Christopher M. Heath

Glenn Research Center, Cleveland, Ohio

Design and Analysis of an Isokinetic Sampling 

Probe for Submicron Particle Measurements at

High Altitude

NASA/TM—2012-215982

February 2012
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Christopher M. Heath

Glenn Research Center, Cleveland, Ohio

Design and Analysis of an Isokinetic Sampling 

Probe for Submicron Particle Measurements at

High Altitude

NASA/TM—2012-215982

February 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
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NASA/TM—2012-215982 1 

Design and Analysis of an Isokinetic Sampling Probe for 

Submicron Particle Measurements at High Altitude 

Christopher M. Heath 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135

Abstract 

An isokinetic tip dilution probe has been designed with the aid of computational fluid dynamics to 

sample sub-micron particles emitted from aviation combustion sources. The intended operational range 

includes standard day atmospheric conditions up to 40,000-ft. With dry nitrogen as the diluent, the probe 

is intended to minimize losses from particle microphysics and transport while rapidly quenching chemical 

kinetics. Initial results indicate that the Mach number ratio of the aerosol sample and dilution streams in 

the mixing region is an important factor for successful operation. Flow rate through the probe tip was 

found to be highly sensitive to the static pressure at the probe exit. Particle losses through the system were 

estimated to be on the order of 50 percent with minimal change in the overall particle size distribution 

apparent. Particle size distributions and number densities from various combustion sources will be used to 

better understand particle-phase microphysics, plume chemistry, evolution to cirrus, and environmental 

impacts of aviation. 

Nomenclature 

Symbols: 

d0 Source diameter (i.e., Nozzle exit diameter = 1.27-cm) 

d*

 Far field equivalent source diameter 

Dratio Ratio of dilution to sample by mass 

P0,∞ Total chamber pressure far from nozzle exit 

T0,∞ Total chamber temperature far from nozzle exit 

ρ0,∞ Total chamber pressure density far from nozzle exit 

Ux Axial velocity x distance from the nozzle exit 

P0,x Total pressure x distance from nozzle exit 

T0,x Total temperature x distance from nozzle exit 

ρ0,x Total density x distance from nozzle exit 

R0,x Specific gas constant distance x from nozzle exit 

P0,Nozzle Total pressure at burner exit 

T0,Nozzle Total temperature at burner exit 

ρ0,Nozzle Total density at burner exit 

PS Static pressure 

TS Static temperature 

mሶ Nozzle Mass flow rate of burner exhaust products 

U∞ Axial velocity of chamber co-flow 

x Axial distance from nozzle exit 

xE Axial distance from virtual origin to jet exit plane 
NASA/TM—2012-215982 2 

Abbreviations: 

CFD Computational Fluid Dynamics 

CPC Condensation Particle Counter 

EC Electrostatic Classifier 

FAR Fuel-to-Air Ratio 

GCM Global Circulation Model 

NCC National Combustion Code 

NASA National Aeronautics and Space Administration 

PAL Particle Aerosol Laboratory 

SGAPE Streamline-based Gas and Particle Emission 

SMPS Scanning Mobility Particle Sizer 

N2 Diatomic nitrogen 

Introduction 

Quantifying the impact of civil aviation emissions on global climate change, local air quality and 

human health has become a growing difficulty for many regulating agencies. General Circulation Models 

(GCMs) have become the primary tool for estimating long-term effects of increasing air traffic on 

climate. Despite efforts, the magnitude and sign of radiative forcing resultant solely from aviation remains 

elusive to the general scientific community. To aid understanding, an improved and comprehensive 

inventory of the current global aerosol background is needed. This data, combined with controlled 

experimental measurements of emission/atmosphere interactions, will offer a step forward in 

understanding how complex particle and gas-phase interactions influence the environment. 

To gain a better grasp on the microphysical and chemical mechanisms that allow contrails from 

aviation to form cirrus, in-situ measurements at well-controlled atmospheric conditions will play a key 

role. The Particle Aerosol Laboratory (PAL) at the NASA Glenn Research Center was established to 

provide just this capability. The facility consists of a flow through altitude chamber that simulates 

standard day atmosphere conditions up to 40,000 ft. Background chamber temperature, pressure and 

relative humidity are all variable parameters. Exhaust from a combustor capable of burning jet fuel is 

injected through a nozzle directly into the altitude chamber. Variable burner settings include fuel 

composition, equivalence ratio, combustor temperature and pressure. Volatile and non-volatile particulate 

matter (PM) may be extracted directly from the burner or chamber via a probe and sampling line. Inside 

the chamber, the probe can be positioned at several axial and radial distances from the exhaust injection 

site. Particle output from the combustor, in terms of number density and size distributions, are used to 

study contrail formation and early evolution. 

Plume characterization is typically performed using a standard Scanning Mobility Particle Sizer 

(SMPS) spectrometer, (TSI, Inc. Model 3936NL76), retrofitted to operate at low pressure. The SMPS 

suite consists of a TSI model 3080 Electrostatic Classifier (EC) and a 3776 Condensation Particle Counter 

(CPC) connected in series. More in-depth information in terms of plume chemistry and particle 

morphology may be acquired by applying non-intrusive methods in concert with additional extractive 

sampling techniques. 

In the past, quantitative particle measurements using standard sampling probes have proven 

problematic at simulated high altitude conditions. Difficulties with extractive measurements have 

included (1) ice accretion on the probe inlet where relative humidity exceeds 100 percent,( 2) non-axial 

flow at the probe inlet resulting in flow separation, turbulence and particle deposition, (3) particle 

transport losses due to electrostatic effects, diffusion, inertial forces, thermophoresis and coagulation, and 

(4) extractive sampling at relatively low pressure near 0.2-atm. Problem (1) is common when sampling 

within short distances from the combustor exit plane due to the high water vapor present in the aircraft 
NASA/TM—2012-215982 3 

exhaust which condenses rapidly and freezes to the probe inlet at cold temperatures. To date, heated 

probes have been tested in the altitude chamber but with limited success (Ref. 1). The higher probe wall 

temperature has been found to cause vaporization of the volatile particle fraction, producing a change in 

the total particle size distribution and overall number concentration measured. Particle output becomes a 

function of probe wall temperature, making measurements difficult to quantify. For the probe designed 

here, neither the effect of ice accretion nor elevated probe wall temperatures were modeled. Problems 2 

through 4 were addressed more directly during the design process. 

Altitude Chamber Facility 

The Particle Aerosol Laboratory (PAL) at the NASA Glenn Research Center consists of a low pressure 

burner whose output is connected to a flow-through altitude simulation chamber. The burner, comprised of a 

combustion chamber and downstream mixing section, typically operates between 1 to 2-atm and burns 

traditional aviation fuel. Alternative and bio-fuel blends such as Fischer Tropsch may also be used. The 

altitude chamber can match atmospheric conditions (temperature and pressure) up to 40,000-ft. Background 

relative humidity can also be varied up to 100 percent for lower altitude operation. Maximum relative 

humidity at higher altitudes (above 35,000-ft) is reduced due to facility limitations. A cold nitrogen supply 

offers the working fluid for the chamber. Combustion byproducts are injected upward from the base of the 

chamber through an insulated transition pipe measuring 2.43-cm in diameter by 1.6-m in length. The 

transition pipe terminates at a 1.27-cm diameter nozzle. A vertical instrumentation plate is located on one 

side of the chamber and contains several 1/4-in. ports at discrete axial distances from the nozzle exit. A 

sampling probe may be inserted through any port and used to extract exhaust products for characterization. 

Three double paned windows are positioned around the remaining circumference of the chamber and 

provide limited optical access. The N2 co-flow and combustion products exit the altitude chamber through 

an exhaust duct located on top. Figure 1 is a diagram of the altitude simulation chamber and low pressure 

burner. More details on the burner and chamber operation, including chamber background temperature and 

pressure profiles, are provided in Reference 1. 

Figure 1.—Particle Aerosol Laboratory (PAL) comprised of an altitude chamber and low-pressure combustor. 
NASA/TM—2012-215982 4 

On-Design Specifications 

An aerosol sampling probe, consisting of an inner sampling line enclosed by a coaxial outer hull 

through which dilution flows, has been designed to extract plume samples from a low pressure, low 

velocity environment. The probe contains a shallow 90° bend so that it can be inserted through any port 

on the side of the altitude chamber and point downward directly into the primary flow. Figure 2(a) shows 

an orthogonal view of the complete dilution probe as well as a full-section view of the probe entrance. 

Both the inner sampling line and outer dilution housing have circular cross-sections. The probe region 

analyzed in this study was the sample inlet and mixing section up to 5.14E-2-m from the probe tip, shown 

in Figure 2(b). The 90° bend, aft probe section, and sampling line to the instruments were not considered. 

Typical operation was expected to be near 40,000-ft standard day conditions, which has proven highly 

problematic in previous experiments due to a number of complications (Refs. 1 to 2). Table 1 lists the 

chamber background conditions from which the probe was designed to operate. 

Figure 2(a).—External view of aerosol sampling probe with full-section view of the sample/dilution mixing region. 

Figure 2(b).—Definition of sample and dilution flow paths.

TABLE 1.—BACKGROUND ALTITUDE CONDITIONS FOR 

SAMPLING DESIGN POINT 

Altitude Chamber Conditions (40,000-ft) 

U∞ 0.0 m/s 

ρs,∞ @ 40,000-ft std. day 0.3016 kg/m3

Ps,∞ @ 40,000-ft std. day 18.753 kPa 

Ts,∞ @ 40,000-ft std. day 216.65 K 

 
NASA/TM—2012-215982 5 

TABLE 2.—BURNER EXHAUST CONDITIONS FOR 

SAMPLING DESIGN POINT 

Burner Exhaust Conditions 

d0 1.27E-2 m 

FAR 0.045 - 

mሶ Nozzle 2.76E-3 kg/s 

Ts,Nozzle 540 K 

Ps,Nozzle 18.753 kPa 

ρs,Nozzle 0.121 kg/m3 

Burner conditions were set based on previous experimental measurements. Exhaust specifications at 

the exit of the burner are listed in Table 2, where d0 is the physical diameter of the nozzle exit through 

which exhaust products enter the chamber.

The distance from the nozzle exit to the probe tip, denoted x, was defined as 0.71-m for the on-design 

sampling condition. Previous measurements show the chamber flow is momentum-dominated in this 

region, meaning buoyancy effects may be considered negligible (Ref. 1). Using the nozzle exit and 

background chamber conditions, momentum-dominated jet scaling laws were applied to approximate 

temperature, pressure and density along the jet centerline where sampling will occur. Static pressure at the 

nozzle exit was assumed to equal that of the chamber (ܲ௦,௫ ൎ ܲ௦,ஶሻ. The chamber coflow velocity was 

near 0.7-m/s and assumed negligible ሺܷஶ ൎ 0ሻ compared to the average velocity of the jet. Exhaust 

products exiting the nozzle were assumed to exhibit a top-hat profile. 

For a non-reacting turbulent jet issuing from a nozzle, the finite-area source (i.e., nozzle exit area) 

may be approximated by an equivalent ideal point source located upstream of the nozzle exit plane. The 

location of the ideal point source is referred to as the virtual origin, making xE the axial distance between 

the virtual origin and the nozzle exit plane. For momentum-dominated jets, self similarity scaling has 

shown that a conserved scalar (i.e., normalized temperature difference) is inversely proportional to the 

normalized distance from the virtual origin (Ref. 3). 

For example, 

1

* , Nozzle ,

, , 5.4





 



 



   



d

x x

T T

T T E

S s

s x s (1) 

where xE may be approximated by Equation (2). For the on-design conditions in Tables 1 and 2, xE has 

been calculated previously to be 0.28-m (Ref. 1). For jets issuing with a uniform velocity profile, constant 

density and from a circular nozzle, d* is the far-field equivalent source diameter given by Equation (3). d*

was first proposed to account for the density differences between an axisymmetric turbulent jet and the 

surrounding ambient fluid assuming no heat release occurs (Ref. 3). 

x 3.6d * E  (2)

,Nozzle

,

0

Nozzle ,

,

0 *

S

S

s

s

T

T

d d d    

  (3)

Rearranging (1) provides a solution for the jet centerline temperature at distance x downstream of the 

nozzle exit. 
NASA/TM—2012-215982 6 

 

  

  



  S ,

E

S S ,

S ,x T

x x

c T T d *

T , Burner (4) 

The axial velocity along a jet centerline may be calculated by applying Equation (5), as given in 

Reference 4. 





 



  x

d *

Ux . U0 6 5 (5)

The density along the jet centerline at downstream distance x from the nozzle exit may be calculated 

by applying the ideal gas law and assuming the specific gas constant in the jet equals that of the chamber 

background (ܴ௫ ൎ ܴஶሻ. This assumption is typically within the accuracy of the self-similar jet scaling 

laws applied. 

x s,x

S ,x s,x R T

P

  (6)

On-design sampling conditions in the plume, as determined from Equations (1) to (6), are listed in 

Table 3. 

TABLE 3.—PROBE INLET CONDITIONS FOR SAMPLING DESIGN POINT 

Sampling Probe Inlet Conditions (40,000-ft) 

x 0.71 m 

xE 0.028 m 

d* 8.044E-3 m 

Ux 13.288 m/s 

Ts,x 235.77 K 

Ps,x 18.753 kPa 

ρs,x 0.277 kg/m3 

Based on prior analysis of high dilution sampling probes for ground-level emission measurements, a 

dilution ratio near 20:1 was selected for standard operation. High dilution using an inert gas is known to 

rapidly quench chemistry that tends to occur as a sample is drawn through a probe and/or transport line.

Parametric Sampling Probe Geometry 

A three-dimensional parametric model for a high dilution sample probe was developed in CATIA V5 

R20. Figure 3 defines the standard geometry. Design parameters include the inner diameter of the sample 

inlet, inner diameter of the sample exit, and outer diameter of the sample exit. Changing the sample inlet 

and sample exit inner diameters for any given design allows the sample flow to be accelerated, 

decelerated, or held constant. Figure 3 shows a design in which the sample inlet flow is accelerated. 
NASA/TM—2012-215982 7 

Figure 3.—Description of variable geometric parameters for the high dilution sampling probe design. 

Design Problem Formulation 

The sampling probe was designed to ensure incoming flow streamlines remained isokinetic, flow 

through the probe remained steady and absent of strong turbulent eddies, and the shape of a given particle 

size distribution did not change significantly throughout the probe. 

Design Objectives 

 Average flow upstream of sample inlet θ  0.5° from axial direction (7) 

 Mixing region absent of turbulent eddies (8) 

 Shape of particle size distribution at sample inlet remains consistent at probe exit (9) 

To keep designs practical in terms of cost and manufacturability, the internal and external tube 

diameters were forced to be consistent with standard stainless steel tube dimensions. The internal 

sampling tube was assumed to have a 1/4-in. outer diameter with 0.035-in. wall thickness. The outer 

probe housing confining the dilution flow was assumed to have a 0.375-in. outer diameter with 0.035-in. 

wall thickness. By setting both tube diameters, the only custom feature of the sampling probe became the 

tip, which was designed to interface directly with the standard stainless steel tube sizes. 

The following physical constraints were placed on the probe tip parameters to eliminate the potential 

for impossible or difficult to manufacture geometries: 

Sample Exit Outer Diameter  0.305-in. (10) 

 Sample Exit Outer Diameter – Sample Exit Inner Diameter > 0.04-in. (11) 

 Sample Inlet Inner Diameter > 0.025-in. (12) 

 Sample Exit Inner Diameter > 0.025-in. (13) 

Equation (10) ensures that the outer diameter of the sample exit is less than the fixed inner diameter 

of the internal sampling tube. Equations (11) to (13) ensure manufacturability and material strength are 

maintained. 
NASA/TM—2012-215982 8 

First-Order Modeling 

A single point design strategy was applied by combining a first-order mixer model developed using 

the Numerical Propulsion System Simulation (NPSS) with a Computational Fluid Dynamics (CFD) code. 

The National Combustion Code (NCC) served as the flow solver. The probe was modeled in NPSS using 

six separate flow elements. Figure 4 contains a block diagram of the NPSS model with description of 

inputs and outputs for each flow station. Model outputs were used to predict probe geometry and flow 

station boundary conditions assuming ideal one-dimensional mixing. 

The NPSS model contains two independent flow streams: a sample inlet flow extracted from the 

plume centerline (Station 1) and a higher pressure and temperature dilution flow (Station 3). A two 

percent pressure loss was assumed between Stations 1 and 2 due to viscous effects. Upon exiting the 

sample region (Station 2), the sample flow is mixed with a dilution stream (Station 4) assuming ideal 

isentropic mixing. The NPSS mixer element assumes that the exit flow area of the mixer is equal to the 

sum of the flow areas of the two mixing streams. During mixing, the flow simultaneously undergoes an 

expansion due to the absence of the wall which initially divides Stations 2 and 3. This was modeled by 

applying a diffuser after the mixer which expands the fluid to match the physical probe exit area 

(Station 5). Because the expansion occurs almost instantaneously, no pressure drop was assumed in the 

diffuser element. Output values from Station 5 were used to predict probe exit boundary conditions 

(Station 6). 

 
NASA/TM—2012-215982 9 

Exploration of the design space using the NPSS model produced a large number of feasible designs, 

many of which exhibited non-ideal mixing characteristics when analyzed with CFD. Performance was 

found to be sensitive to the Mach number ratio between the two mixing stream and to the back pressure on 

the probe exit. A significant difference in the two Mach numbers was found to produce a stagnation region 

just downstream of the sample exit location. This in turn would be accompanied by an internal pressure loss, 

causing the probe exit pressure boundary condition (estimated from NPSS) to be higher than necessary. A 

high pressure boundary condition applied at the probe exit tends to produce an expansion upstream of the 

probe tip, further diluting the incoming sample stream. This type of operation could cause a large 

discrepancy between the number density of an incoming sample measured and the actual number density of 

a sample just upstream of the probe tip. Through trial and error, a ratio of sample to dilution Mach numbers 

around 0.4 was found to produce acceptable internal flow dynamics and mixing. 

By adjusting the probe geometric parameters and Mach number ratio between the sample and dilution 

flows, the design in Table 4 was identified and found to have acceptable on-design performance when 

analyzed using the National Combustion Code (NCC). Note, the geometric probe parameters are specified 

in English units to be consistent with standard stainless steel tube dimensions. 

TABLE 4.—SAMPLING PROBE DESIGN FROM NPSS MODEL 

Probe Geometric Variables 

Sample Inlet Inner Diameter 5.52E-2 in 

Sample Exit Inner Diameter 4.14E-2 in 

Sample Exit Outer Diameter 8.38E-2 in 

Sample Inlet Flow Station 

Sample Inlet Ps 18.753 kPa 

Sample Inlet Ts 235.68 K 

Sample Inlet Mach Number 0.044 - 

Sample Inlet Mass Flow Rate 5.80E-6 kg/s 

Sample Exit Flow Station 

Sample Exit Ps 18.296 kPa 

Sample Exit Ts 235.378 K 

Sample Exit Mach Number 0.080 - 

Dilution Exit Flow Station 

Dilution Exit Ps 18.296 kPa 

Dilution Exit Ts 293.14 K 

Dilution Exit Mach Number 0.121 - 

Dilution Exit Mass Flow Rate 1.16E-4 kg/s 

Mixer Exit Flow Station 

Mixer Exit Ps 18.298 kPa 

Mixer Exit Ts 290.41 K 

Mixer Exit Mach Number 0.118 - 

Probe Exit Flow Station 

Probe Exit Ps 18.352 kPa 

Probe Exit Ts 291.23 K 

Probe Exit Mach Number 0.099 - 
NASA/TM—2012-215982 10 

Computational Fluid Dynamics Modeling 

Numerical Methods 

A steady-state, non-reacting computation was performed to analyze design point operation using the 

National Combustion Code (NCC) (Refs. 5 and 6). The NCC uses a cell-centered finite-volume spatial 

discretization for unstructured grids along with pseudo-time preconditioning. An explicit four-stage 

Runge-Kutta scheme was applied to advance the solution in pseudo time. Local pseudo-time stepping and 

residual smoothing were applied to accelerate the convergence. Turbulence was modeled by applying a 

high Reynolds number k-ε model (Ref. 7). NCC was compiled to use the MPI message passing libraries 

to leverage the parallel processing capabilities of the code. All computations were executed on a single 

remote compute node containing 48 AMD Magny-cours processing cores operating at 2.2 GHz. 

Simulations were set to terminate after a global mass imbalance of 1E-4, determined by Equation (14), 

had been achieved for 5,000 successive iterations. 

Mass In

Mass In Mass Out Mass Imbalance   (14)

Mesh Definition 

Calculated flow station parameters from the NPSS model were used to define the probe geometry and 

initialize the boundary conditions for performing CFD. To reduce computational expense, the probe flow 

domain was modeled as a three-dimensional 10° sector. An unstructured hexahedral mesh consisting of 

around 250,000 grid points was applied to the flow domain. The complete mesh is shown in Figure 5(a). 

Cell density just upstream of the probe tip and inside the probe was increased to resolve boundary layer 

effects (see Fig. 5(b)). Figure 6 shows boundary conditions applied to each external surface of the 

domain. The face upstream of the probe was modeled as a flow inlet where a normal velocity component 

was applied and assumed equal to the free stream jet velocity at 0.71-m downstream of the nozzle exit. 

Both angled sides of the domain were tagged as symmetry boundaries through which no fluid could enter 

or exit. The top face was tagged as a flow exit in which the ambient static pressure predicted in the 

chamber from self similar jet scaling laws was applied. The same condition was used for the adjacent 

vertical surface downstream of the primary flow inlet. The dilution channel entrance face was tagged as a 

mass flow inlet in which the flow rate was set to 20 times that of the incoming sample flow rate predicted 

by the NPSS model. Note, the prescribed dilution mass flow rate in Figure 6 is 1/36th that of the value in 

Table 4 because the flow domain is a 10° sector. The downstream probe exit face was tagged as a flow 

exit and the static pressure NPSS predicted at that flow station was prescribed. 

Figure 5(a).—Unstructured hexahedral mesh of the probe flow domain consisting of ~250,000 grid points. 
NASA/TM—2012-215982 11 

Figure 5(b).—View of refined mesh near the probe inlet and mixing region. 

Computational Fluid Dynamics Results 

Figures 7 to 10 display CFD results for the on-design sampling condition. Streamlines have been 

added to each contour to indicate axial and radial flow directions. Almost no flow was found to exist in 

the azimuthal direction. From the streamlines, only minor variation in the flow direction upstream of the 

probe tip is evident. The probe was found to be streamlined and not cause significant disruption to the 

incoming fluid. On average, the upstream flow diverges from the axial direction by 0.17°, meaning all 

fluid entering the probe tip remains isokinetic. The local Mach numbers shown in Figure 7 indicate all 

flow remains subsonic through the entire domain. Flow field results for the mixing fraction (Fig. 10) 

indicate the sample and dilution inlet streams mix rapidly and within a short distance from the probe 

entrance. By the end of the flow domain, both streams appear to be well-mixed. Total residence time 

through the domain modeled is relatively short and on the order of 0.25-ms. 

 
NASA/TM—2012-215982 12 

Figure 7.—Vertical plane showing in contour the flow Mach number through the probe inlet. 

Figure 8.—Vertical plane showing in contour the static pressure through the probe inlet. 

Figure 9.—Vertical plane showing in contour the static temperature through the probe inlet. 

Figure 10.—Vertical plane showing in contour the ratio of dilution to sample flow. 
NASA/TM—2012-215982 13 

TABLE 5.—SAMPLING PROBE FLOW STATION PARAMETERS FROM CFD 

Sample Inlet Flow Station 

Sample Inlet Ps 18.753 kPa 

Sample Inlet Ts 235.8 K 

Sample Inlet Mach Number 0.034 - 

Sample Inlet Mass Flow Rate 4.47E-6 kg/s 

Sample Exit Flow Station 

Sample Exit Ps 18.381 kPa 

Sample Exit Ts 235.4 K 

Sample Exit Mach Number 0.061 - 

Dilution Exit Flow Station 

Dilution Exit Ps 18.393 kPa 

Dilution Exit Ts 292.5 K 

Dilution Exit Mach Number 0.120 - 

Dilution Exit Mass Flow Rate 1.16E-4 kg/s 

Probe Exit Flow Station 

Probe Exit Ps 18.354 kPa 

Probe Exit Ts 291.0 K 

Probe Exit Mach Number 0.098 - 

Flow station parameters calculated by NCC are provided in Table 5 and were found to be typically 

within 25 percent of the predicted values from the NPSS solution. Reasons for the difference may be 

attributed to assumptions regarding ideal mixing in the NPSS model and incorrect pressure loss estimates 

for the sample inlet. The NPSS model did not account for boundary layer effects, or other general loss 

mechanisms, but it does provide a place to represent such effects based on the CFD results. The most 

significant difference between the NPSS model and CFD solution is the sample inlet mass flow rate. The 

NPSS model appeared to over-predict the probe exit pressure required to keep the sample inlet velocity 

equal to the free stream velocity of the jet. This caused the sample mass flow rate in NPSS to be nearly 

23 percent higher than that predicted by CFD. This discrepancy led to an increase in the dilution-tosample ratio, Dratio, from 20:1 in NPSS to nearly 26:1 in the CFD solution. Tuning of the low-order model 

flow station parameters based on CFD results would allow more accurate predictions to be made by 

NPSS. This may prove useful for future variations on the design. Despite this difference in Dratio from the 

intended design, the internal flow field and mixing characteristics remain acceptable.

Streamline-Based Particle Modeling 

The Streamline-based Gas and Particle Emissions (SGAPE) model was applied to analyze trace 

chemistry and microphysics occurring inside the sampling probe. SGAPE is a particle deposition and 

evolution code containing many of the same chemical and microphysical solvers as NCC. To reduce 

computational expense of a complete reacting CFD model, the software relies on reduced profile 

information post-processed from a non-reacting CFD flow field. For this work, average velocity, pressure 

and temperature were extracted along the flow direction from the three-dimensional NCC solution and 

used as input to the SGAPE code. A finite rate kinetic mechanism involving 29 species and 73 reaction 

steps was used to model combustion exhaust and atmospheric chemistry interactions. Details on the 

species and particle transport models used in SGAPE are provided in References 8 and 9. Microphysics 

considered included soot coagulation, soot activation, condensates forming on soot, scavenging between 

soot and H2SO4 – H2O droplets, H2SO4 – H2O nucleation, H2SO4 – H2O coagulation, and condensation on 

H2SO4 – H2O droplets. 
NASA/TM—2012-215982 14 

Exhaust Composition 

Prescribed sample inlet exhaust conditions were assumed to represent a worst-case scenario for 

general aviation fuel. This means that an unexpectedly high concentration of sulfuric acid was artificially 

added to the fuel surrogate model to ensure soot activation. The non-volatile particle distribution was 

based on prior measurements conducted in the PAL using JP-8. The number density of soot particles 

contained in the sample exhaust was assumed to be 1E12 per cubic meter. A continuous lognormal 

distribution was prescribed with median diameter of 20-nm and standard deviation of 1.5. This 

polydisperse distribution was discretized into 24 unique size bins, described in Table 6. The diluent was 

assumed to be pure nitrogen which enters the probe 2.17E-2-m downstream of the sample inlet. The 1:26 

dilution ratio as predicted by the NCC flow solution was also specified in the SGAPE model. 

TABLE 6.—INITIAL DISTRIBUTION OF PARTICLE SIZE AND NUMBER DENSITY 

Bin Number Particle Diameter (nm) Number Density (#/m3) 

1 5.00 1128264111.96 

2 6.98 11506806045.05 

3 9.75 69465113965.64 

4 13.61 213010240128.56 

5 19.00 331784413632.47 

6 26.52 262502381084.44 

7 37.03 105495098724.19 

8 51.70 21535407173.25 

9 72.18 2233035671.80 

10 100.77 117614266.09 

11 140.68 3146633.15 

12 196.41 42761.60 

13 274.22 295.18 

14 382.85 1.03 

15 534.51 0.00 

16 746.25 0.00 

17 1041.86 0.00 

18 1454.58 0.00 

19 2030.79 0.00 

20 2835.26 0.00 

21 3958.41 0.00 

22 5526.48 0.00 

23 7715.71 0.00 

24 10772.17 0.00 
NASA/TM—2012-215982 15 

Particle Transport Loss Model Results 

Analysis of the non-volatile particle fraction was performed considering changes in the particle size 

distribution due to diffusion, coagulation, thermophoresis, and nucleation. The flow domain was divided 

into two segments (see Fig. 11). For segment 1, average pressure, temperature and velocity measurements 

were extracted from the full CFD solution at 10 axial locations from the probe inlet and supplied as input 

for the SGAPE model. The axial profile data is listed in Table 7. The predefined exhaust composition was 

input at the initial station and particle losses by diffusion were tracked at all downstream locations 

through the end of the segment. Evolution of the particle number for the input particle size distribution is 

shown in Figure 12. Similarly, segment 2 was divided into 14 axial locations at which the pressure, 

temperature and velocity were again specified from the CFD solution. All average station values for 

segment 2 are also listed in Table 7. The output particle size distribution from segment 1 was used as 

input for segment 2. In addition, a N2 dilution mass fraction of 26:1 was specified at the first station of 

segment 2. Evolution of the particle number for segment 2 is shown in Figure 13. 

Figure 11.—Division of probe into two segments for analysis of particle transport losses.

TABLE 7.—AVERAGE FLOW FIELD DATA EXTRACTED FROM NCC SOLUTION FOR PROBE SEGMENTS 1 AND 2 

Distance From 

Sample Inlet (m) 

Cross Sectional Area 

(m2

) 

Temperature 

(K) 

Pressure (Pa) Axial Velocity 

(m/s) 

0.00E+00 4.27E-08 235.62 18731.25 10.56 

2.29E-03 4.04E-08 235.61 18709.69 11.13 

4.57E-03 3.82E-08 235.61 18685.42 11.77 

6.86E-03 3.58E-08 235.60 18658.21 12.49 

9.14E-03 3.45E-08 235.59 18627.89 13.23 

1.14E-02 3.28E-08 235.58 18593.60 14.04 

1.37E-02 3.04E-08 235.57 18554.92 14.98 

1.60E-02 2.86E-08 235.55 18510.98 16.04 

1.83E-02 2.64E-08 235.53 18461.33 17.22 

2.06E-02 2.46E-08 235.51 18403.73 18.46 

2.17E-02 4.57E-07 285.19 18386.41 33.85 

2.40E-02 4.57E-07 287.54 18406.76 33.39 

2.63E-02 4.57E-07 289.35 18410.59 33.37 

2.86E-02 4.57E-07 290.11 18411.26 33.37 

3.09E-02 4.57E-07 290.45 18409.12 33.38 

3.31E-02 4.57E-07 290.65 18406.00 33.38 

3.54E-02 4.57E-07 290.78 18401.00 33.39 

3.77E-02 4.57E-07 290.86 18396.00 33.39 

4.00E-02 4.57E-07 290.92 18390.00 33.40 

4.23E-02 4.57E-07 290.96 18384.00 33.41 

4.46E-02 4.57E-07 290.98 18378.00 33.42 

4.69E-02 4.57E-07 290.99 18372.00 33.42 

4.92E-02 4.57E-07 290.99 18365.00 33.43 

5.14E-02 4.57E-07 290.98 18359.00 33.44 

 
NASA/TM—2012-215982 16 

Figure 12 indicates a significant loss in particles through segment 1, which is mostly attributed to 

diffusion. Because the sample inlet accelerates the incoming fluid stream to better match the velocity of the 

dilution flow, particle losses of up to 55 percent are realized within the first 2.17E-2-m for all size bins. 

Despite this large loss, a significant number of particles remain at the sample exit and the overall shape of 

the particle size distribution remains, for the most part, unchanged. Once dilution flow is added at the start 

of segment 2, the particle concentration decreases rapidly, but the shape of the size distribution still remains 

fairly consistent, as evident in Figure 14. Some broadening of the spectrum is also noted, but microphysical 

effects seem to be significantly reduced by the end of segment 2.

Figure 12.—Particle penetration through segment 1. 

Figure 13.—Particle penetration through segment 2.
NASA/TM—2012-215982 17 

Figure 14.—Evolution of the particle size distribution through the probe entrance region.

Off-Design Performance 

Probe off-design performance was analyzed assuming operation at 30,000-ft standard day conditions. 

A description of the background chamber and sampling probe inlet conditions for this case are provided 

in Table 8. Burner exhaust conditions were assumed consistent with those from before. The probe 

geometry and Dratio of 20:1 were maintained for off-design operation. Figure 15 displays the off-design 

NPSS model block diagram, including input and output flow station parameters. Mach numbers at the 

dilution and sample exits were allowed to vary to keep the ratio of diluent to sample equal with the ondesign case. 

TABLE 8(a).—BACKGROUND ALTITUDE CONDITIONS FOR 

OFF-DESIGN SAMPLING POINT 

Altitude Chamber Conditions (30,000-ft) 

U∞ 0.0 m/s 

ρs,∞ @ 30,000-ft std. day 0.458 kg/m3

Ps,∞ @ 30,000-ft std. day 30.089 kPa 

Ts,∞ @ 30,000-ft std. day 228.71 K 

TABLE 8(b).—BURNER EXHAUST CONDITIONS FOR 

OFF-DESIGN SAMPLING POINT 

Sampling Probe Inlet Conditions (30,000-ft) 

x 0.71 m 

xE 0.029 m 

d* 8.265E-3 m 

Ux 8.509 m/s 

Ts,x 247.57 K 

Ps,x 30.089 kPa 

ρs,x 0.424 kg/m3 
NASA/TM—2012-215982 18 

TABLE 10.—SAMPLING PROBE DESIGN FROM NPSS MODEL (OFF-DESIGN) 

Sample Inlet Flow Station 

Sample Inlet Ps 30.089 kPa 

Sample Inlet Ts 247.54 K 

Sample Inlet Mach Number 0.028 - 

Sample Inlet Mass Flow Rate 5.78E-6 kg/s 

Sample Exit Flow Station 

Sample Exit Ps 29.434 kPa 

Sample Exit Ts 247.38 K 

Sample Exit Mach Number 0.051 - 

Dilution Exit Flow Station 

Dilution Exit Ps 29.434 kPa 

Dilution Exit Ts 293.67 K 

Dilution Exit Mach Number 0.075 - 

Dilution Exit Mass Flow Rate 1.16E-4 kg/s 

Mixer Exit Flow Station 

Mixer Exit Ps 29.435 kPa 

Mixer Exit Ts 291.47 K 

Mixer Exit Mach Number 0.073 - 

Probe Exit Flow Station 

Probe Exit Ps 29.469 kPa 

Probe Exit Ts 291.57 K 

Probe Exit Mach Number 0.061 - 

The output solution from executing the NPSS model in off-design mode is presented in Table 10. 
NASA/TM—2012-215982 19 

Computational Fluid Dynamics Results 

Similarly to the on-design case, off-design mode operation was analyzed using CFD. Boundary 

conditions were modified to reflect the values from Tables 8 to 10. Contour plots showing the Mach 

number, static pressure, static temperature, and mixing fraction for off-design operation are presented in 

Figures 16 to 19. In general, most flow characteristics compare well with those from the design point. The 

flow upstream of the probe tip is very close to axial, with an average divergence of 0.14°. Note the fluid 

velocity entering the probe is reduced compared to the 40,000-ft case to match the slower plume velocity 

Figure 16.—Vertical plane showing in contour the flow Mach number through the probe inlet with streamlines. 

Figure 17.—Vertical plane showing in contour the static pressure through the probe inlet with streamlines. 

Figure 18.—Vertical plane showing in contour the static temperature through the probe inlet with streamlines. 

Figure 19.—Vertical plane showing in contour the ratio of dilution to sample with streamlines. 
NASA/TM—2012-215982 20 

in the chamber. To maintain the same dilution to sample ratio by mass, the mass flow rate of the dilution 

has also been reduced in the model. The local area Mach numbers for the flow field are typically much 

lower than for the design case. The most significant difference in off-design operation is that the dilution 

to sample Mach number ratio has decreased. This allows the incoming sample flow to penetrate further 

into the dilution stream before mixing occurs. By the exit of the probe segment modeled, the dilution and 

sample streams have remained separate and are not well-mixed. Additionally, the residence time through 

the probe has increased allowing chemistry more time to influence the sample distribution. It is expected 

that the dilution and sample streams would become well-mixed within a relatively short distance beyond 

the flow domain. More extensive analysis is required to confirm this hypothesis. 

For comparison, average flow field values were extracted from the CFD solution at locations 

representative of the flow stations in the NPSS off-design model. This data is presented in Table 11. For 

this case, the probe seemed to entrain additional sample flow, causing the sample flow rate from the CFD 

case to be higher than that predicted by the NPSS model. The Dratio for off-design mode operation from 

the CFD solution was predicted to be near 13:1. 

TABLE 11.—SAMPLING PROBE FLOW STATION PARAMETERS 

PREDICTED FROM CFD (OFF-DESIGN) 

Sample Inlet Flow Station 

Sample Inlet Ps 30.089 kPa 

Sample Inlet Ts 247.58 K 

Sample Inlet Mach Number 0.045 - 

Sample Inlet Mass Flow Rate 9.06E-6 kg/s 

Sample Exit Flow Station 

Sample Exit Ps 29.499 kPa 

Sample Exit Ts 247.2 K 

Sample Exit Mach Number 0.079 - 

Dilution Exit Flow Station 

Dilution Exit Ps 29.497 kPa 

Dilution Exit Ts 293.4 K 

Dilution Exit Mach Number 0.075 - 

Dilution Exit Mass Flow Rate 1.16E-4 kg/s 

Probe Exit Flow Station 

Probe Exit Ps 29.470 kPa 

Probe Exit Ts 291.1 K 

Probe Exit Mach Number 0.063 - 
NASA/TM—2012-215982 21 

Particle Transport Loss Model Results 

Similarly to before, the SGAPE tool was applied to the resultant flow solution obtained from NCC. 

Table 12 gives the average information extracted along the length of the probe and provided as input to 

the SGAPE code. 

TABLE 12.—AVERAGE FLOW FIELD DATA EXTRACTED FROM NCC SOLUTION FOR PROBE SEGMENTS 1 AND 2 

Distance From 

Sample Inlet (m) 

Cross Sectional 

Area (m2

) 

Temperature 

(K) Pressure (Pa) Axial Velocity 

(m/s) 

0.00E+00 4.27E-08 247.40 30028.83 14.08 

2.29E-03 4.04E-08 247.39 29997.82 14.75 

4.57E-03 3.82E-08 247.38 29961.52 15.60 

6.86E-03 3.58E-08 247.36 29921.21 16.56 

9.14E-03 3.45E-08 247.34 29875.79 17.54 

1.14E-02 3.28E-08 247.32 29824.19 18.61 

1.37E-02 3.04E-08 247.29 29765.93 19.86 

1.60E-02 2.86E-08 247.27 29699.96 21.26 

1.83E-02 2.64E-08 247.23 29623.94 22.82 

2.06E-02 2.46E-08 247.18 29536.73 24.46 

2.17E-02 4.57E-07 288.60 29492.69 21.88 

2.40E-02 4.57E-07 289.68 29512.40 21.52 

2.63E-02 4.57E-07 290.06 29513.42 21.51 

2.86E-02 4.57E-07 290.25 29512.00 21.51 

3.09E-02 4.57E-07 290.42 29510.00 21.51 

3.31E-02 4.57E-07 290.57 29507.00 21.52 

3.54E-02 4.57E-07 290.70 29503.00 21.52 

3.77E-02 4.57E-07 290.80 29499.00 21.52 

4.00E-02 4.57E-07 290.88 29495.00 21.53 

4.23E-02 4.57E-07 290.95 29490.20 21.53 

4.46E-02 4.57E-07 291.00 29486.00 21.53 

4.69E-02 4.57E-07 291.03 29482.00 21.53 

4.92E-02 4.57E-07 291.06 29477.46 21.54 

5.14E-02 4.57E-07 291.08 29473.00 21.54 

 
NASA/TM—2012-215982 22 

Figures 20 and 21 show the particle penetration through segments 1 and 2 for the off-design case. The 

general results are nearly identical to those in Figures 12 and 13 for the on-design solution. Particle 

penetration is slightly better in off-design operation and near 49 percent for all size bins through 

segment 1. Despite the reduced DRatio for the off-design case, particle penetration through segment 2 

seems to be comparable with the results presented for the on-design simulation. 

Figure 20.—Particle penetration through segment 1. 

Figure 21.—Particle penetration efficiency through segment 2.
NASA/TM—2012-215982 23 

Figure 22.—Evolution of the particle size distribution through the probe entrance region. 

Figure 22 shows the particle size distribution at the sample inlet, sample exit, and probe exit for 

sampling at 30,000-ft conditions. As in the on-design case, some broadening of the initial particle size 

distribution is evident by the probe exit. Despite this, the overall output distribution is fairly consistent 

and losses appear to decrease after dilution is added. 

An additional factor not considered in this analysis is the effect of an elevated probe wall temperature 

on particle transport and loss mechanisms. Probe wall temperature can play a significant role in terms of 

thermophoretic losses. For this study, wall temperature was assumed to be consistent with the temperature 

of the internal fluid stream, meaning no heating was applied. During sampling, however, exhaust products 

must be kept warm to reduce the formation of condensates. If the temperature of an extracted sample 

becomes too high during transport, volatile particles may evaporate, leading to a large reduction in the 

overall particle number density measured. Further analysis is required to better understand the trade-offs 

between the volatile and non-volatile particle modes. Once these trade-offs are better characterized, 

detailed design can be performed to define the best operating practices to measure all submicron 

particulate matter. 

Probe Dimensions 

Figure 23 contains drawings of the probe tip design identified and analyzed in this report. As 

mentioned prior, the probe tip is the primary custom part of this design as it will interface directly with a 

coaxial pair of thin walled stainless steel tubes. 
NASA/TM—2012-215982 24 

Figure 23.—Dimensions of the final probe tip design, analyzed in on- and off-design modes.

Conclusions 

A dilution probe for extracting combustion-generated particles from a high altitude environment has 

been designed at the preliminary level. The probe operates partially as an eductor/mixer with high 

velocity dilution flow generating a portion of the vacuum force that draws the sample in through the 

probe tip. Analysis of the design space was performed using a low-order mixer model developed in the 

Numerical Propulsion System Simulation (NPSS) code. Detailed analysis was performed by applying 

computational fluid dynamics to each feasible design output from the NPSS model. Once an acceptable 

CFD solution was obtained, a data reduction technique was used to extract flow field information along 

the length of the probe. The average flow field data was supplied as input to a first-order streamline based 

gas and particle emissions analysis code. A representative soot distribution was prescribed at the inlet of 

the probe and the particle transport losses due to several common loss mechanisms were estimated along 

the probe length. 

 

All dimensions are in inches unless otherwise specified. 
NASA/TM—2012-215982 25 

Following the design point analysis, an off-design case was also analyzed. The off-design flow field 

was found to be acceptable, but mixing rate of the diluent and sample streams was reduced from the ondesign case. To match the slower velocity of the jet, the mass flow and Mach number through the offdesign probe were reduced. This led to an increase in sampling residence time, which permitted increased 

chemistry to occur.

Overall, the particle transport model for on- and off-design operation predicted losses near 50 percent 

for all size bins within the first 2.17E-2-m of the probe entrance. This loss was considered high and 

thought to be a result of diffusion losses due to the increasing velocity through the sample inlet region. 

More work may be required to confirm this result and understand why the particle losses through such a 

short segment are so large. A full CFD solution including an increased reaction mechanism may offer 

additional insight. 

Future Work 

This analysis has exposed a series of complications with operating a fixed geometry probe 

isokinetically in a variety of environments. Careful control of the probe exit pressure is required to ensure 

that a representative sample is captured from the free stream. When operating with high dilution, the ratio 

of Mach numbers between the sample and dilution streams must be within a reasonable range to reduce 

the formation of turbulent eddies inside the probe and upstream of the inlet. Future work is planned to 

study more advanced probe concepts that include a feedback controller to adjust the sample to dilution 

mass flow ratio. After further investigation, the final probe design will be manufactured and undergo 

preliminary testing in the Particle Aerosol Laboratory. Comparison of the predicted performance will be 

made with the actual performance to validate the success of the design process. 

References 

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Troposhperic Conditions,” Proceedings of the ASME Turbo Expo 2010: Power for Land, Sea and 

Air, June 4–8, 2010, Glasgow, Scotland. 

2. Ram, Michael, Cain, Stuart, & Taulbee, Dale, “Design of a Shrouded Probe For Airborne Aerosol 

Sampling in a High Velocity Airstream,” Journal of Aerosol Science, Volume 26, Issue 6, September 

1995, pp. 945–962. 

3. Thring, M.W. & Newby, M.P. 1953 Combustion length of enclosed turbulent jet flames. Proc. 4th Intl. 

Symp. on Combustion, pp. 789–796, Williams & Wilkins, Co., Baltimore. 

4. Tacina, Kathleen M. & Dahm, Werner J.A., “Effects of Heat Release on Turbulent Shear Flows. 

Part 1. A General Equivalence Principle for Non-buoyant Flows and its Application to Turbulent Jet 

Flames,” Journal of Fluid Mechanics, Volume 415, January 2000, pp. 23–44. 

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33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, July 6–9, 1997, Seattle, 

WA. 

6. Chen, K.-H., Norris, A.T., Quealy, A., and Liu, N.-S., “Benchmark Test Cases for The National 

Combustion Code,” 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, July 

13-15, 1998, Cleveland, OH. 

7. Shih, T.-H., Chen, K.-H., and Liu, N.-S., “A Non-Linear k-epsilon Model for Turbulent Shear 

Flows,” AIAA Paper 1998-3983, 1998. 

8. Wey, Thomas & Liu, Nan-Suey, “Modeling of Aerosols in Post-Combustor Flow Path and Sampling 

System,” NASA/TM—2006-214397, September 2006. 

9. Wey, Thomas & Liu, Nan-Suey, “Streamline-Based Modeling of Aerosol Microphysics in the PostCombustor Flow Path and Sampling System,” AIAA Paper 2007-0794, 45th AIAA Aerospace 

Sciences, Meeting and Exhibit, January 8-11, 2007, Reno, NV.