometry (LDA) which is also referred to Laser Doppler 

Velocimetry (LDV) has been developed more than thirty years, since it was invented by Yeh and 

Cummins in 1964. It is non-intrusive with high accuracy and very high spatial resolution and has 

been extensively applied for fluid mechanics measurements. Before introducing the extended 

techniques of LDA, we briefly review the principle of LD A and its applications in gas-solid flow. 

3.1.1 The brief principle of LDA 

The Laser-Doppler method is based on the measurement of the Doppler shift of laser light 

scattered from small particles carried along with the fluid. The basic components and a dual beam 

structure of a LDA system are illustrated in Figure 3.1. This system usually comprises a 

VES017062 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

continuous wave Argon-Ion-iaser, fibre optics, a frequency shifter and several lenses to construct 

an optical probe, which can produce a measurement volume or interference fringes formed bytwo beams with the same light intensity and a different phase modulated by a Bragg cell. The size 

and position ofthis ellipsoidal crossing point is dependent on the focal length of the probe front 

lens. 

-

* t*i(M i 

Th» pnnapWa of Laser Doppler Anefnometry 

S- IDA is B mn irvnA M1 npticiL 

rmwmv t (•finti J? W 

•iveslKiBliiVj jduc J O at a port n n 

raw LUAcan oeused narfr+ft c 

tlqufl f w m h npo J ur -P SS IA I W 

rMHEuiWINt po-n:puMlHCl it* 

* . • •» w t • •• i 

Ut JWinVMICi.il H>HkoljLA JUlTt* 

w hw \KSV S I MI -t rl- •iit.u'ii kur 

pift*urr ii .i.!"*riTi ipii iHanclrlrtrK 

pi IP* Vwtfl r j ;vih- If". crtt'r*. .rrf' 

r Mii«^ &' IT r,-f i * i"»n i .jinn-r 

• rhrbn~k vitwacl-n** jctjp 

i rar cy dw> FisarF low er flow its 

- _ A phoiamuNipiiv corvens 

W l>ght nuntely fl jeli eiium to aire 

Lncal wgn*!s which a n in Dim con 

verted to valoci'y inTorimilinn In Pie 

BSA prrx(5*Kv ihe frKibencv ol ite 

'mlwig 'Kjhi ftVpnlrr irAq-jnney! n 

prupor jural lu ilv Dxm JBioutj el the 

• tH.iU.UiTVII flUM 

I he processing resub are twided 

tynr RSA lirw <irrfrwMi> Ti m com 

rx*->rn«; of +i» vnocty vftctcv car bn 

dcijjecl i»rma anwusly, dapnrjn^ 

on ine systen ronhqurativi 

Figure 3.1 The schematic diagram of a laser-Doppler anemometry (DANTEC, 2008) 

In Figure 3.1, if the two beams with the same wavelength X intersect at an angle G, the distance dr 

between the interference fringes can be theoretically deduced as: 

V (3.1) 

2 sin (67 2) 

When a particle passes through the measurement volume, the scattered light will be distributed 

over the whole of space. The scattered light is characterized by a Doppler shift, the Doppler 

frequency f, which is proportional to the velocity component perpendicular to the bisector ofthe 

two laser beams. An optical fibre mounted behind the front lens of the probe is used to receive 

and deliver the scattered light to a photomultiplier. Then the intensity, phase as well as the 

amplitude fluctuation of the scattered light can be detected and analyzed in the BSA signal 

processor. If we know the time t which a particle takes to travel the distance between two fringes 

(it is also called the fringe spacing), its velocity can be directly calculated by equation (3.2). 

u„ = 

LL 

t 

(3.2) 

In this way, the task of measuring the velocity of a particle is modified to determine the spent 

time t or frequency (f=l/1) of a particle passing through the fringe, which is evaluated in the BSA 

signal processor by using spectral analysis. The relationship between the frequency and particle 

28 

VES017063 
Chapter 3 Extending LDA for particle flow measurement in pipes 

velocity is given as equation (3.3). 

/=^sin(%) (3.3) 

The above explanation is called the Fringe model of LD A in which the Doppler signal is 

interpreted as several single bursts interspersed with periods of low-power shot noise. 

Correspondingly, the principle of LD A can be also explained by the frequency demodulator 

model in which the Doppler signal is considered quasi-continuous. The detailed contents of these 

two models were reviewed by Buchhave et al. (1979) who deduced the model equations and 

summarized the physical parameters affecting the Doppler signal, such as Doppler ambiguity, 

bias correction, refractive index effects and minimizing and eliminating the effects of phase 

fluctuations on turbulence measurement. 

Since a velocity measured by LD A is calculated from the Doppler shift signal produced by a 

moving particle, this measured velocity is essentially the particle velocity rather than fluid 

velocity. As a result, Buchhave et al. (1979) suggested a criterion for particle selection for fluid 

measurement. Empirically, seeding particles with light density should be chosen less than 17 urn 

for measuring air velocity and 49 urn for water fluid assessment. Consequently, when these 

particles are used as the seeding particles, the velocities measured by LD A will be regarded as the 

fluid velocity. Thus, a velocity measured by LD A is considered as particle velocity or fluid 

velocity depending on the Stokes Number of the seeding particles. 

3.1.2 Literature view of LDA applications in gas-solid flow 

Gas-solid flow like pneumatically conveyed bulk solids is frequently met in many industrial 

fields. Instrumentation for detailed gas-solid flow measurements is highly desirable in order to 

understand physical phenomena and mechanisms for optimum design and operation of gas-solid 

systems. Since it is a non-intrusive and highly accurate velocity measurement instrument, LD A 

has been widely applied to investigate characteristics of gas-solid flow individually or combined 

with other instruments. As a result, there are numerous papers in which some interesting results 

have been published and some valuable techniques used for extending the functions of LD A have 

been reported as well. 

Since the key function of LD A lies in velocity measurement, papers related to the velocity 

measurement of a single phase in gas-solid flow are listed in Table 3. From the 1970s, the 

velocity profiles of a single phase in gas-solid flow in vertical or horizontal pipes (channels) and 

Circulating Fluidized Bed have been measured along a pipe radius or any specific line by using 

LDA. The effect of characteristics such as particle velocity, the roughness effect on the 

development of a stabilized laminar boundary layer, the effect of inter-particle collisions and the 

29 

VES017064 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

effect of particle-wall collisions have been completely or partially explored through analyzing the 

velocity data measured by LDA. In addition, several papers also used LD A to calibrate other 

instruments like Electrical Capacitance Tomography (ECT) for velocity measurement. 

Since the velocity of a single phase is just one of the basic parameters to characterize gas-solid 

flow, it is most interesting to obtain the other possible parameters such as solid concentration and 

particle size as well as their distribution on a cross-section or along the fluid direction. To fully 

characterize unsteady gas-solid flow, it is important to measure particle velocity and other 

parameters simultaneously. Table 3.2 lists the previous papers in which LD A was combined with 

other instruments like a pyrometer and a laser-particle-sizing system to obtain additional 

information like particle temperature and particle size as well as measuring particle velocity. 

Moreover, there have been extensive studies for extending the standard single point velocity 

measurement of LD A to multifunctional operation, which include multi-point LDA, simultaneous 

two phase velocity measurement and particle size measurement by using an individual LD A 

device. For example, several papers describe a similar novel method of setting two threshold 

amplitude values to classify Doppler signals for measuring velocities of two phases at the same 

time. The principle of this method is based on the fact that light scattered by a large particle has a 

large pedestal amplitude of Doppler signal. Additionally, several numerical papers demonstrate 

how to assess the stability of signal processing in LD A and analyze the possible errors of LD A 

using a digital method to measure the frequency of Doppler signals. Table 3.3 lists the 

corresponding principle and the brief results of these papers from the 1980s. 

With regard to the optical arrangements in the experiments listed, three general types can be 

categorized as: in an open environment; in a bounded region with a slot or hole as the 

measurement window; and in a transparent bounded region without the additional flat window on 

the wall. Obviously, no light refractive effect occurs in an open environment and there is only a 

light shift phenomenon on a flat window. But it is necessary to consider the refractive effect 

which results in changing the direction of a laser beam at a curved surface. As for the refractive 

effect, Tsuji et al. (1982, 1984) mentioned it in his horizontal and vertical pipe flow 

measurements. But no detailed information was given for analyzing the deviation of the LD A 

measuring point from its expected position in pipes. It was assumed that the influence of 

refraction could be neglected so long as the traversing of the measurement point was within a 

specific range in a pipe. Most papers give no clear information on dealing with the refraction 

effect. Many experiments in pipes or channels were carried out to measure particle velocity along 

a pipe diameter line rather than on a whole cross-section. Thus, it is a basic and essential task to 

study the refractive effect due to pipe curvature for measuring particle velocity in gas-solid pipe 

30 

VES017065 
Chapter 3 Extending LDA for particle flow measurement in pipes 

flow. 

In the previous section (3.1.1), we have discussed the fact that data recorded by LD A represent 

the velocity of a particle passing through the measurement volume. With regard to gas-solid 

measurement, particles of solid phase are directly regarded as the seeding particles. Consequently, 

the measured results are normally considered as a group of instantaneous particle velocities 

within a specific interval. The local velocity of the solid phase at this measurement point is 

usually averaged from this time series data. However, particle size of the solid phase may be 

distributed widely and particle density may also differ for multi-material solid phases such as 

pneumatically conveying a fuel mixture of pulverised coal and biomass particles, ln these cases, 

the accurate time-averaged value will be strongly dependent on obtaining enough sample data in 

the recorded data set of particle velocity. In particular, particle velocity varies with its Stokes 

Number which is related to particle size, particle density and local air velocity at a sample point. 

Hence, considering the proper sample rate of LD A is also essential and necessary to gas-solid 

pipe flow measurement. 

For the above discussed reasons, we will study the refraction effect and sample rate in the next 

two sections. 

31 

VES017066 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

Table 3.1 The applications of an individual LDA (Single phase measurement) 

Reference Experimental rig Flow condition 

Measurement 

window type 

Results 

Birchenough 

and Mason 

(1976) 

Barrent et 

al. (1995) 

3m long, <|>49.4mm vertical 

glass tube 

An 800mm wide and 1100mm 

long flat plate with an elliptical 

leading edge section and porous 

suction surface mounted in the 

centre of wind tunnel (800mm 

width, 600mm height,1600mm 

length) 

A jet (its central ij)40mm and its 

annular (j)68mm) was mounted 

into a <|>2000mm cylindrical 

container 

An in-line tube bank consisting 

of a 6*6 array of carbon steel 

tube was arranged in a wind 

tunnel 

<f- The rig is same as the 

above one for measuring 

the instantaneous incident 

and rebound particle 

velocities by using two 

dimensional LDA in the 

immediate vicinity of the 

tubes surface. 

Parkhomov A 2500mm long vertical 

Fan et al. 

(1996) 

Tu et al. 

(1997) 

Morsi et al. 

(2004) 

Air conveying velocity, 16-

48m/s 

-v* Solid loading ratio less than 2.2 

-v- Alumina powder with mean 

size 21pm 

-v" Air velocity less than 105 m/s 

•v- Seven kinds of particles ranging 

from 0.5 to 1.5mm 

Primary air velocity 20m/s; 

Secondary air velocity ranging 

from 30 to 60 m/s 

Solid loading ratio, 0.5-1.5 

Silica gel powder (its mean size 

50 and 200 pm) 

11.2m/s air conveying velocity 

•s> two kinds of glass beads (66 

and 95 pm) 

*v- solid loading ratio 0. lkg/kg 

<• Air velocity (8~13.7m/s) 

-v" Glass beads (means size 35,66 

and 93 pm) 

*v- Solid loading O.lkg/kg 

Direct measurement 

without considering the 

effect of light refraction 

on the tube surface 

No information about 

arranging the 

measurement window 

Flat glass measurement 

window on the wall of 

the container 

A glass measurement 

window of 

360*500mm2 was open 

on the bank wall. 

No information about 

arranging the 

measurement window 

Reynolds number 15300 No mention 

-v- With increasing solid loading, the experimental 

results display that the particle velocity profiles 

become flattened 

There is negative particle slip at the pipe wall. 

•4- Examining the effect of isolated surface roughness 

elements on the development of stabilized laminar 

boundary layers by three- component LDA. 

"v" The critical speed at which disturbances are 

propagated downstream from a surface has been 

found to be characterized by a constant value of 

roughness Reynolds number (about 800). 

The results indicate that the diffusion of the 

particle phase in the radial direction is much 

slower than that ofthe gas phase and the mean 

velocity profiles of the particle phase are flatter 

than those of the gas phase. 

The collisions between particles and the tube 

surface had considerably influence on particle 

motion, which resulted in substantial differences in 

the flow patterns between two phases through the 

tubes. 

-y" For a single tube, the particle rebounding flow 

pattern in the vicinity of the wall was mainly 

affected by particle inertia (Stokes Number). The 

gas flow at high Reynolds number was found to 

suppress the rebound tendency ofthe particles. 

•v- For tube bundle structure, particles rebounding 

from the approaching tubes upstream migrated 

downstream and impinged the tubes in an 

extremely complex and random disposition. 

A significant anisotropy of fluctuations of particle 

32 

VES017067 
Chapter 3 Extending LDA for particle flow measurement in pipes 

(2007) stainless steel pipe with 446mm 

Datta et. al., A 7 m long horizontal pipe 

(2007) section (<|)57mm) and a 3 m 

long vertical section. 

•v- Glass beads (mean size 50pm) 

-v- Tracer particles 2~3pm 

Air velocity ranging from 1.0 to 

5.0m/s 

Polyamide chips approximate 

(3mm long, 3 mm wide and 

1mm thick) 

No mention 

velocity has been found. The amplitude of 

turbulent fluctuations of particle velocity in the 

axial direction is much higher than that in the 

radial direction 

For low solid concentration, the pixel values ofthe 

images measured by ECT were low as well, there 

was not adequate signal to obtain valid velocity 

measurement from the instantaneous correlation 

function. This was observed from the velocity 

distribution profile obtained by ECT compared 

with the data of LDA 

Table 3.2 The applications of LDA combined with other instruments for gas-solid flow measurements 

Reference Experimental rig Flow condition Additional instrument and 

Measurement window type 

Results 

Fincke et al. Powders of a metal (Ni-Al) Air spray velocity A two-color pyrometer Measured particle size, velocity and temperature 

(1990) and a metal oxide (AI2O3) 100~300m/s for measuring particle simultaneously 

were presented for Particle size within temperature The particle velocities and temperature were 

studying the plasma spray- 5-100 pm A scattered-light insensitive to the particle injection rate over the 

coating process Particle mass flow rate particle size loading range. 

within 0.25 to 5 measurement Particle injection geometry and particle velocity and 

kg/hour No measurement mass significantly affect the spatial distribution of 

window (in open particle size in the plasma plume and thus can alter the 

environment) characteristics of the deposited coating. 

Huber and In a dilute pneumatic Air conveying velocity Phase Doppler The experimental results indicated that the pipe wall 

Sommerfeld conveying system, the ranging from 10 to Anemometry (PDA) roughness had a considerable influence on the particle 

(1994) measurements were 30m/s for particle size motion after a wall collision and, especially for small 

performed in horizontal Solid loading ratio measurement collision angles. 

and vertica vertical pipes between 0.2 and 1.0 Laser-light sheet An increased conveying velocity reduced segregation 

kg/kg technique for in a horizontal pipe due to the higher flow turbulence 

Glass beads with mean measuring the solid established. 

diameter 40pm concentration within The motion of large particles (larger than 100pm) was 

the whole pipe cross- strongly governed by inertial effects and no marked 

section segregation was observed in the different pipe 

No information about elements considered. 

the light refractive 

effect on pipe surface 

33 

VES017068 
An investigation of particle dynamics in a dilate phase pneumatic conveying system 

Van de wall and 

Soo (1994) 

A cyclone-standpipe 

recirculating pipe flow 

system 

A horizontal pipe (|>127mm 

Air conveying velocity 

20m/s 

<- Glass beads <|>44-62um 

<• Solid loading ratio 

0.4-2.2 kg/kg 

-v* Phase Doppler Particle 

Analyzer (PDPA or 

PDA) 

No information about 

the effect of the light 

refraction on the pipe 

surface 

-0-

Determination of instantaneous local particle velocity 

and density 

The experimental data confirmed that PDPA provides 

a means of determining the instantaneous local 

particle density when the averaging procedure 

satisfied the criterion of continuum counterpart of 

time-averaging and length scale relation. 

The local averaging time should include on the order 

of 102 particles for the continuum approximation and 

the path length should be smaller than the 

characteristic dimension of the physical system 

Bao and Soo 

(1995) 

The same as the above rig Air conveying velocity 

20 m/s 

Solid loading ratio 

1.92,2.04 and 2.5 

kg/kg 

Glass beads ranging 

from 0~80pm with the 

mean size 45 pm 

PDPA 

<*• No detail information 

about the effect of 

refraction at the pipe 

wall 

Used a two-dimensional Laser Doppler Velocimeter 

combined with a phase Doppler particle analyzer to 

determine average and fluctuating properties and 

Reynolds stresses. 

The traverse of the measuring volume along a vertical 

diameter gave results showing the gravity effect in the 

flow of a gas-particle suspension 

The instantaneous local particle cloud density and 

velocity showed the nature of distribution in the 

particle phase. 

Table 3.3 Development of LDA multifunction for gas-solid flow measurement 

Reference Research Objectives and 

Experimental rig 

Flow condition Multifunction principle and 

Measurement window 

Results 

Tsuji et al. A horizontal glass pipe 

(1982) (<|)30.5mm) in a pneumatic 

conveying system 

Air conveying velocity 

from 6 to 20m/s 

Two kinds of plastic 

particles (1000 kg/m3) 

with 0.2 and 3.4mm mean 

size respectively 

Solid loading ratio less 

than 6 kg/kg. 

Ammonium chloride 

Principle: determining 

the pedestal component 

of the burst signal of 

LDA, the larger a particle 

is, the larger the pedestal 

component is. 

Window: considering the 

refractive effect at the 

pipe wall, but without 

Measured the velocities of air phase and particles 

simultaneously 

As the loading ratio increased and the air velocity 

decreased, mean-velocity distributions of both 

phases became more asymmetrical tendency. 

The effects of the solid particles on air-flow 

turbulence varied greatly with particle size. That is, 

3.4mm particles increased the turbulence markedly, 

while 0.2mm ones reduced it. 

34 

VES017069 
Chapter 3 Extending LDA for particle flow measurement in pipes 

Tsuji et al. 

(1984) 

Dupont et al. 

(1991) 

Chao et al. 

(1992) 

A vertical glass pipe 

(<t>30.5mm) in a pneumatic 

conveying system 

A circular cylinder in a 

wind tunnel for studying 

the wake developing 

behind it 

The experiments were 

performed in the shear 

layer of turbulent jet flow 

issued from a large 100 

and 10 mm nozzle 

Grechikhin Numerical simulation of 

and analyzing the errors of the 

Rinkevichyus signal processor of a LDA 

(1993) using digital methods to 

measure the frequency of 

the Doppler signal. 

•0-

•0-

(<j>0.6pm) as tracers 

Air conveying velocity 

from 8 to 20m/s 

Five kinds of glass beads 

(mean size 3.43,2.78,1.0, 

0.5 and 0.2 mm) 

Solid loading ratio less 

than 5 kg/kg. 

Air velocity 4.5 and 61 

m/s 

No information for tracers 

Seeding particles of 

paraffin oil with a nominal 

diameter of lpm 

Air velocity 4m/s for the 

small nozzle and 1.8m/s 

for the large nozzle 

any detailed explanation. 

The same as the above 

Used spectral analysis 

method to assess the 

sample data obtained by 

LDA 

Window: in open 

environment 

Used an unequally 

spaced fractal 

interpolation method to 

reconstruct the random, 

intermittent, low data 

rate LDA signals for 

spectral analysis 

Window: in open 

environment 

Considered the photodetector output represented as the sum 

of particle velocity signal and noise component of a steady 

Gaussian process with zero average value 

The errors related to the frequency difference of the 

interfering beams, the period of the interference pattern and 

the effective radius of the measuring region 

*v- The smaller the particle size, the flatter was the 

mean air velocity profile for the same mass flow 

ratio of solids to air. Large particles increased air 

turbulence throughout the pipe section, while the 

small particles reduced it. 

•v- Both effects of promotion and suppression of 

turbulence were observed at the same time in the 

presence of particles of medium size, that is, the 

turbulence was increased around the pipe centre and 

reduced near the wall. 

•v" A simple model of PSD (power spectral density) 

was introduced to determine whether the particles 

used for fhe seeding were able to follow the 

characteristic frequencies of the flow 

-4* It has been shown that the PSD can be relied on up 

to frequencies much greater than the mean sampling 

rate as long as there are well pronounced peaks in 

the spectrum. 

•v" The reconstructed result not only augmented the 

data points to improve the statistics but also retained 

the turbulent fractal dimension ofthe flow. 

-v" The results showed that the fractal reconstruction 

method could properly restore the characteristics of 

peak frequencies of the flow from low data rate 

LDA signals or from decimated data, while the 

conventional sample-and-hold method failed to 

capture any of the major peak frequencies. 

- V s Developed a method for evaluating the period of the 

high-frequency filling of the LDA signal from the 

coordinates of the "zero" of the process and for 

estimating the signal frequency from the position of 

the maximum value of the spectral density. The 

results reported show that the limiting values of the 

total error of measurement of the LDA signal 

35 

VES017070 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

Chen and 

Kadambi 

(1995) 

Frank et al. 

(1996) 

Werther et al. 

(1996) 

A 3.05m long PVC pipe 

(<|)25.4mm) connected with 

a glass test section 

A horizontal and vertical 

glass pipe in a heat transfer 

station 

A cold model of 

Circulating Fluid Bed 

(CFB) with inner diameter 

of the riser 0.4m and 

15.6m height. The 

measurement region 

located at a height of 6m 

above the distributor 

4-

Silica gel particles of 

mean size 40pm 

50% sodium iodide 

solution as the liquid 

phase whose refractive 

index is similar to the 

silica particles 

Used the dimensionless 

parameters without clear 

data about fluid 

conditions 

Superficial gas velocity 

4m/s 

Quartz sand with mean 

size 166pm 

Solids volume 

concentration 1.2% 

•v* Used the matched 

refractive index of the 

solid and liquid to 

discriminate the signal 

amplitude according to 

the histograms of 

measured velocity data 

"v- Without consideration of 

the refraction effect on 

the pipe wall 

Velocity-area methods 

are characterized by 

dividing the cross-section 

into a number of annuli 

of equal area. 

Without infonnation 

about effect of light 

refraction on the pipe 

wall 

The LDA probe was 

contained in a 

miniaturized measuring 

head which could be 

inserted into the CFB 

Compared with data 

measured by fibre-optical 

probe 

Window: no refraction 

effect due to the Probe 

being inserted into the 

CFB 

frequency can be decreased to 0.1-0.3%. 

Discriminated between solid and liquid velocities in 

a refractive index matched solid-liquid slurry flow 

by identifying the histogram peaks associated with 

the solid and liquid velocities respectively, 

•v- The experimental data demonstrates that this 

discrimination technique is suitable to be applied to 

slurry flows flow of low turbulence intensity (less 

than 10%) with volumetric concentration as high as 

25% in the heterogeneous and saltation flow 

regimes 

«v- Developed a multi-point technique of LDA for flow 

rate measurement in asymmetric and unsteady pipe 

flow 

The uncertainty of flow-rate measurement was 

decreased less than 1% by using velocity-area 

methods. The minimum number of radii for slightly 

asymmetric flow is four, otherwise six or eight radii 

are used 

•v- The design of miniaturized low-cost multi-point 

sensors with just one laser and signal processor is 

shown to be possible. 

•v" The experimental data confirmed that the LDA 

technique is a reliable and precise tool for particle 

velocity measurements in a CFB 

"v- It also demonstrated that the results of both LDA 

and the Fibre-optical probe yielding in the CFB riser 

were in close agreement 

Mathiesen et A clear PVC riser •fr Air superficial velocities -fr The Shape -v- Developed a modified Shape Discrimination 

36 

VES017071 
Chapter 3 Extending LDA for particle flow measurement in pipes 

al. (1999) Oj.50.8mm, 2.75 height) in 

a CFB was used and the 

test section was located 

1.12m over the elbow inlet 

0.7 and 1.4m/s 

FCC catalyst with density 

1700 kg/m3 and size range 

from 11 to 180 pm (Sauter 

mean diameter 55pm) 

-v- Solid concentration in 

CFB 0.5 and 2% 

Discrimination technique 

based on knowledge 

about Doppler burst 

signal was used to 

determine particle size 

through processing the 

Doppler signal. 

A pair of flat glass 

measurement windows 

were mounted on the 

wall 

Technique to extend the LDA technique to measure 

particle size directly 

The experimental results showed that there was no 

significant agglomeration in the gas-solid flow 

system. Typical annulus-core flow behavior was 

observed 

The largest particles showed the highest fluctuating 

velocity. This behavior was caused by significant 

gas-solid interactions. 

Hussainov et 

al (2000) 

Hart et. al. 

(2001) 

Lain et al. 

(2002) 

A vertical two-phase windchannel with a closed test 

section. The test section 

was made of plexiglass 

about 2m length. Its initial 

upper cross section 

200x200 mm2 and its 

downstream cross-section 

was 205x205 mm2 

Developed a seedless 

equivalent of LDA for use 

in low-turbulence or 

supersonic flows or 

elsewhere where seeding is 

undesirable or impractical 

A horizontal channel of 6m 

length with the section 

35x350mm2 

0-

Air conveying velocity -v* 

9.5/s 

Glass beads with mean 

size 700pm 

Solid loading ratio less 

than 0.1 kg/kg 

Ti02 particles (2pm) as 

tracers 

Used two laser powers of 

26 and 10 mw to form 

two channels for 

measuring gas (tracers) 

and particle phase (glass 

beads) velocities 

simultaneously 

Flat measurement 

window 

The principle of Heterodyne Non-resonant Laser-induced 

Thermal Acoustics (LITA) is to analyze the Doppler shifts 

ofthe acoustic waves produced by a short pulse pumped 

laser in the medium for measuring temperature and velocity. 

Neither molecular nor particulate seed is added to the flow. 

Air volume flow rate from 

507 to 1002 m3/h and 

maximum air conveying 

velocity 30m/s 

Solid loading ratio up to 2 

kg/kg 

Five kinds of glass beads 

with mean size 60,100, 

195, 625 and 100pm 

respectively. 

Measured two phase 

velocities simultaneously 

based on the signal phase 

discrimination, which 

was introduced in the 

paper ofQiu etal. (1991) 

A glass plate of 

measurement window 35 

by 350 mm was inserted 

at the top wall. 

•v* The coarse particles considered, attenuated the gridgenerated turbulence resulting in an increase ofthe 

turbulent energy in the sub-range of the energy 

containing eddies. 

• Y * The influence ofthe Stokes number on the 

turbulence modulation of the carrier flow is strong. 

"v- A compact, portable, single-component heterodyne 

LITA Velocimeter was constructed. 

•4- A comparison ofthe experimental data with Pitotstatic probe measurements showed that they agreed 

with each other. Repeated measurement of various 

flow profiles showed excellent stability. 

-y- The experimental data validated the Reynolds stress 

turbulence model in conjunction with a wall 

roughness model and a stochastic inter-particle 

model, which were introduced by this author. 

-y" It also demonstrated that both inter-particle 

collisions and wall roughness had a very strong 

effect on the development of the particle phase 

properties. 

A slight enhancement of the turbulent kinetic energy 

was caused by particle collisions 

37 

VES017072 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

•v- Spherical tracer particles 

with a nominal size 4pm 

Furunich et A development of a two- A two-dimensional flow 

al. (2002) component velocity channel of 160 by 160mm 

profiler using a fiber multi- was used to simulate 

point LDA and a CCD area 0.065m/s water flow 

image sensor "5* Tracer particles of glass 

beads with 100pm mean 

size. 

Combined the multipoint LDA system and a 

CCD area image sensor 

to measure a twocomponent velocity 

profile simultaneously as 

a function of time 

•v- A novel method to measure a two-component 

velocity profile simultaneously was developed by 

using combined system of the CCD area image 

sensor and the multi-point LDA. 

Fitzpatrick 

and Simon 

(2005) 

An isothermal jet at a 

Mach number of 0.75 

Measured the velocities 

near the end of the 

potential core of the jet 

• Air velocity 0.75 Mach 

No detail on tracer 

particles 

Used the sample-andhold reconstruction 

technique for 

determining the cross 

power spectra of 

turbulence 

-v- The results obtained by the proposed method were 

much better than the mean sample frequency for the 

low LDA data rates and to the mean sample 

frequency for the higher rates. 

The computational time required for the new 

approach is significantly less than for the traditional 

slot correlation method in LDA signal processing 

Li et al. A development of multi-

(2005) point LDA 

4- By employing two acoustic optic modulators for both beam 

splitting and frequency shifting, a multiple measuring 

volumes could be formed by intersecting the multiple 

diffraction orders generated by the two modulators. Each 

measuring volume had its frequency shift, which was 

proportional to the corresponding velocity at the local point 

-v- In contrast to the other multi-detector system, the 

structure ofthis multi-point LDA system was 

significantly simplified. 

As the multi-channel information was demultiplexed by a single-channel signal processor, 

the processing speed needed to be increased. 

38 

VES017073 
Chapter 3 Intending LDA for particle How measurement in pipes 

3.2. EXPERIMENTAL APPARATU S 

•-f-i 

M;r out 

Notice 

—H 

(1) Cyclone 

i2) i>crev°/Feeder 

ii) Centrifugal Fin 

Figure 3.2 Sketch ofthe experimental rig 

In order to characterize the cross-sectional characteristics of the solid phase in different pipe 

elements of a dilute pneumatic conveying systems, a pilot scale positive pressure dilute 

pneumatic conveying system was built which involved horizontal and vertical pipes connected by 

an elbow (R/D=1.3). The schematic diagram ofthe experimental apparatus is shown in Figure 3.2. 

The pipe inner diameter and outer diameter are d>75 and <j)85mm respectively. The effective length 

of the horizontal glass pipes is 4m and the vertical is 2m long. A T-piece is used to connect a 

screw-feeder with the horizontal pipe and several plastic tubes are used to connect a centrifugal 

fan and a cyclone. This cyclone is used to separate particles from the air at the end of the system. 

Then particles separated from air are collected in a storage reservoir for recycling in further 

experiments. Additionally, every metal connector between the pipes is earthed to reduce 

electrostatic risk due to particle-wall collisions. 

Air flow is provided by a centrifugal fan with a maximum 0.05 m3/s volume flowrate under 

against a 2.2kPa back-pressure. Air conveying velocity can vary from 10 to 42m/s by adjusting 

the pipe back-pressure and the butterfly valve at the inlet of the fan. The maximum particle mass 

flow rate of the feeder was experimentally determined to be 1.0x10"' kg/s for glass beads with 

actual density 2550 kg/m3 and bulk density 1559 kg/m3. 

39 

VES017074 
An mvcMig;tlic)ii of p;irliclc dynamics in a dilute phase pneumatic conveying system 

According to the numerical results of Chapter 2, which states that particles with different 

materials in gas-solid pipe flow have similar properties at equal Stokes Number (St), three types 

of glass beads and a smoke are selected in our experiment. Glass beads were used to simulate 

gas-solid pipe flow and the smoke was used as the seeding particle for air fluid velocity 

measurement. The manufacturer's data (Worf Glaskugeln, 2008) indicate that the first type of 

glass beads range from 0 to 50pm, the second within 70-110pm and the third varied from 

180-300 um. 

Since the pipeline is made of glass, the rig can be used to investigate properties of gas-solid flow 

in a horizontal and vertical pipe as well as in an elbow by using LD A or any other optical 

imaging system. 

3.3 DEVELOPING THE SPATIAL TRANSFOR M MATRIX FOR LDA PIPE 

FLOW MEASUREMEN T 

The review of LDA applications in gas-solid flow discussed in the section of 3.1.2 indicates that 

few papers discuss the refraction effect which causes the measurement volume of LDA to deviate 

from its expected position in the pipe when the crossing point of the laser beams passes through 

the curved surface of the pipe. In order to access the whole area of a pipe cross section, it would 

better to evaluate theoretically this deviation. Consequently, a shift coordinate matrix was 

devised to map actual positions of the measurement volume in the pipe. This is called the 

transform matrix. 

In this section, the basic parameters of LDA used in this study are introduced. Then we will 

demonstrate how to calculate a spatial coordinate matrix for a given experimental rig and a fixed 

optical arrangement of the LDA. 

3.3.1 Parameters ofthe LDA system 

Two FibreFlow probes with a front lens of f=800mm are used to measure three dimensional 

velocity in a 3D Dantec LDA system. One probe combines two pairs of beams with wavelengths 

of 488 nm (blue) and 514.5 nm (green) respectively. The planes produced by the different 

coloured beams are perpendicular to each other. Another probe was integrated with one pair of 

476.5nm beams (violet). As a result, the 3D particle velocity could be measured simultaneously 

by using these two probes when three pairs of beams focus on the same point. A 40M shifting Hz 

frequency of a Bragg cell is selected to detennine the particle velocity direction. Table 3.4 lists 

the essential parameters of this three-component LDA. 

40 
Chapter 3 Intending LDA for particle flow measurement in pipes 

Table 3.4 LDA parameters 

Blue Green Violet 

Nurnber of fringes 22 22 22 

Fringe spacing (pm) 5.281 5.568 5.672 

Beam half angle (deg) 2.648 2.648 2.648 

Width of measuring volume (mm) 0.121 0.128 0.122 

Length of measuring volume 2.624 2.767 2.832 

(mm) 

... . in the horizontal 6-20 -5-5 -5-5 

Velocity / / \ pipe 

range (m/s) . . 

in the vertical pipe -5-5 6-20 -5-5 

In order to accurately locate the beams' crossing point, the LD A probes are mounted on a 

computer-controlled 3-D traversing system whose minimum step is 0.5mm in all three orthogonal 

directions. A Burst Spectrum Analyser (BSA) is used automatically to process the LD A burst 

signals. Every record of the experimental data includes particle order number, transient time in 

the sample volume, three components of particle velocity and the related status. Then statistical 

values of particle velocity and particle number are calculated from the recorded set in postprocessing. 

3.3.2 Optic principle of a ray crossing a pipe wall 

Optical axis 

Figure 3.3 Refraction of a ray at a curve 

(the incident angle 0j; the refractive angle 0r; the curve radius R and the curve centre C; y j and y, are 

equal to the distance from the refractive point to the oplical axis; (y;, at) is the coordinale of a 

inbound ray and (yr, a,) is the coordinate of its refraction ray ) 

It is known that three components of a particle velocity can be measured by using three pairs of 

laser beams with the same intersection point where the particle crosses. Every velocity 

component is proportional to the corresponding Doppler frequency originating from the fringe 

intersected by a pair of laser beams. However, in pipe flow measurement, the crossing point 

intersected by three pairs of different coloured laser beams may be separated into three crossing 

points or no crossing point when this point at which the laser beams converge on the outside is 

shifted inside the pipe. Since a hollow pipe comprises two curved surfaces, refraction occurs at 

41 
Ail investigation of particle dynamics in a dilute phase pneumatic conveying system 

Ihe pipe outer and inner surfaces when a beam traverses the pipe wall. A pair of beams may be 

convergent dependent on their incident conditions. Before discussing the optical refractive effect 

on a pipe wall, let us consider the refraction of a beam at a cylindrical surface dividing media of 

refractive indices n, and nr, which is shown in Figure 3.3 (The subscripts of i and r mean incident 

and refractive respectively). 

According to the paraxial form of Snell's law, the optical relationship between the incident beam 

and its refractive one can be given as (Pedrotti et al., 2007): 

"1 0 

R\n.. n,. 

where the first item on the right side is defined as the refraction matrix M R. 

, ; "v 

Optical axis 

(3.1) 

Figure 3.4 The geometric sketch of a beam translating a distance L 

With regard to a simple translation in a homogeneous medium (as shown in Figure 3.4), the 

relationship of a beam passing through two points with a distance L can be described as: 

(3.2) 

"1 L 

0 1 

where the first item on the right side is called as the transfer matrix M T 

Consequently, the matrix description of a ray passing through a pipe wall can be simplified as 

occurring twice (refractions at each surface) with a translation between the surfaces. The optical 

characteristic of this process is represented by the system matrix of equation (3.3): 

S = MK2MTMK, (3.3) 

where the subscripts of 1 and 2 mean pipe outer and inner surfaces. The matrices are multiplied 

together from right to left. 

The above equations are limited to two dimensional optical calculations and are not directly used 

in three dimensional optical calculations in pipe flow measurement, for the laser beams of the 

LDA and the normal line of the pipe surface may not be in an identical plane and their spatial 

42 

VES017077 
Chapter 3 Hxtcnding LDA for particle flow measurement in pipes 

position should be expressed by three dimensional vectors. 

5 -

5 0 

Figure 3.5 The schematic of a ray crossing a pipe 

Figure 3.5 illustrates the optical path of a ray crossing a pipe wall. This ray is assumed to 

originate from the point M| . P] is its first refraction point at the pipe outer surface and Vi is its 

second refraction point at the pipe inner surface. Oi and Oi are the circle centres of the pipe 

outer cross-section through P| and the pipe inner cross-section through P2. These two sections 

are perpendicular to the x axis. As such, the vector M tP t means the ray outside the pipe. The 

vector PIP2 shows the ray crossing the pipe wall and the vector P;M, displays the track of the 

ray inside the pipe. Since a pipe cross-section is a standard circle, the normal vector of a point on 

its perimeter is the pipe radius pointing outward (we use the Capital Bold Italic to express a 

vector). 

In the Cartesian coordinate system of Oxyz, we introduce three spatial unit vectors (V„ V,„ Vr) 

of a ray describing its refraction at the point Pi on the pipe outer surface. PI is (he spatial 

reference point. F, is the unit vector of the incident ray. V„ is the unit vector of the normal ray at 

point PI of the surface and Vr is the unit vector of the refractive ray. According lo Snell's 

refraction law, these three vectors should locate within the incidence plane formed by the vector 

product of V; and V-,. The relationship among these three vectors can be expressed as 

V,.-(-F„ ) = cosf?,. 

Vr-{VixVn)=0 

i i 

«;sin<9. = « sinf? 

(3.4) 

where (n„ nr) are the refractive index of the medium outside and inside the pipe wall. Thus, the 

unit vector of the refracted ray can be deduced from equation (3.4) when the incident unit vector 

43 

VES017078 
An investigation ol'particle dynamics in a dilute phase pneumatic conveying system 

and Ihe normal unit vector are known. If the refraction point Pi is also known, the optical path of 

a beam crossing a pipe wall can be fixed in Cartesian coordinates. This refraction point Pi is the 

crossing position of the beam intersecting with the pipe outer surface. Subsequently, the 

coordinate ofthis point can be concluded from equation (3.5) 

2 ^ 2 Dl 

V + Z = 

(3.5) 

where (a, b, c) are three components of the incident unit vector of a beam and (x0, yo, z 0 ) is its 

initial position related to a Cartesian coordinate of the origin point on the pipe centre line. The 

dependent variables (x, y, z) are each given functions of an independent variable t in the 

parametric form of a beam's mathematic equation. D is the pipe outer diameter. 

It is clear that there will be four steps as shown in Figure 3.6 to determine the optical path of each 

beam of the LDA crossing a pipe wall by using equation (3.5) and (3.4). Then the actual 

coordinates of the measurement volume formed by a pair of laser beams can be calculated by 

solving the crossing point of two lines described by the refractive unit vector inside the pipe and 

the refraction point on the pipe inner surface for each beam. 

The vector of the 

incident beam (vj) 

is known from the prearranged LDA setting 

The equations of pipe 

outer and inner 

surfaces (S„, S,) can 

be obtained by the 

fixed experimental rig. 

Determining the crossing point P, 

at the pipe outer surface 

(1) Equation (3.5) 

Determining the refractive unit 

vector of (Vr) within the pipe wall 

(2) Equation (3.4) 

Determining the crossing point P2 

at the pipe inner surface 

(3) Equation (3.5) 

Determining the refractive unit 

vector of (Vr) inside the pipe 

(4) Equation (3.4) 

c End 

44 

VES017079 
Chapter 3 Hxtcnding LDA for particle flow measurement in pipes 

Figure 3.6 Flow chart of determining the optical path of a ray crossing a pipe wall 

3.3.3 Optic paths of different colour beams through a pipe 

According to the parameters of LDA listed in Table 3.4, the half intersecting angle of each of the 

pairs of laser beams in the probes of LDA is 2.648°. Now we construct a Cartesian coordinate 

system with origin at the centre of a pipe cross-section which is the measurement region in our 

experiments. With regard to the probe combining two pairs of beams, the plane of the probe 

window and the two planes formed by two pairs of beams are orthogonal with each other. In this 

case, the probe axis is not only the normal line of the probe window but also the crossing line of 

two planes fonned by two pairs of laser beams. Now the blue beams of the LDA are arranged to 

measure one velocity component in the x axis or the horizontal direction. The green and violet 

beams arc built for detecting the other two components of velocity in the plane Oyz. Therefore, 

we can follow the flow chart in Figure 3.6 to calculate the spatial position of each beam of LDA 

inside the pipe when the angle of the probe axis intersecting with the horizontal plane of Oxz is 

known and the crossing point of three pairs of laser beams focus on the point of the perimeter 

curve of the pipe cross-section intersecting with the horizontal plane. 

For example, in the horizontal pipe of our experimental rig shown in Figure 3.2, the pipe inner 

and outer diameters are <j)75mm and (|)85mm respectively. Under the condition ofthe zero probe 

angle (meaning that the plane formed by the blue beams lies on the horizontal plane of Oxz, the 

crossing point of the blue beams is initially adjusted to locate at the point (0, 0, 42.5) mm, which 

is the crossing point of the perimeter curve of the pipe cross-section (x=0) intersecting with the 

plane Oxz. It is clear that the cross point of the blue beams will arrive at the coordinate origin 

when the probe is moved about the distance (0, 0, -42.5) mm along z axis if there is no refractive 

surface. However, considering the refraction effect of the pipe, Figure 3.7 shows the optical paths 

of two blue beams through the pipe wall and the crossing point is moved to the point (0, 0, -1.67) 

mm rather than the origin (0, 0, 0) mm. This indicates that there is a deviation of -1 67mm on z 

axis due to refraction when the cross point of blue beams moves from the point (0, 0, -42.5)mm 

to the expected point (0, 0, 0) at a probe angle of 0°. The crossing point of the green beams can 

reach the coordinate origin (0, 0, 0) when the probe is shifted a distance (0, 0, -42.5) mm from 

the initial point (0, 0, 42.5) mm at the probe angle of 0°. The optical paths of two green beams 

are shown in Figure 3.8. Comparing Figure 3.7 with Figure 3.8, we can observe that the crossing 

point of the blue and green beams outside the pipe is separated when it moves into the pipe. 

45 

VES017080 
An invcshizalion of particle dynamics in a dilute phase pneumatic conveying system 

Probe angle= 0-GO ^ ~ 

cross-point ( 0.00 0.00-1 67)imm 

50 

Figure 3.7 Crossing point of blue beams shifting towards the pipe centre line (Units: mm) 

40 J 

' - - Probe angle= 0.00 

! ~ - , cross point (0.00 0.00 0.00) mm 

20 J 

Figure 3.8 Cross point of green beams moving to the pipe centre line (Units: mm) 

As a result, the burst-and-coincidence mode of LDA was no longer available for pipe flow 

measurement due to the different crossing points of the different coloured laser beams. Otherwise, 

we would need an additional optical arrangement to calibrate or adjust each pair of different 

colour beams to converge at the same point in the pipe. 

3.3.4 Developing the coordinate transform matrix for LDA pipe flow 

measurement 

In the previous section, we demonstrate that there is a deviation between the actual coordinate 

and the expected position when a crossing point of beams is shifted into a pipe due to the 

refractive effect at the pipe outer and inner surfaces, The value of this deviation is dependent on 

probe angle, beams initial conditions and pipe geometric parameters. As for the specific 

experimental rig shown in Figure 3.2 and the LD A system discussed in the section of 3.3.1, only 

46 
Chapter 3 Extending LDA for particle flow measurement in pipes 

Ihe probe angle affects the deviations. 

A 

y A . . 

Beams' positions on 

the probe front lens 

x 

the probe coordinate 

locates at the probe 

center 

Figure 3.9 The experimental coordinate (Oxzy), the probe coordinate (O'x'yV) 

and the initial arrangement of the probe 

In Figure 3.9, we construct an experimental coordinate system at the centre of a pipe crosssection and define the x axis along the pipe centre line, the y axis opposite to the gravity direction 

and the z axis across the pipe. Z | is the crossing point of the z positive axis intersecting the pipe 

outer surface and Z 2 is the point of the z positive axis intersecting the pipe inner surface. Z| is 

also selected as the reference point to locate the beams' spatial coordinates when the probe angle 

is known. Thus, the initial position ofthe crossing point of all colour beams locates at Z|. The 

point O is the origin point of the system coordinate. As such, the angle 0 which the plane of blue 

beams intersects with the horizontal plane Oxz of the coordinate system is defined as probe angle. 

In other words, the probe coordinate which is formed by two pairs of beams is parallel to the 

reference coordinate transformed by rotating the experimental coordinate about the probe angle 

along x axis. 

When a three dimensional traverse system carrying the probe moves from Z | to Z z to cross the 

cross-section along the z axis (as shown in Figure 3.9), the actual positions ofthe crossing point 

of blue beams against the different probe angles are shown in Figure 3.10, which illustrates the 

deviations of (y'-y) and (z'-z) against z under the different probe angle 0. The variations of (z'-z) 

show that there always exits a shift between an expected point and its corresponding actual 

position in the z direction no matter what the probe angle is. But a larger the probe angle results 

in a larger deviation ofthe crossing point away from its expected position (y'-y) on the y axis and 

a smaller shift of (z'-z) in the z axis. This is clearly shown in the pipe centre region. For smaller 

the probe angles like 0° and 5°, the variation of (z'-z) against z is almost like a straight line which 

means that the z coordinate of an actual blue beams' crossing point has a -1.67mm shift to its 

expected z coordinate. In contrast, the variations of (y'-y) clearly display that the larger the probe 

47 

VES017082 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

angle is, the larger the shift between y and y' is. For example, the difference between y' and y is 

almost equal to zero under the probe angle zero. And at every probe angle, the maximum 

deviation in y direclion occurs at shifting the blue beams' crossing point to the section centre or 

the coordinate origin point. 

-1.55 

-1.6 

-1.65 

-1.75 

-30 

0 

-0.2 

E 

^ -0.4-

-0.6 

-0.8 

0 10 

z (mm) 

Probe Angle(°) 

0" 

.- 5 o 

10° 

15° 

20= 

-20 -10 0 10 20 30 40 

z (mm) 

Figure 3.10 Variations of the crossing point of blue beams against probe angle 

Note: (y, z) is the expected position and (y\ z') is the actual position 

Under the same shifting process, Figure 3.11 displays the deviations of (z'-z) and (y'-y) of the 

green beams at different probe angles when their crossing point moves from Z | to Z 2 to cross the 

pipe along the z axis (as shown in Figure 3.9). In general, the variations of (z'-z) show a similar 

tendency at the different probe angles, in which the minimum shift occurs in the pipe centre and 

the maximum shift happens close to the pipe wall in the z axis. By contrast, the variations of (y'-

y) show that there is almost no shift between y' and y under smaller probe angles like 0° and 5°. 

But for probe angles larger than 5°, there exists a large discrepancy between y' and y when the 

green beams' crossing point is further from the pipe centre. Comparison of Figure 3.11 with 

Figure 3.10 shows that the identical crossing point of blue and green beams outside the pipe will 

be separated when the crossing point moves into the pipe. 

48 

VES017083 
Chapter 3 Intending LDA for particle flow measurement in pipes 

E -0.5 

E 

Probe Angle(°) 

-1.5 ' • 1 i 0° 

-30 -20 -10 0 10 20 30 

z (mm)

 5 ° 

10= 

0.01 

E

 0 

£ 

-0.01 

15» 

20° 

-0.02

 ! 1 < 

-30 -20 -10 0 10 20 30 40 

z (mm) 

Figure 3.11 Variations of the crossing point of green beams against probe angle 

Note: (y, z) is the expected position and (y\ z') is the actual position 

Considering the probe angle as a constant of -5°, Figure 3.12 (a) and (b) show the actual 

coordinates of the blue and green crossing points in the horizontal section of the rig respectively, 

when the traverse system carrying the probe is linearly shifted to access the whole pipe cross 

section. Generally, two colour crossing points have less discrepancy from their normal positions 

in the pipe mid part than in the pipe top or bottom regions. Al l the crossing points lie in the same 

plane of Oxz. Figure 3.13 (a) and (b) display the spatial positions of blue and green crossing 

points in the vertical section of the rig pipe separately over the entire cross-section at a probe 

angle of -5°. Both two colour crossing points have more deviations from their expected position 

in the right or left region than in the pipe centre. Comparing Figure 3.13 with Figure 3.12, we can 

see that both colour crossing points in the vertical pipe not only deviate from their normal 

positions but also are no longer on the same plane of a pipe vertical cross-section, which means 

that there is a deviation in the y axis, although the movement ofthe 3D traverse system is limited 

to the plane of Oxz. Therefore, in order to compensate the deviation in y axis, we should add an 

additional y value at each sample point for keeping sample points on a pipe vertical cross-section, 

when setting the shift coordinate of the traverse system. 

On the other hand, there also exists a practical over-current problem for the photomultipliers in 

the LDA, which is caused by the strong intensity of the reflected light on the pipe outer surface. 

The over-current condition will make the linear scale capability of the photomultipliers worse. In 

addition, the laser source of the LD A system had to be powerful enough to measure the velocity 

49 

VES017084 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

of a micro particle like smoke (size less than 30 p.m), which is used to trace fluid due to its small 

Stokes number. In our experiments, ihe trade-off between laser power and the probe angle was 

tested to select two data sets of (0.8w, 0°) and (1.6w, -5°). A zero probe angle minimizes the 

crossing point deviation due to the refraction effect. 

Green Beam. P'obe Angle (-5' ) 

(a) Coordinates of the blue beams' crossing point (b) Coordinates of the green beams' crossing point 

Figure 3.12 In the horizontal pipe at the -5° probe angle 

According to the above discussions, a total of 1047 sample points are arranged to occupy the 

whole area of a horizontal or vertical cross section by shifting the traverse system. The spatial 

resolutions of these sample points were defined as 1mm in the y direction and 2mm in the z 

direction respectively. Thus a transform matrix can be calculated in advance, which is used to 

index a shift coordinate of the traverse system with a sample point in pipe. 

Green Be2m. Probe Angle (-5"iBiue Seam. Probe Angle"!-:, I 

(a) Coordinates of ihe blue crossing point (b) Coordinates ofthe green crossing point 

Figure 3.13 In the vertical pipe at the -5° probe angle 

3. 4 SAMPLE RATE OF LD A FOR GAS-SOLID FLOW MEASUREMEN T 

According to Shannon's sampling theorem, it is necessary to sample a continuous signal at a rate 

greater than twice its highest frequency component for measuring it exactly. The twice highest 

frequency of a continuous signal is also called Nyquist frequency or Nyquist rate (Smith, 2003). 

50 

VES017085 
Chapter 3 Extending LDA for particle flow measurement in pipes 

However, particle signals recorded by LD A at a measurement point inside a pipe are discrete with 

a random fluctuation due to the local fluid conditions and particles impacting states. These 

signals can be considered as a numerical event like a discrete-time random process which is 

illustrated in Figure 3.14. 

AtK 

P 

m k I 

O - O o o o- o o o 

I K+1 

k M 

T 

Figure 3.14 The sketch of particle signals recorded by LDA 

With regard to LD A measurement, Figure 3.14 shows the signal characteristics of particle 

velocities recorded by LDA. A total of N particles are assumed to cross the LD A fringe at the 

sample point P within a time interval T. The parameter u p expresses a particle instantaneous 

velocity. The parameter K is used to label the order of particles passing through P and At*c means 

the time interval between particle K and particle (K+1). When these particles are very small, for 

example, seeding particles like ammonium chloride smoke (mean size 0.6 pm), the measured 

L D A data represent the local velocity of the air phase at the point P. Since seeding particles are 

fme with a small Stokes Number and have a good ability to follow the fluid, the time-series 

signals of {up} recorded by LD A are similar and only dependent on the local air phase state. In 

the case of a steady flow measurement, the set of {up} should have a small deviation. Hence, the 

total number of signals {up} would have less influence on calculating its mean velocity. However, 

for gas-solid flow measurement, particles of solid phase are considered as the seeding particles 

for LD A measurement and they are different in terms of size and initial conditions, as well as 

density, for a multi-material solid phase. The signals {up} recorded by LD A at the point P are not 

only related to the local air phase state but also dependent on the particle properties. Items in the 

set {up} may have large discrepancies. Compared with measuring the air phase velocity, particle 

signals for solid phase measurement are discrete with an unequal separation distance. Then the 

local mean velocity of solid phase will be affected by particle number in the velocity set {up }, 

which can be described as equation (3.6) 

AM 

«P = {upk,k = l,A ,N} (3.6) 

up = 2X * N 

where T is the measurement period and N is the total number of particles crossing the 

measurement fringes. In the group equation (3.6), the first expression means the sum of the time 

intervals between particles equals the total measurement time. The second means that the 

51 

VES017086 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

measured instantaneous velocity would be one of the items in the particle velocity countable 

sample space. The third is the equation for calculating particle mean velocity whose accuracy is 

obviously related to the total number and the deviation of particle velocity set {up }. Then, if t mj n 

is the minimum temporal interval between two adjacent particles crossing the measurement point, 

it can be described as 

Thus, it would be possible to record each item in the sequence of particles shown in Figure 3.9 if 

the sample frequency of LD A is equal to or greater than l/tmi„ and the other time interval At k can 

be assumed as Att/tmb^ (n is an integer). Since the minimum temporal interval at a local point 

is much affected by the local solid concentration and the corresponding boundaries around the 

point, it will change point by point on a whole pipe cross-section. In contrast to continuous-time 

signals, which have a minimum sample rate of twice its highest frequency for avoiding aliasing, 

there is no Nyquist frequency for a discrete-time signal. As such, we will demonstrate how to 

determine the suitable sample frequency of LD A for our experiments discussed in Chapter 4. 

In this chapter, LD A applications for gas-solid flow research have been reviewed, which shows 

that there is still a lack of papers discussing the refractive effect and the sample rate in gas-solid 

pipe flow measurement by using LD A in detail. Since the refractive effect due to the pipe's 

curved surface can not be ignored, we have developed a group of mathematical equations and 

demonstrated fhe calculations of the transform matrix for mapping the actual positions of the 

measurement volume of LD A in tlie pipe by shifting the traverse system according to probe angle. 

In addition, particle size in gas-solid flow has a very wide distribution and particle velocity is 

dependent on many factors like Stokes Number, drag coefficient and local air velocity etc. When 

L D A is applied to measure the local velocity of solid phase, which is averaged from velocities of 

particles passing through a sample point, a sequence of discrete-time signals can be recorded. In 

order to calculate accurately the time-averaged velocity at a local point, the sample frequency of 

L D A should be experimentally determined. Theoretically, this sample rate should be at least 

equal to or greater than l/tmi„ which is described in equation (3.7). 

(3.7) 

3.5 SUMMARY 

52 

VES017087 
Chapter 4 

Experimental studies of cross-sectional 

characteristics of air velocity in pipes using the 

extended LDA Technique 

In this chapter, the extended LDA technique was applied to measure the cross-sectional distributions 

of air velocity on two horizontal pipe cross sections and eight vertical pipe cross-sections behind a 

horizontal-to-vertical 90° bend (R/D=1.3) in the rig. On the horizontal cross-sections, the 

experimental contours of the axial air velocity (Vox) show an annular feature. On the vertical crosssections, each cross-sectional distribution of the axial air velocity (Uay) has been shown to be like a 

horseshoe. A secondary flow including two circulations has also been shown in the cross-sectional 

distributions of the radial air velocity (U^) on the vertical cross-sections close to the bend outlet. At 

the further downstream sections, both the above features explored by Uay and Vox gradually become 

weak. A comparison of the experimental data and the numerical results by CFD indicate good 

agreement. 

4.1 INTRODUCTION 

In the previous sections, we have discussed how to calculate the transform matrix for mapping 

the actual positions of the measurement volume of LD A in a pipe according to the shifting 

coordinate of the traverse system and the importance of a proper LD A sample rate for gas-solid 

flow measurement. With regard to the experimental rig shown in Figure 3.2, the transform 

metrics at the 0° and -5° probe angles respectively, which had been calculated in advance 

according to the principle discussed in Chapter 3, were used to arrange the shift coordinates of 

the traversing system to make the crossing point of the laser beams able to access a whole pipe 

cross-section. 

At the 0° probe angle, the experimental coordinate is parallel to the orthogonal probe coordinate 

(as shown in Figure 3.9) which is formed by two pairs of the laser beams (blue and green). The 

measured signals using the blue beams and the green beams are corresponding to the axial air 

velocity (Ua x ) and the vertical air velocity (Ua y ) in a horizontal pipe respectively. By contrast, the 
Chapter 4 Hxpcrimental studies of cross-sectional characteristics of air velocity in pipes 

measured signals using the blue beams and the green beams are corresponding to the horizontal 

air velocity (U M ) and the axial air velocity (Ua y ) in a vertical pipe. When the probe angle is not 

equal to 0° (like -5°), this probe coordinate will be parallel to the reference coordinate 

transformed by rotating the experimental coordinate about -5° along x axis. In this case, the data 

measured by the blue beams still represents the air x velocity (Ua x ), but the data measured by the 

green beams are not directly equal to the air vertical velocity (Ua y ), for there is the -5C 

intersection angle between them. 

In order to avoid uncerlainty due to the separation phenomenon of the crossing points of different 

colour beams (as discussed in Chapter 3), the 0° probe angle was used to investigate the 

horizontal and vertical characteristics of air velocity and the -5°probe angle was only applied to 

study the horizontal characteristics of air velocity in the following sections. 

A total of 1047 measurement points were arranged over a pipe cross-section by shifting the threedimensional traverse system according to the pre-calculated transform coordinates. A suitable 

sample rate of LDA was selected as the same as that for particle velocity measurement. It was 

10K Hz determined by the experiments in Chapter 5. For air velocity measurement, the seeding 

particles were produced by burning abundle of incense. The smoke is less than lOpm (Fang et al., 

2002). 

4.2 MEASURING THE CROSS-SECTIONA L CHARACTERISTICS OF THE 

AXIAL AIR VELOCITY (Ua x ) IN A HORIZONTAL PIPE 

•200 R/D=1.3 

Umt. mnrj 

H y| 

(R/D=1.3, R is the bend radius 100mm and D is the pipe diameter 75mm) 

Figure 4.1 2D schematic diagram of two selected cross sections in the rig horizontal pipe 

In the horizontal pipe of our rig shown in Figure 4.1, two cross-sections which are 200mm and 

300mm away from the elbow inlet respectively were selected as the measurement regions. Here, 

the experimental coordinate locates at the centre of the elbow inlet, fhe x positive axis denotes 

54 

VES017089 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

fluid direction on the pipe centre line. The y negative direction is on the gravity direction and the 

z positive axis signifies inward towards the paper. 

It is known that the turbulent velocity profile in a circular pipe can be described by an empirical 

equation (4.1), which was summarized from J. Nikuradse's experimental data (Schlichting H., 

1955). 

where u is the local time averaged velocity; u m i K is the maximum value of u,; R is the pipe 

radius; r is the distance from the pipe centre and n is the empirical exponent factor dependent on 

Reynolds number. This equation is also called 1/n-th power law of air velocity profile in pipe. 

According to J. Nikuradse's experimental data, the value of the exponent n is n=6 at the lowest 

Reynolds number Re=4xl03; it increases to n-7 at Re=110xl03. For our fluid condition of air 

conveying velocity I2m/s in rj)75mm pipe, its Reynolds number is equal to 56x101 for air density 

1.12kg/m3. As such, the n is chosen as 7 in our experiments. In order to compare conveniently the 

cross-sectional data of air velocity with the empirical profile estimated by equation (4,1), the 

experimental data and the empirical profile are plotted together. 

Under the condition of air conveying velocity 12m/s and the -5° probe angle, the axial 

characteristics of air velocity were investigated and the experimental data are illustrated in Figure 

4.2 and Figure 4.3. 

ln order to compare conveniently the cross-sectional data of air velocity with the empirical 

profile estimated by equation (4.1), the experimental data and the empirical profile are plotted 

together. Figure 4.2(a) shows air velocity profiles along the y axis on two cross-sections at x=-

200 and -300mm. Similarly, profiles of the axial air velocity (Ux ) along the z axis on the two 

sections are displayed in Figure 4.2(b). The maximum value of air velocity (U max ) which is used 

to normalize the experimental data of air velocity is selected from the data of air velocity on the 

cross-section x=-300mm. As such, the maximum normalized value of air velocity on the other 

cross-section x=-200mm is equal to 0.97 rather than 1. This discrepancy is possibly due to 

experimental random error, since the measurements on the two cross sections were carried out 

separately without coincidence. Comparison of the experimental data with the empirical velocity 

profile indicates that they have a good agreement and a similar shape. The total deviation 

between the experimental data and the value given by Eq. (4.1) is less than 10%. The maximum 

discrepancy occurs at the pipe boundary (r/R>0.82). It is also observed that the air velocity 

profile along the y axis (z=0) is closer to the empirical one-seventh power profile than air 

(4.1) 

55 

VES017090 
Chapter 4 Hxperimcntal studies of cross-sectional characteristics of air velocity in pipes 

velocity profile along the z axis (y=0). Moreover, the air velocity profile on the y axis (z=0) is 

almost symmetrical along the pipe centre line, but the air velocity on the z axis (y=0) is 

somewhat skewed. There is a bias in the centre velocity towards the pipe bottom. 

Air wlocity profile at y=0 

1 

0.8; 

0.6i 

0.4| 

0.2 

c: oi-

-0.2 j 

-0.41 

•0.61 

-0.81 

x=-200mm, mean U =11.89 m/s 

x=-3O0mm, mean U =11,50 m/s 

one-se«nth power profile 

-1 L 

1 

0.8 

0.6 

0.4 

0.2 

0 

-0.2 

-0.4 

0.8 0.85 0.9 

U

x ' U TM 

(a) 

Air yslocity profile at z-0 

x=~200mm, mean U =11.50 m/s 

x=-300mm, mean U =11.11 m/s 

one-seventh power profile 

-1: 1 i 

0.7 0.75 0,8 0.85 0.9 0.95 1 

U

>:/Uma!< 

(b) 

Figure 4.2 Velocity profiles ofthe axial air velocity (U*) at x=-200, -300 mm along 

(a) y axis and (b) z axis, under air conveying velocity 12m/s 

With regard to the cross-sectional distribution of air velocity, Figure 4.3 clearly shows the 

contours of air velocity on two pipe cross-sections on which two sub contour diagrams display an 

annular nature of the cross-sectional distribution ofthe axial air velocity. It is clear that the axial 

air velocity decreases gradually from pipe centre to pipe wall on each cross section. However, the 

fastest region of the axial air velocity does not exactly locate at the pipe geometric centre, having 

a small deviation towards the z-negative direction. But in the y direction, there is no obvious 

deviation. This kind of deviation indicates that the actual air flow in the rig is not ideal. 

56 
An mvcsligitiiort of particle dynamics in a dilute phase pneumatic conveying system 

40: 

20 

-20

 s 

x=-200mm 

-40

 s 

-40 -20 0 20 

z(mm) 

4 0 -

20 

1 °!; ? 

-20 | 

40 

-40 > 

-40 

x=-300mm 

-20 0 20 

z(mm) 

40 

Figure 4.3 Cross-sectional distribution of the axial air velocity distribution (Ux ) at two horizontal cross-sections 

of x=-200 and -300mm from the bend inlet respectively, under air conveying velocity 12m/s) 

4.3 AIR VELOCITY IN THE RIG VERTICAL PIPE AFTE R A 90° BEND 

Since the effect of gravity on air can be ignored, the velocity characteristics of single phase air in 

a horizontal or vertical pipe should be identical if fluid conditions at inlet are the same. However, 

large discrepancies will occur when a vertical pipe is mounted downstream of a connecting elbow. 

So far, there have been many studies to investigate the elbow effect on gas-solid flow. This effect 

is also called the roping phenomenon. Forkeer et al. (2004) reviewed the related papers. However, 

there is still lack of experimental air velocity data for single phase flow in a vertical pipe behind 

an elbow. In this section, we will publish our experimental data of air velocity distribution on a 

scries of cross-sections along the vertical part of the rig. 

R/D=l .3 

Sinn: rnni 

(R/D=l .3, R is the bend radius 100mm and U is the pipe diameter 75mm) 

Figure 4.4 2D schematic diagram of cross sections along the vertical pipe behind a 90° bend (R/D=1.3, R is the 

bend radius 100mm and D is pipe diameter 75mm; 0D is 15mm away from the bend outlet). 

In the experiments, air conveying velocity was adjusted to lOm/s and the incense smoke was 

57 

VES017092 
Chaplcr 4 H\perimcnlal studies of cross-sectional characteristics of air velocity in pipes 

again used as Ihe seeding particles. A total of eight cross-sections from OD to 7D downstream of 

the elbow exit, with a 75mm interval spacing equivalent to one pipe diameter (D), were selected 

as measurement regions along the vertical leg. The origin of the reference coordinate was chosen 

to be the centre ofthe cross-section OD, as shown in Figure 4.4. The section OD is about 15mm 

away from Ihe bend outlet. On each section, a total of 274 sample points with a 4mm interval 

distance in the x and y directions were selected to grid a whole cross section. 

The experimental data on a series of cross-sections along the vertical pipe are illustrated in Figure 

4.5 for the axial air velocity U a y and Figure 4.7 for the radial air velocity Uax respectively. The 

corresponding numerical data, which have been simulated for single phase air by using the 

standard k-e model in the commercial Computational Fluid Dynamics software Fluent 6.3.26 

(Fluent Inc., 2006), are plotted in Figure 4.6 and Figure 4.8. 

Compared with the annular nature of the axial air velocity Uax on a horizontal pipe cross-section 

shown in Figure 4.3, cross-sectional distributions of the axial air velocity U a y in the vertical pipe 

do not retain such annular structure due to the bend effect. Especially, on the section OD, the 

slowest regions of Uo y (the dark blue colour) are located at the pipe centre and the region close to 

the pipe inner wall (the x negative axis). The fastest region of U u y (the dark red colour) is around 

the region close to the pipe outer wall (the x positive axis). Generally, a horseshoe-like feature is 

shown on the cross-sectional distributions of U a y at OD. This horseshoe can be also observed on 

cross-sectional distributions of U a y at the further downstream sections. Compared with that at the 

section OD, the axial air velocity U a y is gradually accelerated inside the horseshoe and 

decelerated outside it from the section ID to 7D. Consequently, the difference of U a y between 

inside and outside the horseshoe is narrowed. The total scale number of isolines is decreased, 

which shows the homogenous area within a certain isoline enlarged. 

With regard to the numerical simulations of air single phase pipe flow, the evolution ofthe axial 

air velocity (Ua y ) shown in Figure 4.6 is illustrated through cross-sectional distributions of U a y 

along the vertical pipe behind an elbow (R/D=1.3). Here, the geometric parameters of the 

numerical model are the same as those of the experimental rig shown in Figure 4.4. Air 

conveying velocity at the pipe inlet is lOm/s in the smooth glass piping of the rig. The detailed 

content of numerical equations and the related numerical parameters are those discussed in 

Chapter 2. It is clear that the elbow apparently affects the axial air velocity distributions in the 

vertical pipe behind a bend. When the main direction of air flow is changed from x positive axis 

to y positive axis, the annular structure of the cross-sectional distributions ofthe axial air velocity 

(Ua x ) in a horizontal pipe is also modified into a shape like a horseshoe in the vertical pipe, 

where air runs faster in the pipe outer wall than in the region ofthe pipe centre or the pipe inner 

58 

VES017093 
An invesligalion of particle dynamics in a dilute phase pneumatic conveying system 

wall The slow region of U, l y is marked as the blue on the section OD and the green on the section 

of 2D. Then its colour is developed as the little dark yellow on the section of 7D. All these colour 

variations mean increase tendency of U a y in the pipe centre region. 

cross-sectional distributions of U (m/s) av 40 1 

50 50 

40 

20 

0 

-20 

-40 

-50 0 

z(mm) 

50 

40 

20 

0 

-20 

-40 — 

-50 

3D 

0 

z(mm) 

50 

Figure 4.5 Experimental cross-sectional distributions ofthe axial air velocity (Ua y , m/s) along the vertical pipe 

behind a 90° bend (R/D=1.3, R is the bend radius 100mm and D is pipe diameter 75mm; 0D is 15mm away from 

the bend outlet). 

59 

VES017094 
(.'hapter 4 Kxperimenlai studies of cross-sectional characteristics of air velocity in pipes 

1 2 . Q • 

11. 5 ' 7 D 

11. 0 

10. 5 

• 6 D 

i a . a 

9 . 5 

g . a 5 D 

B . 5 

8.0 AO 

7 . 5 

7 . a 3 D 

B . 5 

B.O ^liiSii^ 

" 2 D 

5 . 5 

5 . 0 1 D 

- 4. 5 u H E 

• 4. 0 

O D 

Contour s of Y Velocit y (m / s ) De c 01 , 200 8 

' F L L Eh T 6.3 [3d , pbns , ske ) 

Figure 4.6 Numerical cross-sectional distributions of the axial air velocity (U,„ , m/s) along the vertical pipe 

behind a 90° bend (R/D=l .3, R is the bend radius 100mm and D is pipe diameter 75mm; OD is 15mm away from 

the bend outlet). 

The comparison of Figure 4.5 and Figure 4.6 not only shows the similar structure of the axial air 

velocity (Ua > ) distribution on each section but also illustrates the similar evolution of U a y from 

0D to 7D. On each section, the numerical data validate that the axial air velocity is faster on the 

outside of the horseshoe than inside it. Also, the axial air velocity inside the horseshoe gradually 

increases and decreases outside the horseshoe. As such, the total scale number of U a y decreases 

and the difference of U a y between inside the horseshoe and outside it tends to lessen, which 

shows the cross-sectional distributions of U a y inclining to the homogenous during air flow 

development further downstream. 

60 

VES017095 
An investigation of pnrltcie dynamics in a dilute phase pneumatic conveying system 

cross-sectional distributions of U (m/s) 

40 40 -

OD 1D 

20 20 

o ! 

V i i 

IT 

; " E 0 c 

-20 % /I -20 \;-

-40 

-50 0 50 

-40-' 

-50 

40 

20 

0-

-20 

^0 

2D 

-50 

z(mm) 

* bis* 

0 

z(mm) 

50 

40-

20-

0-

-20 

-40 

3D 

-50 0 

z(mm) 

50 

50 

40 

5D 

20 L-m - • 

0 1 w 

-20 ' 

-40 

-50 0 

z(mm) 

40 

7D 

20 

0 

-20 

-40 

< | 3 § 

-50 0 

z(mm) 

50 

50 

Figure 4.7 Experimental cross-sectional distributions of the horizontal air velocity (Ua x , m/s) along ihe vertieal 

pipe along the vertical pipe behind a 90° bend (R/D=1.3, R is the bend radius 100mm and D is pipe diameter 

75mm; 0D is 10mm away from the bend outlet). 

As for the radial air velocity (Ua x ) in the x axis. Figure 4.7 and Figure 4.8 respectively show the 

experimental and numerical distributions of U a x on the eight cross-sections from 0D to 7D in the 

vertical pipe behind the 90° bend (R/D=1.3). In Figure 4.7, the experimental cross-sectional 

61 
Chapter 4 Kxpcrimenlal studies ofcross-sectinnal characteristics of air velocity in pipes 

distribution of U a x on each section can be classified into three sub-regions according to the 

direction of U a x . These sub-regions include the mid strip along the x axis and two sectors outside 

the strip towards the pipe left and right walls. According to the colour bar in Figure 4.7, the 

mixed colour of yellow-green represents Uax close to zero velocity. The dark blue stands for the 

fastest U a x in the x negative direction and the dark red indicates the fastest U a x in the x positive 

direction. As such, U a x is positive or zero in the mid strip and negative in the two sectors. 

An interesting phenomenon is shown in the cross-sectional distribution of U a x on the section OD, 

where the positive maximum U a x (3m/s) locates on the mid strip close to the pipe inner wall (the 

negative x axis) and U a x tends to be zero at the pipe outer wall. In two sectors, U ^ changes its 

direction from the positive to the negative when air flow reaches the pipe outer wall. Then U a x is 

gradually accelerated towards the pipe inner side till it reaches the maximum magnitude of U a x in 

the negative x axis (-3m/s). This process illustrates a secondary flow resulting from the 

momentum transfer among three components of air velocity due to the 90° bend at the section OD. 

This secondary flow includes two circulations. Each them starts from the pipe outer wall (Ua x 

tends to be zero). Then one is accelerated along the pipe left wall counter clockwise while U„ 

shows an increasing tendency in the negative x direction By contrast, the other is accelerated 

along the pipe right wall in the negative x direction clockwise. When both subflows meet at the 

pipe inner wall, they are mixed and run towards the pipe centre (the positive x direction), U a x 

reaches its maximum positive value (dark red colour) before the pipe centre. Then U a x gradually 

decreases to be nearly zero in the mid strip close to the pipe outer wall. These two circulations 

demonstrate a symmetrical feature in the cross-sectional distribution of U a x . Perhaps, this 

symmetry is due to air flow coming along the x axis in the horizontal pipe and there being no 

additional force in the z axis. 

On the downstream sections ID, 2D and 3D, the cross-sectional distributions of U a l i can be still 

observed to be similar to that on the section OD. But the symmetric feature of U a x shown at the 

section OD is skewed. The region with the maximum positive radial air velocity (Ua x ) shifts from 

the pipe inner wall towards the pipe centre. Compared with that on the previous sections, the U a x 

scale number is obviously reduced on the sections from 4D to 6D. The difference of U a x between 

the mid belt and either of the two sectors becomes smaller and the positive peak of U a x decreases 

as well. However, on the section 7D, the cross-sectional distribution of U a x shows that U a x in the 

mid belt becomes positive again and the positive maximum U a x occurs close to the pipe centre. 

This is unexpected and does not agree with those on the previous cross-sections due to unknown 

reasons. Overall, variations of cross-sectional distributions of U a x from OD to 7D indicates that 

the bend effect on the momentum transfer between the axial air velocity (Ua y ) and the radial air 

velocity (U M ) is still active as far as the section 7D. 

62 

VES017097 
An investigation of particle dynamics in a dilute phase pneumatic conveying system 

7D 

6D 

5D 

4D 

3D 

2D 

1D 

OD 

Contour s of X Velocit y (m/s ) De c 0 1, 2 0 08 

FLUEN T 6.3 (3d , pbns , ske ) 

Figure 4.8 Numerical cross-sectional distributions of the horizontal air velocity (U,„) along the vertical pipe 

behind a 90° bend (R/D=1.3, R is the bend radius 100mm and D is pipe diameter 75mm; OD is 10mm away from 

the bend outlet). 

Figure 4.8 shows the numerical distributions of U a x on the sections from OD to 7D. Compared 

with Figure 4.7, a similar structure of the Uax cross-sectional distribution can be observed, 

especially on the section OD. The symmetrical feature along the z axis is clearly shown in the 

cross-sectional distribution of U „ on each section. However, the symmetrical structure of the 

experimental data shown in Figure 4.7 is not regular like those predicted by the numerical data in 

Figure 4.8. In addition, the difference of U a x between the mid strip and its two outsides gradually 

becomes smaller, for the magnitude ofU a x estimated by the numerical data decreases either in the 

mid strip or in the two sides of the strip. Consequently, the numerical prediction of the secondary 

flow described by U a x becomes weaker and weaker from the section 0D to 7D. By contrast, the 

experimental secondary flow shown by U ^ is observed to retain a larger magnitude of the 

63 

VES017098 

J.Q 

~> r 

2.0 

] .5 

i . a 

0.5 

O.Q 

-fl.5 

- i . a 

-1. 5 

- 2 . a 
I hapler 4 experimental studies of cross-sectional characteristics of air velocity in pipes 

negative U a x in the boundary ofthe pipe left and right walls on the section 6D. 

U /<U > profiles On the x axis 

2.5 

2 

v_ 

_ » 1 i 

0 

2.5 

6D 

U IU profiles On the z axis 

6D 

0.6 

1.2 : 

4D 

Ol 

2.5 

0 

2.5 , 

4D 

2D 

C F D 

IDA i 

-1 -O.f 

OD 

-0.2 0.2 0.6 

x/R 

0.6 t1.2 

2D 

0.6

 : 

1.2 

0D 

C FD 

LDA 

-1 -0.6 -0.2 0.2 0.6 

2/R 

Figure 4.9 Profiles of U;,* and U n > along the x axis at four selected vertical cross-sections 

(<U» > is the mean absolute value of U„ ; U n is the air conveying velocity I Om/s) 

In order to quantitatively and clearly compare the experimental data with the numerical results, 

the numerical and experimental profiles of U a x and U a y on the x axis and on the z axis are 

displayed in Figure 4.9 and Figure 4.10 respectively. 

Figure 4.9 shows the evolution of profiles of U a x and U a y in the x axis at the four selected crosssections. With regard to U a x , the numerical U a x

 profile shape is similar to the experimental one 

on each section. Both the numerical and the experimental data show that U a x in the negative x 

axis is faster than in fhe positive x axis. The width of U a x profile gradually expands from 0D to 

64 

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