3

Drexel

University

College of Engineering

Department of

Mechanical Engineering

and Mechanics

Philadelphia, PA 19104

(215)895-2352-53

(NASA-CR- 176564)1 COMBUSTION CHABACXERISTICS

IN THE TBANSI1ICN BEGIC N Of IIQUIt lUEL

SPHAYS Final Report, Jan. 1963 - Sep. 1985

(Drexel Univ.) 56 p He AG4/M F AO1 CSCL 21B

G3/25

N8I6-20517

Unlclas

05(582
COMBUSTION CHARACTERISTICS IN THE

TRANSITION REGION OF LIQUID FUEL SPRAYS

Final Report

January 1983 - September 1985

N. P. Cernansky, I. Namer and R. J. Tidona

January 1986

Research Supported by the NASA-Lewis Research Center

NASA Research Grant Award NAG 3-382

Opinions, findings, and conclusions or recommendations

are those of the authors and do not necessarily reflect the views of NASA

MECHANICAL ENGINEERING AND MECHANICS DEPARTMENT

DREXELUNIVLRSMY

PHILADELPHIA, PA 19104
ABSTRACT

A number of important effects have been observed in the droplet size

transition region in spray combustion systems. In this region, where the

mechanism of flame propagation is transformed from diffusive to premixed

dominated combustion, the following effects have been observed: (1) maxima

in burning velocity; (2) extension of flammability limits, (3) minima in ignition

energy; and (4) minima in NOX formation. Unfortunately, because of differences

in experimental facilities and limitations in the ranges of experimental data, a

unified description of these transition region effects is not available at this

time. Consequently, a fundamental experimental investigation was initiated in

January 1983 to study the effect of droplet size, size distribution, and

operating parameters on these transition phenomena in a single well controlled

spray combustion facility.

A monodisperse aerosol generator has been used to form and deliver a well

controlled liquid fuel spray to the combustion test section where

measurements of ignition energy have been made. The ignition studies were

performed on monodisperse n-heptane sprays at atmospheric pressure over a

range of equivalence ratios and droplet diameters. A capacitive discharge

spark ignition system was used as the ignition source, providing independent

control of spark energy and duration

Preliminary measurements were made to optimize spark duration and spark

gap, optimum conditions being those at which the maximum frequency or

probability of ignition was observed. Using the optimum electrode spacing and

spark duration, the frequency of ignition was determined as a function of spark

energy for three overall equivalence ratios (0.6, 0.8, and 1.0) and for initial

droplet diameters of 25, 40, 50, 60, and 70 urn. An LDA system was used to

n
determine the actual equivalence ratio at the spark gap, which varied from 1 5

to 47. The spark energy at which the frequency of ignition was 90 percent was

defined as the minimum ignition energy.

These data indicated that the ignitability of the sprays was enhanced as

the equivalence ratio was increased, but was diminished as the droplet size

was increased. The increase in minimum ignition energy with increasing

droplet size and fixed equivalence ratio was nearly linear over the range of

parameters studied. However, the effect became smaller with increasing

equivalence ratio.

Optical in situ particle sizing of fuel droplets has been developed to better

characterize droplet size distribution, number density, and extent of

prevaporization at the test section (spark gap in the case of the minimum

ignition energy studies). Numerical calculations using a newly developed Mie

scattering computer code have been made to provide information critical to

optimizing the optics for the laser-based droplet sizing technique. This

method is a single particle counter which measures near forward scattering

and uses light detection at 90 degrees to the laser beam to constrain the probe

volume to the central portion of the beam only. This technique can provide

accurate measurement of droplet sizes in the 10-100 ^m range, is relatively

insensitive to particle index of refraction, and can handle realistic spray

number densities.

iii
TABLE OF CONTENTS

Page

ABSTRACT 11

TABLE OF CONTENTS iv

LIST OF FIGURES AND TABLES v

I. INTRODUCTION AND PROGRAM OVERVIEW 1

A. Introduction 1

B. Project Summary 3

C. Recommendations for Future Work 6

D. Research Personnel and Activities 7

E. Reports and Publications 7

II. EXPERIMENTAL FACILITY 9

A. Monodisperse Spray Generator 9

B Spark Ignition Apparatus 11

C. Laser Doppler Anemometer 13

D. Droplet Size Measurement 15

III. EXPERIMENTAL RESULTS AND DISCUSSION 21

A. Effect of Spark Duration on Minimum Ignition Energy 21

B. Spark Gap Equivalence Ratio Determination 21

C. Optimization of Spark Gap Width 23

D. Droplet Size and Equivalence Ratio Effects on Minimum

Ignition Energies 25

E Droplet Sizing Method 31

F. Summary and Conclusions 42

IV REFERENCES 45

APPENDIX PUBLICATIONS AND PRESENTATIONS 47

iv
LIST OF FIGURES AND TABLES

Page

Figure 1 Schematic of Aerosol Generation and Ignition Test

Facility 10

Figure 2. Schematic of Spark Generation Circuit 12

Figure 3. Schematic of Laser Doppler Anemometer System 14

Figure 4 Optical Arrangement for Droplet Sizing 17

Figure 5. Schematic of Signal Processing System for Laser

Droplet Sizing Apparatus 19

Figure 6. Fffect of Spark Duration on the Minimum Ignition Energy of

N-Heptane Sprays: D0 = 68 \un; 0 = 1.0; Spark Gap = 2.0 mm 22

Figure 7. Radial Profiles of Local Equivalence Ratio (01) Across the Test

Section for N-Heptane Sprays: D0 = 50 )im;

Spark Gap = 3.0 mm; Spark Duration = 77 us 24

Figure 8 Effect of Spark Gap Width on the Frequency of Ignition for

N-Heptane Sprays: D0=70 urn; 0=10, Spark Duration = 76 us;

Spark Energy = 0.8 mJ 26

Figure 9. Effect of Gap Equivalence Ratio (0G) and Spark Energy on the

Frequency of Ignition for N-Heptane Sprays:

DQ = 60 urn; Spark Gap = 3.0 mm; Spark Duration = 77 us 28

Figure 10. Effect of Gap Equivalence Ratio (0G) and Initial Droplet Diameter

(DQ) on the Minimum Ignition Energy of N-Heptane Sprays 29

Figure 11. Col lection Geometry 33
Figure 12. Computed Response Function Using Mie Theory,

Optimized System ...................................................................................... 35

Figure 13. Computed Response Function Using Mie Theory,

Non-Optimized System ............................................................. 36

Figure 14 The Effect of Collection Aperture ( 4>|_) on the Computed

Response Function ........................................................................................ 38

Figure 15. The Effect of Collection Angle (<J>C) on the Computed

Response Function ...................................................................................... 39

Figure 16. The Effect of the Imaginary Part of the Index of Refraction

on the Computed Response Function ...................................................... 41

Table 1. Computational Matrix .................................................. 34

vi
I. INTRODUCTION AND PROGRAM OVERVIEW

A. Introduction

The combustion of liquid fuel sprays Is expected to remain a major source

of energy well into the twenty-first century. However, fuel properties and

quality are currently deteriorating. It is anticipated that the fuels of the

future may have properties which are significantly different from today's

fuels. In order to effectively design fuel injectors, combustors, and igniters to

accommodate broadened specification and synthetic fuels, a more fundamental

understanding of the combustion of liquid fuel sprays in the so called

transition region has been undertaken at Drexel University.

The transition region is usually associated with droplets in the 10-100 urn

diameter range. The following important effects have been observed in the

transition region: (1) maxima in burning velocity; (2) extension of

flammability limits; (3) minima in ignition energy requirements; and (4)

minima in NOX formation. Therefore, combustion In the transition region is an

attractive design goal for practical combustors.

Previous research has shown that, in the transition region, control of the

combustion process is transformed from diffusive to premixed domination. It

has been suggested that this transformation in controlling mechanism is

responsible for the observed transition region phenomena. More specifically,

these phenomena are associated with the complex interactions between

droplets In the size and number density range where the presence of

neighboring droplets can significantly affect the evaporative and combustion

environment of the fuel spray. Overall, the combustion behavior is affected by

the relative fraction of fuel initially in a prevaporized and premixed state, the

average interdroplet spacing, and the fuel properties. Although considerable

research has been and is being undertaken in this area by a number of
investigators,a consolidated picture of these transition region phenomena and

their mechanisms is not yet available. This is due in part to the very different

aerosol generating techniques (generally poorly controlled polydisperse

sprays), limited and dissimilar operating ranges (e.g., equivalence ratio),

different combustion systems, etc. used by the various experimental

investigators.

Adequate predictions of droplet interaction effects on spray ignition in the

transition region are not available yet. It remains difficult to directly compare

detailed model predictions with experimental data because there are little data

from spray configurations which correspond to those that have been modeled.

Furthermore, much of the experimental ignition data to date has been for

polydisperse sprays where the drop sizes which may control the evaporation

and interactions are not well characterized. Where monodisperse sprays have

been used, there has generally been poor control of other variables. More

specifically, several important ignition studies were for polydisperse sprays

with droplets in the diameter range of 30 to 120 microns. Thus, it has not been

possible to ascertain the droplet diameter at which the ignition energy

requirement is truly minimized from the previous work. Also, previous work

has not covered the entire transition range of droplet diameters and

equivalence ratios. For these reasons, a complete and comprehensive picture of

ignition energy variation in the transition region is not available. Thus, there

is a need for comprehensive studies examining droplet size effects on minimum

ignition energy in more detail, particularly where better control can be placed

on the experimental variables.
B. Project Summary

A fundamental study was initiated in January 1983 with the support of

NASA Grant Award NAG 3-382 to study the droplet size effects on (1) minimum

ignition energy, (2) maximum burning velocity, and (3) extension of the

flammability limits and to determine the optimum droplet diameters (if they

exist). The use of a single spray combustion facility for all of this work

insured uniformity in the interpretation of data and is expected to lead to the

formulation of a comprehensive model of combustion in the transition region.

This program followed the study of Cernansky and Sarv (1983) on NOX

formation in monodisperse fuel spray combustion.

NASA support was prematurely terminated because of funding constraints.

Therefore, the effort was limited to the study of the droplet size effects on

minimum ignition energies. The burning velocity studies and flammability

limits study had to be abandoned at this time without proper support.

The experimental program as originally planned had the following specific

objectives:

(1) To map the minimum ignition energy for a monodisperse fuel spray

combustion system in the transition region (10-80 u.m initial

droplet diameter) for various fuels of interest and for other

important variables such as equivalence ratio, flow velocities, and

fuel type;

(2) To determine the optimum droplet diameter at which the ignition

energy is minimized;

(3) To study the effects of changing the evaporation environment on

the minimum ignition energy;
(4) To characterize the ignition requirements of polydisperse fuel

sprays in terms of the droplet size distribution, and

(5) To determine the effects of droplet size and size distribution on

flame speed and flammability limits.

In the first year of the project, the design, purchase and construction of a

new droplet ignition system and a new droplet sizing system were completed

and the existing burner arrangement was modified. The flow geometry was

changed to prevent flashback in the new configuration. The burner system now

accommodates an ignition system which allows the control and measurement of

ignition energy Also, the initial design of the optical diagnostics for droplet

size and velocity measurements was completed.

Since January 1984, work has concentrated on (1) the further development

of the spray ignition facility to provide more repeatable data at low ignition

energies; (2) minimum ignition energy experiments with n-heptane sprays at

various equivalence ratios and with several droplet sizes; (3) the development

of a detailed Mie scattering computer code to predict the performance

parameters for the optical spray sizing system, and (4) the development of the

spray sizing apparatus.

Because of initial difficulties in obtaining reliable and repeatable spark

energies at the lower energies of interest, the minimum ignition energy

experiments required more time than had originally been planned. Further, it

had been expected that a pre-existing Mie scattering code could be readily

adapted to the present optical diagnostic system. However difficulties in

getting this code to run successfully on the Drexel VAX computer forced the

development of an in-house code This required significantly more time than

originally planned However, the extra time spent on the development of the

ignition facility and the Mie scattering code has resulted in a more reliable and
repeatable spray sizing technique than could have been obtained without the

extra effort. The result of these efforts is a highly reliable and repeatable

facility for the study of spray ignition phenomena and a well-developed

expertise in optical diagnostics for droplet sizing, thus providing a solid

foundation for the completion of the remaining objectives of the program

These objectives include quantifying the effects of droplet size and burner

operating variables on flammability limits and flame speeds.

Since the first year of work on the project (January 1983 through January

1984) was covered in detail by the last annual progress report (Cernansky et

ai, 1984), this final report will focus on the program efforts since that period.

Section II contains a discussion of the development of the experimental

facility including the monodisperse aerosol generator, spark ignition apparatus,

laser Doppler anemometer, and droplet sizing system. Section III is a

discussion of the experimental results for n-heptane sprays. Spark duration

and gap width optimization procedures and equivalence ratio and droplet size

variations are discussed in terms of their effects on minimum ignition

energies. A discussion of the theory behind the droplet sizing techniques and of

the results of some numerical calculations for optimizing the optical system

design is given also. Section IV contains the summary and conclusions drawn

from the present study.
C. Recommendations for Future Work

In order to build on the accomplishments of the present experimental and

numerical efforts, we anticipate continuing our investigation of the following

specific areas.

(1) Minimum ignition energy studies of several different pure fuels,

binary mixtures, and at least one actual distillate fuel;

(2) Evaluation of flammability limit effects of droplet size and spray

burner operating variables for the same fuels used in the minimum

ignition energy experiments;

(3) Studies of droplet size and spray burner operating variable effects

on flame speeds for the above fuels;

(4) Development of a pressurized spray burner facility with reactant

preheat capability to extend the results of the atmospheric

studies to conditions approaching those in real engines;

(5) All of the above studies are to incorporate the new optical droplet

size and size distribution measurement technique developed under

the present program to make in situ measurements of actual

droplet size. This will provide detailed information which will

enable much more precise determination of local equivalence

ratios and extent of prevaporization as well as droplet size and

size distribution.
D. Research Personnel and Activities

The research project has been conducted under the supervision of

Professors N. P. Cernansky and I. Namer as Co-Principal Investigators. They

shared the responsibility for overall management of the program including

conducting, directing, and reporting the research efforts. The primary

responsibility for carrying out the details of the experimental and analytical

efforts was shared by two graduate research assistants, Mr. Allen M. Danis and

Mr. Daniel L. Dietrich, both Ph.D. candidates. Also contributing to the initial

design and construction of the modified spray facility and the ignition

apparatus were Dr. Hamid Sarv (then a Ph.D. candidate) and Mr. Shawn Smolsky

(undergraduate research assistant). Mr. Robert J. Tidona (Laboratory Research

Engineer) provided day-to-day guidance in the design and development of the

experimental apparatus and testing program, as well as technical assistance in

the collection and interpretation of data.

E. Reports and Publications

In addition to various interim progress reports and this final report, work

conducted as part of this research program has resulted in the following

publications and papers presented.

(1) Danis, A. M., N. P. Cernansky and I. Namer, "Transition Region Ignition

Characteristics of N-Heptane Fuel Sprays," Paper No. CSS/WSSCI

85-I-6B, presented at the 1985 Spring Technical Meeting of the

Central and Western States Sections/The Combustion Institute, San

Antonio, TX, 22-23 April 1985.

(2) Dietrich, D. L., I. Namer and N. P. Cernansky, "Optimization of a Droplet

Sizing Apparatus for ]n situ Measurements," Paper No. ESSCI 85-79,

presented at the 1985 Eastern States Section/Combustion Institute

. Technical Meeting, Philadelphia, PA, 4-6 November 1985.
(3) Danis, A. M., I. Namer and N. P Cernansky, "Minimum Ignition Energies

of Rich N-Heptane Fuel Sprays m the Transition Region," Paper No

ESSCI 85-26, presented at the 1985 Eastern States

Section/Combustion Institute Technical Meeting, Philadelphia, PA,

4-6 November 1985.

(4) Sarv, H. and N. P Cernansky, "Analysis of the Effects of Fuel Spray

Characteristics on NOX Formation," ASME Paper No

85-WA/HT-47, presented at the ASME Winter Annual Meeting, Miami

Beach, FL, 17-22 November 1985.

(5) Dams, A. M., N P Cernansky and I. Namer, "Ignition Characteristics of

N-Heptane Fuel Sprays in the Transition Region," ASME Paper No

85-WA/HT-46, presented at the ASME Winter Annual Meeting, Miami

Beach, FL, 17-22 November 1985

8
II. EXPERIMENTAL FACILITY

A. Monodisperse Sorav Generator

Most practical combustors burn fuel sprays composed of droplets

distributed over a wide range of diameters. However, in order to accurately

determine droplet size effects on ignition energy, a spray of monosized

droplets was used for the current work. The spray was produced by a

Berglund-Liu Model 3050 Vibrating Orifice Monodisperse Aerosol Generator

(Berglund and Liu, 1973). The spray is generated by applying a periodic

disturbance to a liquid jet emerging from an orifice plate. This breaks the jet

into discrete droplets with diameters within approximately 1% standard

deviation of the mean diameter. Since one droplet is generated per cycle of

disturbance, the droplet diameter can be calculated from the liquid flow rate

(Q) and the frequency of disturbance (f), and is given by:

D =(6Q/TTf)l/3.

A given size orifice is only capable of producing monodisperse droplets in a

discrete range of diameters. However, this range can be extended by using

several orifices of various diameters. For this work, orifices of 12.5, 15, 20,

22.5, 25, 30 and 35 urn diameter were used.

A schematic drawing of the aerosol generator and ignition test facility is

given in. Figure 1. This facility is described in detail by Dams e_tal (1985),

The fuel flow to the aerosol generator is supplied by a high infusion syringe

pump (Harvard Apparatus Model 901), and the disturbance frequency is applied

by a function generator (Hewlett Packard model 3310A). After generation, the

fuel droplets are kept from coagulating by a flow of dispersion air as they

leave the dispersion cup. A dilution airflow is then added to the spray in the
Mounting Plate For 3-D Traversing Mechanism

\

Drain Plug

Liquid Syringe

Pump

AC Signal Generator

Oropiet Generator

Combustion Tunnel—-

••—Dilution Air

Dispersion Air

-Perforated Plate

Reducing

Section

Electrode Traversing Assembly

Figure 1. Schematic of Aerosol Generation and Ignition Test Facility

10
flow reducing section to achieve the desired overall spray stoichiometry.

Both the dispersion and dilution air flows are controlled by flow controllers

(Tylan Model FC-260) and monitored by mass flow meters (Hastings Model

ALL-5K). The air/fuel aerosol then passes through the test section, a 1 3 cm

i.d. pyrex tube 20 cm in length, and is vented to the atmosphere.

Electrodes are used to deliver an ignition spark to the spray in the test

section at a point 8.25 cm below the orifice plate. These electrodes protrude

into the test section with their main axes normal to the spray direction, and

are mounted on a micrometer traversing assembly which is used to adjust the

spark gap The electrodes are 5 mm diameter stainless steel rods, with the

spark-producing ends machined to 1 mm diameter and 6 mm length The

high-voltage electrode tip was pointed, while the ground-side tip was flat.

B. Spark Ignition Apparatus

A capacitive discharge spark generation system capable of independent

variation of spark energy and spark duration was developed for the present

study of droplet size effects on the minimum spark ignition energy of

monodisperse fuel sprays. A schematic of the spark generation circuit is given

in Figure 2 The spark energy and duration are independently controlled by

varying the charging voltage (E0), the capacitance (C), or the resistance (R) of

the circuit. Spark energies ranging from 0.6 to 30 mJ are attainable, as well as

spark durations from 5 to 150 us.

The spark generating procedure is as follows. The capacitor is charged to

15-25 kV by closing switch 5C. The charging resistor, RC, limits the charging

current to protect the capacitors. After charging for 3 seconds, the
I

 KC

— WW

3 no

E0=15-25kV

PROBE 1

C R

0.1-10 nF 1-150 kO

*• PROBE 2

Rs S 15 MO ELECTRODES

Figure 2. Schematic of Spark Generation Circuit

12
high voltage source is disconnected by opening Sc. The grounding switch, Sg,

and the discharge switch, Sd, which are open while charging the capacitor, are

then closed. This causes a large voltage difference to form across the

electrodes and a spark is generated. The shunt resistor, Rs, provides an

alternative path for the capacitor to discharge in the event the spark cannot

jump the spark gap Sc> Sd and Sq are high voltage solenoid switches, and the

charge-discharge cycle is controlled by timing relays.

To measure the spark energy, two high voltage probes were connected to

the circuit across the resistor, as seen in Figure 2, and the voltage traces of

the spark were recorded on a digital oscilloscope (Norland Prowler) The

voltage difference across R divided by the value of R gave the spark current, I

The spark energy, E, was determined from:

E = 0f'v(t)l(t)dt

where V was the measured voltage drop across the electrodes during the spark,

as measured by probe 2, and t was time. The upper limit of the integration, t,

was the spark duration. This was taken equal to four times the time constant

of the current trace. The above integral was evaluated using the waveform

processing capabilities of the digitial oscilloscope.

C. Laser Doppler Anemometer

Due to the nature of the spray generation system used in this work, the

droplet density, and therefore the local equivalence ratio, was not radially

uniform across the test section. An approach whereby a laser Doppler

anemometer (IDA) system was used to determine the actual equivalence ratio

of the spray in the spark gap was developed. A detailed schematic drawing of

the LDA system used is given in Figure 3.

13
MICROCOMPUTER

A/D

CONVERTER

FREQUENCY

COUNTER

TIMER

SIGNAL

CONDITIONER

PINHOLE

PHOTODETECTOR

PROBE VOLUME

LASER

BEAM SPLITTER

MIRROR

FOCUSING LENS

FOCUSING LENS

Figure 3. Schematic of Laser Doppler Anemometer System

14
The light source was a 35 mW Spectra Physics He-Ne Laser (Model 124B)

The beam passed through a beam splitter and focusing lens to the probe volume

m the spray. The backscatter mode collecting optics consisted of a collimatmg

lens, mirror, pinhole and T5I Model 962 Photodetector The signal was

processed by a TSI Model 1984 signal conditioner and a TSI Model 1985 timer

One output of the timer led to a Monsanto Model 100B Frequency Counter for

droplet rate measurements. A second output of the tinner led to an A/D

converter in a DEC LSI-11/2 microcomputer, which performed mean droplet

velocity calculations

The LDA was used to measure the mean droplet velocity (v) and the droplet

rate (n) simultaneously at discrete radial positions across the test section. At

each radial location, the ratio n' = n/v was proportional to the local

equivalence ratio ( 01). The average, <n'>, of n1 over the entire cross section

was proportional to the overall equivalence ratio, 0, so the local equivalence

ratio was: 0' =(n' / <n'>)0.

D Droplet Size Measurement

An apparatus has been designed which will provide the capability for

measuring droplet size, and size distribution in the minimum ignition energy

measurements. The technique is similar to those reported by Yule et al

(1977), Ungut et al (1978), Holve and Self (1979), and Holve (1980) to obtain

particle size distribution. Droplets which pass through a laser probe volume

scatter light which is collected in the near forward direction. The

measurement of the peak of the scattered signal is then related to the particle

size. Due to the non-uniform illumination of the probe volume (Gaussian in the

mode) and the characteristics of the collection optics, the scattered

15
light signal depends both on particle size and its trajectory through-the probe

volume. Holve and Self (1979) have developed a deconvolution technique to

unfold the particle size distribution from the distribution of measured peak

signals from a large number of particles. In the present method a second gate

detector is used to ensure that only particles passing through the center of the

measurement volume are measured. The scattered light signal is then a unique

function of particle size.

The method chosen is a non-intrusive technique capable of accurately

measuring droplet size in the 10-100 um range with a minimal dependence on

the index of refraction. The computation of droplet size distribution becomes

straight forward without the need for a deconvolution technique which

invariably introduces error in the measurement of the particle size

distribution. The system has good spatial resolution due to the small size of

the measurement volume which is defined by the intersection of the two sets

of collection optics. A detailed description of the hardware follows, and

results of numerical computations to optimize the performance of the system

will be presented in a later section.

The droplet size measuring system consists of a 35 mW He-Ne laser (632.8

nm wavelength), transmitting optics, and light collection optics which focuses

the scattered light onto a photomultiplier tube. A second detector system is

used to gate the signal from the primary detector. A schematic of the optical

setup is shown in Figure 4 The transmission optics consist of a spatial filter

and spherical lens which collimate the laser beam. An ellipsoidal probe volume

is formed at the focus of the cylindrical lens when the collimated beam is

passed through it The dimensions of the probe length can be changed by using

different focal length cylindrical and spherical lenses.

When the spray passes through the focused beam, it scatters the laser

light The near-forward scattered light is collected by the primary detector

16
Gate

^/Optics

Spatial Cylindrical

Filter Lens

Signal

Optics

He-Ne

Laser

Spherical

Lens

Probe Volume

Adjustable

Slit

Optical

Filter

PMT

Collection

Lens

\

Expanded View of Collection Optics

Figure 4 Optical Arrangement for Droplet Sizing

17
optics. This consists of a 55 mm f/1.4 camera lens, a pinhole for spatial

filtering, and a 10 nm bandwidth laser interference'filter. The scattered light

is measured using a RCA 4840 photomultiplier tube. The gate detector system

collects light scattered at 90°. For maximum flexibility, the gate detector

system contains the same components as the primary detection system. Both

detector systems are mounted on precision translation and rotation stages to

allow extremely accurate positioning and alignment of the optical system

The signal processing system for the droplet sizing apparatus is shown in

Figure 5. When a fuel droplet passes through the probe volume, defined by the

intersection of the laser beam and the collecting optics field of view, nearly

simultaneous Gaussian-type current pulses are produced by each of the

photomultiplier tubes Each of these pulses is processed on a single channel

analyzer (SCA). When the signal peak occurs, each SCA sends a TTL-logic pulse

to an adjustable one shot chip which then produces a variable-width TTL-logic

pulse. The output from the two adjustable one shot chips are compared in an

"and" gate. When these pulses overlap in time, the "and" gate outputs another

TTL-logic pulse which triggers an analog-to-digital conversion of the primary

signal detector output by the A/D converter of the LSI 11/2 based

minicomputer. The data can then be stored on disk and later analyzed to obtain

the droplet size distribution. A delay exists from the time the peak in the

signal occurs until tne signal is actually read by the A/D converter This is due

to the processing in the SCA and is of the order of one microsecond Since the

signal generated by the droplet has a width of about 100 microseconds (100

um/1 m/sec) there is only a slight error in measuring peak height with this

apparatus. An estimate of the error in the peak height voltage, V(0), can be

18
Amp

Signal

PMT

Pulse

Height

Analyzer

Pulse

Height

Analyzer

Adjustable

One Shot

Adjustable

One Shot

K A/D Converter

Amp

Gate

PMT

LSI-11

Minicomputer

Figure 5. Schematic of Signal Processing System for Laser Droplet Sizing

Apparatus

19
made by noting that the voltage V(t) measured at a time t after the peak

occurs is

V(8t) * V(0) + V(0) 5 t2/ 2

For a Gaussian signal

V = exp[-t2 /2o2 ] / /(2n a)

where a is the standard deviation and

V"(0) = - V(0)/a2 .

Therefore

{ V(0) - V( 5t)} / V(0) * 0.5 ( 8t / a)2 .

If 5t is 1 uS and o is about 25 ^S.the error in peak voltage measurement is

0 08% which is adequate for this experiment.

20
III. EXPERIMENTAL RESULTS AND DISCUSSION

A. Effect of Soark Duration on Minimum Ignition Energy

The effect of spark duration on ignition energy was tested initially with

n-heptane sprays Using a constant electrode spacing of 2.0 mm, ignition

frequency measurements were made at two overall equivalence ratios (0.5 and

1.0) and two initial droplet diameters (28 and 68 )im) for each of five durations

covering the range from 15 to 150 us. For a given spark duration and spray

condition, the ignition frequency was measured for 3-5 different spark

energies ranging from 0.6 to 10 mJ One hundred sparks were delivered to the

spray at each spark energy, with the criterion for ignition being a visible

flame The spark energy was measured for 20 of the 100 sparks and the mean

spark energy determined. The ignition frequency was then plotted against the

mean spark energy For each duration and spray condition, the spark energy

which produced a 90% ignition frequency, as determined by a least mean

squares straight line fit, was called the minimum ignition energy

Figure 6 is a representative result of this duration study, showing the

minimum ignition energy of a n-heptane spray of 68 |im initial droplet

diameter and 1.0 overall equivalence ratio as determined by using five

different durations ranging from 15-165 \is. As seen in Figure 6, spark

duration had little effect on ignition energy, and this was the case for all

conditions tested. Based on these results, spark durations in the 60-90 us

range were used for all subsequent ignition testing

B. Spark Gap Equivalence Ratio Determination

The equivalence ratio at the spark gap, 0G, was taken as the average of the

local equivalence ratio, 0, over the width of the electrode spacing in the center

of the spray. A representative graph of the variation in 0 with radial position
*•*» ->

E

UJ

z

UJ

o

M

H . o

Z

H

0

58 190 150

SPARK DURATION Gus«c)

280

Figure 6. Effect of Spark Duration on the Minimum Ignition Energy of

N-Heptane Sprays: D0= 68um, 0 =1.0; Spark Gap = 2 Omm.

22
for three overall equivalence ratios for a spray of Initial droplet diameter D0 =

50 urn is given in Figure 7. As seen in Figure 7, the local equivalence ratio had

a parabolic profile across the test section, with the maximum droplet density

at the center of the spray where the electrode spark gap was located

Therefore, 0G was greater than 0 in all cases. From data such as these, 0G was

determined for each spray condition studied.

C Optimization of Spark Gap Width

Previous spark ignition studies by other researchers (Rao and Lefebvre,

1973; Ballal and Lefebvre, 1978) have shown the electrode spacing to have an

effect on the measured ignition energy of fuel sprays, with an optimum spark

gap width observed. This optimum electrode spacing occurs due to the nature

of spark ignition The spark must raise the temperature of some volume of the

fuel vapor/droplet and air mixture to it's flame temperature in order to ignite

it. This volume is called the spark kernel In the case where the electrode

spacing is smaller than optimum, more heat loss occurs from the spark kernel

to the electrodes, and the kernel reaches a lower temperature Conversely,

when the spark gap width is larger than optimum, the volume of the spark

kernel is larger and the kernel again reaches a lower temperature. Therefore,

to accurately measure the minimum ignition energy of a spray, the optimum

electrode spacing must first be determined.

For each spray condition studied, the spark gap width was optimized in the

following manner. While keeping the spark energy constant, ignition frequency

measurements were performed over a range of electrode spacmgs from 1 0 to

50 mm, in intervals of 0.5 mm. One hundred sparks were delivered to the

spray at each spacing, and the spacing which produced the highest ignition

23
b

0 4

H ^

C£

UJ

2 3

UJ

— 1

H

ID

O o

UJ *•

_J

O

o

~* 1

n

A -A

A

A

 A

D D

D

A

n D

n o ° ° o

a ° ° o a -

o o

i .... . r

2 8 2

RADIAL POSITION

Figure 7. Radial Profiles of Local Equivalence Ratio (01) Across the Test

Section for N-Heptane SpraysD0 = 50^m, Spark Gap = 3 Omm, Spark Duration = 77jis

(A-0 = 1 0,0-0 = 0.8; 0-0 = 06)

24
frequency was the optimum. Representative results from this optimization

procedure are given in Figure 8, which shows the effect of spark gap width on

ignition frequency for a spray of 70 \im initial droplet diameter and 1 0 overall

equivalence ratio As seen in Figure 8, 3.0 mm is clearly the optimum. In

fact, 3.0 mm was the optimum spark gap width for all sprays studied with the

exception of the 25 u,m initial droplet diameter sprays, for which 3.5 mm was

the optimum. All subsequent ignition energy testing was done using these

optimum electrode spacings.

D. Droplet Size and Equivalence Ratio Effects on Minimum Ignition Energies

In order to determine the effect of droplet size and equivalence ratio on the

minimum energy required to ignite n-heptane sprays, some criteria for that

minimum had to be established. The ignition phenomenon is such that no sharp

division exists between energies which are sufficient for ignition and those

which are not Instead, there is a range of energies for which ignition becomes

more likely as the energy increases. Therefore, one can speak of a probability

or frequency of ignition for a given spark energy It was decided to denote the

energy at which ignition was observed 90% of the time as the minimum ignition

energy.

The minimum ignition energy was determined for a fuel spray of given

initial droplet diameter and overall equivalence ratio in the following manner

Ignition frequency observations were made over a range of 3-5 different spark

energies which produced ignition frequencies in the 70 to 100% range. At

each spark energy, 150 to 250 sparks were used to determine the ignition

frequency, and the actual spark energy of every fifth spark was measured to

determine the mean spark energy. The ignition frequencies and their

25
X

\s

O

u

o

LJ

O

H

CD

H

88

78

1.5 2.8 2.5 3.8 3.5

SPARK GAP CmrO

4.0 4.5

Figure 8. Effect of Spark Gap Width on the Frequency of Ignition for

N-Heptane Sprays:

D0 = 70um, 0 = 1.0, Spark Duration = 76us;Spark Energy = 08mJ

26
corresponding mean spark energies were then plotted. Representative data

from this procedure are shown in Figure 9, which is for 60 ^m initial droplet

diameter sprays at gap equivalence ratios of 4.5, 3.5 and 2 5 The straight lines

in Figure 9 were fit to the data by a least-squares method, and the minimum

ignition energy (90% ignition frequency) was determined from these lines This

figure clearly shows that the igmtibility of the sprays increased as the gap

equivalence ratio increased.

From data such as that presented in Figure 9, the minimum ignition energy

was determined for sprays with initial droplet diameters of 25, 40, 50, 60 and

70 urn, and gap equivalence ratios of 1.5, 2.5 and 35 (as well as two

measurements at 00 > 4.0). Figure 10 shows the effect of varying droplet

diameter on the minimum ignition energy for 0G » 1 5, 25 and 3.5 As seen in

Figure 10, the minimum ignition energy of the sprays increased linearly as the

initial droplet diameter increased from 25 to 70 urn at a given equivalence

ratio. Also, the minimum ignition energy decreased as the gap equivalence

ratio increased from 1.5 to 35 at a given droplet diameter These results are

in general agreement with the predictions based on an ignition model proposed

by Ballal and Lefebvre (1981). The relevant part of their model applies to

a heterogeneous, quiescent fuel/air mixture of monodisperse droplets They

further assumed that there was no prevaponzation of droplets, while some

prevaponzation does occur in our experiments. Both the model and the data

presented here predict a decrease in the minimum ignition energy with

increasing equivalence ratio and decreasing initial droplet diameter The

main qualitative difference is that our experimental ignition energy data are

represented better by a linear dependence on droplet diameter rather than the

diameter cubed dependence predicted by the model. This may be due to the

27
100

o

UJ

Z>

a

UJ

QL

U.

O

H

tH

O

H

1.0 1.5 2.0

SPARK ENERGY CmJ)

2.5

Figure 9. Effect of Gap Equivalence Ratio (0G) and Spark Energy

on the Frequency of Ignition for N-Heptane Sprays:

00 = 60)im; Spark Gap = 3.0mm; Spark Duration = 77u.s.

(0 - 0fi = 45; D - 0r = 3.5; A - 0r = 2.5)

28
CD

QL

LJ

UJ

O

M

I—

M

H

z:

3.0

2.5

2.0

1 .5

1 .0

0.5

0.0

0 10 20 30 40 50 60 70

INITIAL DROPLET DIAMETER CunO

80

Figure 10 Effect of Gap Equivalence Ratio (0G) and Initial Droplet

Diameter (D0) on the Minimum Ignition Energy of N-Heptane Sprays.

(0 - 0G, = 1 5, D - 0G = 2.5; A - 0G = 3.5)

*_

29
comparatively narrow range of droplet diameters studied here: 25-70 urn.

Quantitatively, it was found that the ignition energies predicted by the model

were always lower than the experimental values by 0.4-1.0 mJ.

A final observation concerning the results presented in F-vgure 10 is that

the rate at which the minimum ignition energy increased with increasing

droplet diameter decreased with increasing equivalence ratio. This would

indicate that some optimum equivalence ratio, above 06 = 3.5, exists for

ignition. This may seem to contradict the established convention that fuel/air

mixtures ignite most readily at or near stoichiometnc conditions However,

this stoichiometnc condition only applies to prevaponzed/premixed fuel/air

mixtures, while most of the fuel within the spark gap is still in the liquid

phase in our experiment Therefore the average equivalence ratio in the gas

phase region between the droplets in the spark gap will be considerably

lower than 06, which includes the liquid phase fuel as well For this reason,

one must expect phenomena which are usually associated with stoichiometeric

conditions to occur at considerably richer values of 0Q. This is essentially the

same conclusion which was reached by Aggarwal and Sirignano (1984)

An optimum droplet size for ignition was not observed in this study If

such an optimum exists, It would be below 25 urn initial droplet diameter for

the conditions examined in this work This has also been suggested by work

done by Chan and Polymeropoulos (1981).

30
E Droplet Sizing Method

The most important theoretical aspect of the droplet sizing system is the

sensitivity of the measurement system to droplet size. The sensitivity of the

system to droplet size is a function of-the collection geometry, and the optical

properties of the droplet. A computer code was developed which simulates the

response of the system as a function of droplet size, collection geometry, and

droplet optical properties. A brief discussion of the theory behind the code is

presented, followed by the results of the computations.

The scattered light intensity from a spherical particle can be written as

I = U0 /U2r

2

)} F'( a.<J>', n) (van deHulst, 1957)

where: I0 - Incident intensity on the particle.

k - Wave number of the incident radiation.

a - Scattering parameter ( a = itD/A )

D - Particle diameter

A - Wavelength of the incident radiation.

n - Complex index of refraction

F' - Scattering function

<t> - Scattering angle relative to optical axis

The response function is defined as the scattered light intensity integrated

over the surface of the collection optics, and is proportional to the output of

the photomultiplier tube Using the equation for the scattered intensity above,

the response functon is written as

RF = (l 0 / k 2)-| sF

1

(a,(}),n)d s
The scattering function, F', has been computed from Mie scattering theory,

using subroutines from Wiscombe (1979). Results have also been computed

using Fraunhofer diffraction theory to determine the scattering function

Hodkinson and Greenleaves (1963) found that in the limit of large particles,

Mie theory can be approximated by the separate effects of diffraction,

reflection, and refraction. If a significant portion of the signal is due to

diffraction, the dependence on the index of refraction and polarization of the

incident radiation will be minimized. This is the reason a near forward

geometry is used Holve (1979) investigated a coaxial geometry, but found that

the measurement volume was too large, and errors due to two particles in the

probe volume simultaneously became significant. If part of the lens is masked

off, then it is possible to collect more light at smaller angles, thus capturing

more of the diffracted component. This geometry is shown in Figure 11, along

with all other angles used in the calculations. Computations of the response

function have been made for a wide range of collection geometries and

refractive indices The test conditions are presented in Table 1 Results

representative of the computation for Mie theory are shown in Figs. 12 and 13.

Both of these and all subsequent plots have the computed response function

as a function of the scattering parameter for fixed collection geometry and

droplet refractive index. It should be noted that for He - Ne laser light the

droplet diameter D = a /10. Figure 12 shows that the response function is a

monotomcally increasing function of the scattering parameter. This is the

desired result since the output of the PMT would uniquely define particle size

A different collection geometry and refractive index result in the plot of Figure

13 In this case, the response function will no longer uniquely define particle

size. The reason these computations are performed is to optimize the

design of the system and eliminate or minimize the sizing ambiguity of Fig. 13.

32
Projection in the x-z plane.

ProDe

Volume

Projection in the x-y plane.

Figure 11. Col lection Geometry

33
<

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(0

Q.

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10'

to

0e.0M.l

12.3*

n K0.10)

too

Scattering Parameter

1000

Figure 12. Computed Response Function using Mie Theory,Optimized System

35
to

-1

100

Scattering Parameter

1000

Figure 13. Computed Response Function using Mie Theory,

Non - Optimized System

36
This optimization is based on the minimization of the parameter R defined as

R = 2 {( aL - a s ) / ( QL+ a s ) }

where d|_ and as are the large and small values of the scattering parameter for

which a sizing ambiguity exists The location of the maximum value of R is

also important. For this work the maximum value of R was always found to be

in the a = 10 to a = 100 range ( D s 1 to 10 iim ). The computations have been

performed to determine the effects of collection geometry and droplet

refractive index on the parameter R

The effect of increasing the collection aperture ( 0\_ ) is shown in Figure

14 The response function has been shown as a function of scattering

parameter for different collection apertures and fixed collection angle and

droplet refractive index. For a collection aperture of 5°, the maximum value of

R is 0.51 The maximum value of R decreases to 0 40 as the collection aperture

is increased to 20" The effect of increasing the collection angle is therefore

to improve the resolution of the system Similar collection aperture behavior

is observed regardless of the value of the collection angle or refractive index

Only the magnitude and location of the maximum R will change. However, the

effect becomes more pronounced as the collection angle is increased.

Figure 15 shows the effect of increasing the collection angle ( 0C ) on the

computed response function ( fixed collection aperture and refractive index)

For a collection angle of 10°, R is 0.96, and as the collection angle is decreased

to 1°, the maximum value of R is decreased to 045 The resolution of the

system will improve as the collection angle is decreased.

37
c

o

4-t

o

c

3

c

o

a

<n

o.

o

u

100

Scattering Parameter

Figure 14 The Effect of Collection Aperture (^) on

the Computed Response Function

38
10

o

4>»

u

u.

cn

O

o.

CO

0)

cc

0)

*J

a

a

o

u

100

Scattering Parameter

1000

Figure 15 The Effect of Collection Angle ( 4>c ) on

the Computed Response Function

39
This trend is independent of collection aperture and refractive index although

the magnitude and location of the maximum R does change.

The optimum geometry will then have as small a collection angle and as

large a collection aperture as possible Signal coincidence effects in the probe

volume may require the use of larger collection angles to reduce the size of the

probe volume If this is the case, a larger collection aperture can be used to

partially offset the reduction of sensitivity due to a larger collection angle.

The results have shown that the effect of decreasing the collection angle was

stronger than that of increasing the collection aperture

The effect of the imaginary part of the complex index of refraction on the

response function is shown in Figure 16 This figure clearly shows that when

the imaginary part of the index of refraction of a particle is increased from 0 0

to 0.1, the resolution of the system is improved. For an imaginary part of the

index of refraction of 0 1 the response function becomes a monotonically

increasing function of the scattering parameter for the collection geometries

shown. The calculations have shown that this trend is independent of the

collection geometry and real part of the refractive index Results have been

tabulated for variations of the real part of the refractive index Although the

shape of the response function curve does change, the resolution, as measured

by R, is not affected by variations in the real part of the refractive index

40
o

£

<J

a

CO

c

o

Q.

tn

•o

4)

*j

3

a

E

o

o

100

Scattering Parameter

1000

Figure 16. The Effect of the Imaginary Part of the Index of Refraction on

the Computed Response Function

41
F. Summary and Conclusions

A reliable facility for studying the details of spray combustion for liquid

droplets in the transition region (10-100 u,m initial diameter) has been

developed and minimum spark ignition energies forn-heptane have been

measured. The following conclusions regarding the present ignition

studies can be drawn:

(1) For initial droplet diameters of 28 and 68 ^m and overall equivalence

ratios of 0.5 and 1.0, spark duration had little effect on minimum

ignition energy.

(2) In all cases where local droplet number density was measured, the

spark gap equivalence ratio was found to be greater than the overall

equivalence ratio.

(3) For all but the smallest initial droplet size studied (25 urn), the

optimum spark gap width was determined to be 3.0 mm. This value

increased slightly to 3.5 mm for 25 urn diameter droplets.

(4) For a range of initial droplet diameters of 25-70 iim and gap

equivalence ratios from 1.5 to 3.5, the minimum ignition energy of the

sprays increased linearly with initial droplet diameter and decreased

as the gap equivalence ratio was increased. These results followed the

trends predicted by Ballal and Lefebvre (198 i); however the dependence

of minimum ignition energy on initial diameter was not as strong as

their model predicts (linear dependence observed rather than the d3

dependency predicted).

(5) Ignition energies measured in the present work always exceeded the

predicted values of Ballal and Lefebvre by 0.4-1.0 mj.

42
(6) The rate of increase of minimum ignition energy with droplet diameter

was reduced as spark gap equivalence ratio was increased, indicating

that some optimum equivalence ratio greater the <J>G = 3.5 (highest

value studied) exists. This is essentially the'conclusion drawn for

heterogeneous systems by Aggarwal and Singnano (1984)

Further work on minimum ignition energy is planned for more complicated

spray systems including binary mixtures Also, smaller initial droplet sizes

will be studied (< 25 urn) to determine the dependence of minimum ignition

energy at the lower end of the transition region. Eventually, more precise

measurements of spark gap equivalence ratio and droplet size at the gap will be

provided by the optical droplet sizing technique. These data will provide the

true droplet size effect on minimum ignition energy as well as a better

knowledge of droplet evaporation rates.

The laser-based forward scattering droplet sizing technique has not yet

been implemented to make actual droplet size measurements on the spray

burner. This is due to delays in the development of the complex computer code

which makes the response function calculations. In addition, it was decided to

carry out a series of numerical studies with the code in order to obtain

desirable information for setting up the optics for actual droplet size

measurements Conclusions which can be drawn from these studies are.

' (1) The optimum collection geometry for maximizing the resolution of the

droplet sizing system (minimizing R) should have the smallest possible

collection angle with the largest possible collection aperture

(2) Increasing the imaginary part of the refractive index (absorption) tends

to smooth the resonant structure of the response function curve, and

increase the resolution of the system.

43
(3) The resolution of the droplet sizing system will be a very weak

function of the real part of the refractive index.

In addition to the numerical studies presented, the transmitting optics and

one set of collection optics have been set up and aligned Work in the

immediate future will center on the implementation of the electronic signal

processing equipment to the existing optical set up Upon completion of'this

task,a system calibration will be performed after which the system will be

used to measure droplet size, droplet size distribution, and number density in

the spray experiments

Additional work will also be performed on the computer code to include

probe volume and collection optics effects on the computed response function

The code will also be used to examine the possibility of modifying the present

system to measure droplet size and size distribution using an intensity

ratiomg technique.

44
IV REFERENCES

Aggarwal, 5. K. and Sirignano, W. A.,

"Ignition of Fuel Sprays: Deterministic Calculations for Idealized

Droplet Arrays," Twentieth Symposium (International) on Combustion.

p. 1773-1780, The Combustion Institute, Pittsburgh, PA, 1984

Balla!, D. R. and Lefebvre, A. H,

"Ignition and Flame Quenching of Quiescent Fuel Mists," Proc. R. Soc

Lond.. A364, pp. 277-294, 1978.

Ballal, D. R. and Lefebvre, A. H,

"A General Model of Spark Ignition for Gaseous and Liquid Fuel-Air

Mixtures," Eighteenth Symposium (International) on Combustion, p

1737-1745, The Combustion Institute, Pittsburgh, PA, 1981.

Berglund, R. N. and Liu, B. Y. K,

"Generation of Monodisperse Aerosol Standards," Environmental

Science and Technology 7. 2, 147, 1973

Chan, K. K. and Polymeropoulos, C. E.,

"An Experimental Investigation of the Minimum Ignition Energy of

Monodisperse Sprays," Paper No. ESSCI 81-21, Fall Technical Meeting,

Eastern States Section, The Combustion Institute, Pittsburgh, PA,

October 1981.

Danis, A. M., Cernansky, N. P. and Namer. I.,

"Transition Region Ignition Characteristics of N-Heptane Fuel Sprays,"

Paper No. CSS/W55CI 85-1-6B, Joint Western and Central States

Section, The Combustion Institute, San Antonio, TX, 22-23 April 1985.

Hodkinson, J. R. and Greenleaves, I.,

"Computations of Light-Scattering and Extinction by Spheres

According to Diffraction and Geometrical Optics, and Some

Comparisons withMie Theory," J. Opt. Soc. Am. 53. 5, 577-588, 1963

Holve, D. and Self, S.,

"Optical Particle Sizing for In-Situ Measurements, Parts I and II," J,

Appl. Optics 18, 10, 1979.

45
Holve, D.,

"In-Situ Particle Sizing Technique." J Energy 4. 176, 1980

Rao, H. N. 5. and Lefebvre, A. H.,

"Ignition of Kerosine Fuel Sprays in a Flowing Air Stream,"

Combustion Science and Technology 8. 95. 1973

Ungut, A, Yule, A. J, Taylor, D C. and Chigier, N. A,

"Particle Size Measurement by Laser Anemometry," J Energy 2, 6,

1978.

van de Hulst, H. C,

Light Scattering by Small Particles. Dover Publications, New York, New

York, 1957

Wiscombe, W J.,

"Mie Scattering Calculations. Advances in Technique and Fast, Vector

Speed Computer Codes," Report No. NCAR/TN - 140 * STR, National

Center for Atmospheric Research, Boulder, Colorado, June 1979

Yule, A. J., Chigier, N A, Atakan, S. and Ungut, A.,

"Particle Size and Velocity Measurement by Laser Anomometry," J_

Energy J_, 4, 1977.

46
APPENDIX: PUBLICATIONS AND PRESENTATIONS

In addition to the semi-annual progress reports and this final report,

work accomplished as part of this and related research programs has resulted

in the following publications and presentations:

(1) Nizami, A. A and N P Cernansky, "A Monodisperse Spray Combustion

System and the Measurement of Oxides of Nitrogen," Paper No. W5SCI

78-37, presented at the 1978 Western States Section/Combustion

Institute Spring Meeting, Boulder, CO, 17-18 April 1978

(2) Nizami, A. A., "An Experimental Study of NOX Formation in

Monodisperse Fuel Spray Combustion," PhD. Thesis, Drexel University,

Philadelphia, PA, 1978.

(3) Nizami, A. A and N P. Cernansky, "NOX Formation in Monodisperse Fuel

Spray Combustion," Seventeenth Symposium (International)

Combustion. The Combustion Institute, Pittsburgh, PA, p. 475-483,

1979

(4) Nizami, A. A. and N. P. Cernansky, "Some Aspects of Pollutant

Formation in Spray Combustion of Certain Hydrocarbon Fuels,"

presented and published in the Sixth National Conference on

Combustion and Internal Combustion Engines. I IT Bombay, December

1979

(5) Nizami, A. A., S. Singh and N P Cernansky, "Formation of Oxides of

Nitrogen in Monodisperse Spray Combustion of Hydrocarbon Fuels/'

Combustion Science and Technology 28. 97-106, 1982.

(6) Nizami, A. A., R A. Recchiuti and N P. Cernansky, "Chemiluminescent

Measurements of Oxides of Nitrogen from Combustion Systems,"

Department of Mechanical Engineering and Mechanics Report, Drexel

University, Philadelphia, PA, May 1982.

(7) Sarv, K, A. A. Nizami and N. P. Cernansky, "Droplet Size Effects on NOX

Formation in a One-Dimensional Monodisperse Spray Combustion

System," ASME Paper No. 82-JPGC-GT-10, presented at the 1982 Joint

Power Generation Conference, Denver, CO, 17-21 October 1982.

47
(8) Cernansky, N. P. and H. Sarv, "Air Preheating and Droplet Size Effects

on NOX Formation in a One-Dimensional Monodisperse Spray

Combustion System," Ninth International Colloquium on Dynamics of

Explosions and Reactive Systems, Poitiers, France, 4-8 July 1983.

(9) Sarv, H, "Droplet Size Effects on NOX Formation in a One-Dimensional

Monodisperse Spray Combustion System," PhD Thesis, Department of

Mechanical Engineering and Mechanics, Drexel University,

Philadelphia, PA, June 1985.

(lO)Danis, A M., N P. Cernansky and I. Namer, "Transition Region

Characteristics of N-Heptane Fuel Sprays," Paper No. CSS/W55CI

85-1-6B, presented at the 1985 Spring Technical Meeting of the

Central and Western States Sections/Combustion institute, San

Antonio, TX, 22-23 Apri 1 1985

(11) Dietrich, D. L, N P. Cernansky and I Namer, "Optimization of a Droplet

Sizing Apparatus for in silu Measurements," Paper No ES5CI 85-79,

presented at the 1985 Eastern States Section/Combustion Institute

Technical Meeting, Philadelphia, PA, 4-6 November 1985

(12)Danis, A. M., I. Namer and N P. Cernansky, "Minimum Ignition Energies

of Rich N-Heptane Fuel Sprays in the Transition Region," Paper No

ESSCI 85-26, presented at the 1985 Eastern States

Section/Combustion Institute Technical Meeting, Philadelphia, PA,

4-6 November 1985

(13) Sarv, H. and N. P Cernansky, "Analysis of the Effects of Fuel Spray

Characteristics on NOX Formation," Paper No. ASME 85-WA/HT-47,

presented at the ASME Winter Annual Meeting, Miami Beach, FL, 17-22

November 1985.

(14) Danis, A M., N. P. Cernansky and I Namer, "Ignition Characteristics of

N-Heptane Fuel Sprays in the Transition Region," Paper No ASME

85-WA/HT-46, presented at the ASME Winter Annual Meeting, Miami

Beach, FL, 17-22 November 1985.

48
Copies of these papers or reports can be obtained by writing

Or Nicholas P Cernansky

Department of Mechanical Engineering and Mechanics

Drexel University

Philadelphia, PA 19104 (215)895-2284,2352