WS/CSCIJ85-1-6B

TRANSITION REGION IGNITION CHARACTERISTICS

OF N-HEPTANE FUEL SPRAYS

by

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

College ot Engineering

Department of

Mechanical Engineering

and Mechanics

Philadelphia, PA 1910-4

(215)895-2352-53

Presented at the Joint Western and Central States Section

Combustion Institute Meeting, San Antonio, Texas,-22-2-3 April 1985.

(N&S&-CR- i7§36^|i) I Tfi&NSiTICN'BEGION IGNITION

CHARACTERISTICS OF n-HEPTAN E JOEL S-PflAX&

(Dcexel Univ.) 24 p HC A02/HJ A01 CSCL 2 IB

¥8 6- to 3"f

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AooTPUT ORIGINAL PAGE IS T

 OF POOR OUAUTY

Tj'nlt. Jon studio s wor e per f^rro u on nonodlsperse n-heptane sprays at

atmospheri c pressur e over a r;inj*o of equivalence ratios and droplet diameters.

A capacitivc discharge spark Ignitio n system was used as the ignition source,

providing Independent control of spark energy and duration. Preliminary

measure rents' were rw!c to optimiz e spark duration and spark cap, optimum

conditions heln/^ those at whic h the nnxinum frequency or probability of

ignition was observed. The effect of spark duration on ignition frequency for

several spark energies was determined for equivalence ratios of 0.5 and 1.0

and initial droplet diameters of 28 and 68 microns. Spark duration had little

effect on ignition frequency over the entire 15-170 us range examined. Spark

durations of 70-80 us were used for all subsequent work. The spark gap was

optimized at equivalence ratios of 0.6, 0.8 and 1.0 and initial droplet

diameters of 30, 40, 50, 60 and 70 microns by varying the electrode spacing

from 0.5 to 5.0 mm while maintaining a constant spark energy. The optimum gap

was determined to be 3.0 mm for nearly all conditions.

Using the optimum electrode spacing and spark duration, the frequency of

Ignition was determined as a function of spark energy for the same three

equivalence ratios (0.6, 0.8, and 1.0) and for initial droplet diameters of

40, 50, 60, and 70 urn. The spark energy which led to a frequency of ignition

of 90 percent was evaluated and defined as the minimum ignition energy. These

data indicate that the ignitability of the sprays is enhanced as the

equivalence ratio is increased from 0.6 to 1.0, as evidenced by a decrease in

minimum ignition energy from 2 - 2.6 mJ to 0.6 - 1.0 mJ depending on droplet

size. There is an initial uniform and consistent increase energy with

increasing droplet size (from 40 -50 urn). This is followed by a slight

decrease and then a subsequent increase in minimum ignition energy as the

ii
droplet sine is increased further. This locnl tnlnlnum in the ignition energy

curve occurs near 60 yn for £ = 0.6 and shifts to larger initi.il droplet

diameters wit h increasing equivalence ratios. The effec t of increasing spray

velocity vas to Increase the minimum ignition energy, as expected.

s
ORIGINAL PAGE IS

I. iNTRQPi.-CTioN OF .POOR QUALITY

The cocbustion of liqui d fue l sprnys is expected to rcn.iin a rr.ijor source

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

qualit y are currently deteriorating. It is anticipated that the fuels of the

future nay have properties which are significantly differen t from today's

fuels. In order to effectivel y design fue l Injectors, conhustors, and

1,en I tors to accocrorln te broadened specification and syntheti c fuels, a more

fundamenta l understanding of the ignition of liquid fuel sprays is essential.

To this end, a systenatic program to study the Ignition of monodisperse fuel

sprays In the so called transition region is being undertaken at Drexel

University. Some experimental results of the minimum ignition energy of

n-heptane sprays are presented here.

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

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

transition region: (1) maxima in burning velocity (Polymeropoulos and Das,

1975; Polymeropoulos, 1984; Hayashi e£ £l., 1976); (2) extension of lean

flammability limits (Burgoyne and Cohen, 1954; Hayashi and Kumagi, 1974); (3)

minima in ignition energy requirements (Rao and Lefebvre, 1973; Ballal and

Lefebvre, 1978; Chan and Polymeropoulos, 1981; Chan, 1982); and (4) minima in

NOX formation (Nizani and Cernansky, 1979; Nizami e_t al. , 1982; Sarv et al. ,

1983; Cernansky and Sarv, 1983). Therefore, combustion In the transition

region is an attractive design goal for practical combustofs.

Previous research (e.g., Burgoyne and Cohen, 1954; Cernansky and Sarv,

1983) 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 tho cornplcx interactions between droplets tn the size nnd

number density range where tho presence of neighboring droplets can

significantly affect the evaporative and cor.bustion environment of the fuel

spray. Overall, the combustion behavior is affected by the relative fraction

of fuel initially in a prevaporlzed and prenixed state, the average

interdroplet spacing, and the fuel properties. Although considerable research

has been and is being undertaken in this area by a minbcr of investigators

(e.g., Tal e_t al_. , 1983; Sirignano, 1904; Sirignnno and Soirr.cr, 1934; Bcllan

and Cuffel, 1983; Labowsky, 1976, 1980; Marberry et_ a_l. , 1984; Ray and Davis,

1980, 1981; Peters et^ al_., 1984; and others), 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.

The situation with regard to ignition energy and ignition phenomena in

the transition region is not much better. Again, because of the complex

configuration and geometry of sprays (even monodisperse sprays), adequate

predictions of droplet interaction effects on spray ignition are not

available. 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, Rao

*

and Lofcbvre's (1973) and Ballal and Lefebvre's (1978, 1979) important
Ignition studies were for polydisperse sprays with droplets In the diameter

ranp,e of 30 to 120 microns. Thus, It Is not possible to Isolate the precise

droplet dlaneter at which the Ignition energy requirement Is really minimum.

Also, Chan and Polymeropoulos (1981) used Burgoyne and Cohen's (1954) aerosol

generator and, hence, could only produce droplets over a relatively narrow

ranp// of droplet diameters without changing equivalence ratio. Thus, they

could not perform their experiments over the entire transition range of

droplet dlaneters and equivalence ratios. The reported data Is only for a

droplet diameter range of 10 and 30 ym and equivalence ratios from 0.39 to

1.0. Further, recent data have raised questions about the results of Chan and

Polymeropoulos (Polymeropoulos, 1985). For these reasons, a complete and

comprehensive picture of Ignition energy variation in the transition region is

not available.

The data which are presented here are some results obtained with a spray

facility which-overcomes many of the above problems and limitations. The .

major components of the facility include: a Berglund-Liu droplet generator

which supplies the monodisperse fuel spray to a combustion tunnel; a spark

ignition system with the capability of independently varying the spark

duration, energy, and gap; and a data acquisition system consisting of a

Norland Prowler two-channel digital oscilloscope, capable of data acquistion

at rates up to 20 million samples per second. This facility has been used to

accurately determine the minimum energy required to ignite n-heptane sprays in

the transition region.
II. FXPFRIMFN'TAT. APPARATUS

1. SPRAY1 '

,

A Rer^lund-Llu vibrating orific e aerosol generator was used to supply a

well-defined and characterized spray of fuel droplets for ignition s.tudies.

In this system, droplet size, number density, flow velocities, stoichlome try ,

fue 1/7 type and properties, and extent of prevaporization can be independently

" . v .

varied and their effects isolated and studied. It can be shown that for a

given fuel flow rate and droplet diameter in air, the average interdroplet

spacing is determined uniquely by the equivalence ratio. However, this can be

controlled and modified by employing diluents or pxygen enrichment.

Nonetheless, for the present experiments, only Initial droplet size,

equivalence ratio and spray velocity were varied. The fuel used was

n-heptane. The fuel and air were at 20°C and there was no appreciable

prevaporization prior to ignition.

The aerosol generator provides a spray whose droplet diameters can be

routinely varied between 10-100 urn with a standard deviation. of less than 1

percent of the mean for monodisperse sprays. After generation, the droplets

•are subjected to" a flow of dispersion air to prevent coagulation. The aerosol

then passes through a flow reducing section where dilution air is added to

achieve the desired stoichiometry. Both the dispersion and dilution air flows

are adjusted and monitored by electronic flow controllers. Thus, the

equivalence ratio can be varied over the entire range of flammability.

The basic system is the same as that previously used by Nizani and

Cernansky (1978, 1979), Nizami e_t al. (1982), Sarv e_t al. , (1983) and

Cernansky and Sarv (1983), with some modifications. In particular, a downward

flowing spray orientation has been chosen in order to prevent accumulation of

, fuel droplets in the test section. This provides better uniformity of the
spray, smaller differences between droplet and air flow velocities, and a

nnrgin of safety by preventing fuel accumulation within the burner. Also, the

test section has been fitted with a set of Ignition electrodes, which can be

adjusted for gap spacing, position in the fuel spray, geometry, etc. Further,

the design of the flow reducing section has been changed in order to provide

flot//veloci ties which always exceed the flame speed, thereby preventing

flashback. Special consideration was given to maintaining uniform flow as

well, since a screen flaneholder, such as that used in previous NOX emission

experiments, is not appropriate for use In the present ignition experiments.

Tests have shown that the present spray apparatus achieves a uniform aerosol

flow with virtually no fuel droplet accumulation in the test section. A

schematic of the modified spray burner facility is presented in Figure 1.

2. IGNITION SYSTEM

In order to determine the minimum energy needed to ignite a combustible

mixture, a capacitive discharge spark Ignition system has been developed.

Briefly, the ignition system follows the design of Peters (1981) with a few

additional features added for improved safety and convenience of operation. A

'schematic of the system is shown in Figure 2. The spark energy and duration

can be Independently controlled by varying the charging voltage, the

capacitance, or the resistance of the circuit. (Switches .are used to connect

or disconnect capacitors or resistors from the circuit.) The capacitors are

charged to approximately 20 KV using a high voltage source by closing switch

Sc; after charging and before ignition, the voltage source Is disconnected

from the circuit by opening Sc. The grounding switch, Sg, and the discharge

switch, Sjjj which are open while charging the capacitors, are then closed.

When this happens, a large voltage difference forms across the electrodes and

* a spark Is generated.'
To measure the spark energy, two high voltage probes are connected across

the resistor bank and the voltage traces of the spark are recorded on a

Norland two-channel digital oscilloscope. The probe locations arc indicated

In Figure 2. Typical voltage traces obtained from the two probes are shown In

Figure 3. The voltage differenc e across the resistor bank divided by the

equivalent resistance of the resistor bank gives the current, I. (The voltage

neasurenents are made relative, to ground.) The energy of the spark, E, Is

determined fron E = /c V(t)I(T)d T where V is the measured voltage drop across

o

the electrodes during the spark, as measured by probe 2, and T is time. The

upper limit of the Integration, t, Is the spark duration. This was taken

equal to four times the time constant of the current trace. The waveform

processing capability of the .digital oscilloscope readily provides evaluation

of the above integration.

It should be noted that care has been taken to provide short cable

lengths in order to minimize ground loops and any stray inductance or

capacitance. This was found to be important by Ballal and Lefebvre (1975) and

it did provide Improved energy measurements in this study.

As noted above, stainless steel electrodes were used to deliver the

ignition spark discharge. The electrodes were mounted In the test section

with their main axes normal to the spray direction. This configuration has

been shown by Ballal and Lefebvre (1975) to produce more consistent sparks

than electrodes mounted with their axes parallel to the flow. The electrodes

were mounted on a micrometer traversing assembly which was used to adjust the

spark gap, as shown in Figure 1. The electrodes were 5 mm diameter stainless

steel rods, with the spark-producing ends machined to 1 mm diameter, and 6 mm

length. The high-voltage side electrode tip was pointed, while the

•

ground-side tip was flat, as shown In Figure 4.
III. IGNITION ENKRGY MEASUREMENTS

1. SPARK DURATION OPTIMIZATION

The effect of spark duration on i/Tiition energy was tested initially.

With a constant spark gap of 2.0 mm, Ignition frequency measurements were made

a t two equivalence ratios (0.5-and 1.0) and two initial droplet diameters (28

and 68 urn) for each of five durations from 15 to 150 vs. For a given spark

duration, capacitance and resistance values were varied to produce sparks of

three to five different energies, ranging from 0.6 to 10 mJ. One hundred

sparks were delivered to the fuel spray at each spark energy and duration, and

frequency of Ignition was determined by visual observation. The criteria for

ignition was the observation of a visible, propagating flame. Spark energy

and duration were measured every fifth spark and an average spark energy was

determined with a standard deviation of 20 percent of the mean.

The results showed little effect of duration on ignition energy, as seen

in Figure 5. These results are in basic agreement with the data of Ballal and

Lefebvre (1975), who reported very little effect of spark duration on ignition

energies of propane/air mixtures for durations in the 40 to 100 us range.

However, Rao and Lefebvre (1976) reported a greater effect of spark duration

on ignition energies of kerosene/air mixtures with droplets in the 52 to 72 urn

Sauter mean diameter (SMD) range. By varying the duration from 30 to 100 us

they saw changes of up to 100 percent in minimum ignition energies for

kerosene fuel sprays.

Based on the above results, spark durations in the 70 to 80 us range were

chosen for the present study. The range of variability in spark duration for

a given setting of the ignition system was about ±8 percent of the mean.

2. GAP OPTIMIZATION

The electrode gap spacing was optimized for n-heptane sprays at

equivalence ratios of 0.6, 0.8 and 1.0, and each initial droplet diameter
studied. At a ftivcn condition, the optimu m j;np' wns determine d by varying the

electrode spacing from 0.5 to 5.0 nm In 0.5 urn steps while keeping the spark

-*

energy constant, and observing which gap gave the maximum ignition frequency

for fifty spnrks. A representative graph of the effect of spark gap on

frequency of ignition is shown in Figure 6. These tests showed the optimum

- gap/j/as 3.0 mm for all cases studied except for the 30 urn droplets at <}> = 0.6.

In this case the optimum spark gap was 3.5 mm.

3. MINIMUM IGNITION ENERGIES

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

the minimum energy required to ignite a n-heptane spray, the criteria for that

minimum had to be established. The ignition phenomenon is such that a sharp

division does not exist 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, and one can speak of the

probability or frequency of Ignition for a given spark energy. Some

researchers have designated the energy at which ignition Is observed 50

percent of the time as the minimum ignition energy. Others have used either

the 90 percent value, the 100 percent value, or have not stated their

criteria. For the sake of providing data which is on the conservative side,

it was decided to denote the energy at which ignition is observed 90 percent

of the time as the minimum ignition energy.

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

initial droplet diameter and equivalence ratio in the following manner.

Ignition frequency observations were made for 3 to 5 sets of 50 sparks each at

2 or 3 different spark energies. The spark energies and, hence, the

capacitance and resistance settings of the ignition system were selected such

that the tuition frequency values fell between 60 and 100 percent. The spark
energy wns measured for every fifth or 10 of the fifty sparks; the standard

deviation of these neasurenents was typically 20 percent of the mean energy.

*

These Ignition frequencies and their corresponding spark energies were then

plotted. Representative data Is shown In Figure 7, which Is for 60 un droplet

diameter sprays at equivalence ratios of 0.6, 0.8 and 1.0. Each point in

Flgu/7e 7 represents one set of 50 sparks. The straight lines in Figure 7 were

u

fit to the data by a least-squares method, and the minimum ignition energy (90

percent Ignition frequency energy) was determined from these lines. This

figure clearly shows that Jthe ignltability of the spray increases as the

equivalence ratio increases from 0.6 to 1.0.

From data such as that presented in Figure 7, the minimum ignition energy

was determined for droplet sizes of 40, 50, 60, and 70 urn and equivalence

ratios of 0.6, 0.8, and 1.0. Figure 8-shows the effect of varying droplet

diameter on the minimum ignition energy for the three equivalence ratios which

were investigated. As noted, the data points, on this figure represent the

intersection of the straight-line-fit and the 90 percent ignition frequency on

Figure 7 and similar figures. The error bars on the figure represent the

uncertainty in the determination of the minimum ignition energy (90 percent

ignition frequency). This was determined from the uncertainties in the spark

energy and ignition frequency which were, in turn, taken as the 90 percent

confidence levels for those values. The uncertainty in the minimum ignition

energy was thus determined to be 12 percent.

As seen in Figure 8, the minimum ignition energy increased for all

equivalence ratios as the initial droplet diameter was raised from 40 to 50

urn. Decreasing ignition energies were observed at all equivalence ratios as

the droplet diameter was increased from 50 to 60 urn. From 60 to 70 Jim the

» minimum ignition energy increased at $ = 0.6, remained level at $ = 0.8, and
decreased slightly at $ = 1.0. This indicates the possibility of a local

minimum in ignition energies for nonodisperse nrhcptnne sprnys, located at

about 60 urn initial droplet diameter for <*> = 0.6 and shifting towards higher

droplet sizes with increasing equivalence ratio. Again, this figure

Illustrates- that the Igni ta-bili ty of these sprays is-enhanced as the

equivalence ratio is increased from 0.6 to 1.0.

The 40 to 70 pm sprays used for the data above were all generated with a

fuel flow rate of 0.18 cc/min, which resulted in droplet velocities of 34, 26

and 20 cm/sec for equivalence ratios of 0.6, 0.8 and 1.0 respectively.

Additional measurements were made on 60 and 70 urn sprays with a higher fuel

flow rate of 0.24 cc/min without changing any other parameters. This resulted

In a 28 percent increase in spray velocities, to 45, 34 and 27 cm/sec

respectively. Figures 9 and 10 show a 20 to 25 percent increase in minimum

ignition energies observed for the 60 and 70 micron sprays at the higher spray

velocities.

10
IV. CONCT.PS TON'S

A spray combustion facility has been developed to study the Ignition

characteristics of monodisperse sprays in the transition region. Procedures

for spark duration and gap spacing optimization have been developed. The

system has been used to determine the minimum ignition energy of some

monodisperse n-heptane sprays. In particular, the effects of initial droplet

diameter and equivalence ratio were considered. The data shows that a local

minimum of the minimum ignition energy exists. .The droplet diameter at which

this minimum occurs increases with increasing equivalence ratio. Flow

velocity studies were performed which confirmed the expected result of

increasing minimum ignition energies for increasing spray velocities.

ACKNOWLEDGEMENTS

This work was supported by NASA Lewis Research Center through Research

Grant No. NAG 3-382, Dr. E. J. Mularz, Technical Officer. The authors would

also like to express their appreciation to Douglas Edwards, Hamld Sarv, Shawn

Smolsky, and Robert J. Tidona for all their help.

11
V. RF.KEKr.f.'CKS

Ballal, D.R. and Lofebvre, A.M., 1975,

"The Influence of Spnrk Discharge Characteristics on Minimum Ignition

Energy in Flowing Cases", Combust ion and I-'lane 24, p. 99.

Ballal, D.R. and Lefebvre, A.M., 1978,

"Iftnitfon and Flame Quenching of Quiescent Fuel Mists''^ Proc. R. Soc.

Lond. A364, p. 277-294. . .

Ballal, D.R. and Lefehvre, A.M., 1973,

"Ignition of Liquid Fuel Sprays .at Subatmospheric Pressures", Combustion

and Flane 31, p. 115.

Ballal, D.R. and Lefebvre, A.M., 1979, •

"Ignition and Flane Quenching of Flowing Heterogeneous Fuel-air

Mixtures", Combustion and Flane 35, p. 155.

Bellan, J. and Cuffel, R., 1983,

"A Theory of Nondilute Spray Evaporation Based Upon Multiple Drop

Inter-actions", Combustion and Flame 51, p. 55.

Burgoyne, J.H. and Cohen, L., 1954,

"The Effect of Drop Size on Flame Propagation in Liquid Aerosols",

Proc. of the Royal Society; Series A; Mathematical and Physical Sciences

225, No. 1162, p. 375.

Cernansky, N.P. and Sarv, H., 1983,

"Pollutant Formation in Monodisperse Fuel Spray Combustion", Final Report

prepared for NASA Lewis Research Center, Grant No. NAG-3-1 Drexel

University, Philadelphia, PA.

Chan, K.K., 1982,

"An Experimental Investigation of Minimum Ignition Energy of Monodisperse

Fuel Sprays", Ph.D. Thesis, Rutgers Univ., New Brunswick, N.J.

Chan, K.K. and Polymeropoulos, C.E., 1981,

"An Experimental Investigation of the Minimum Ignition Energy of

Monodisperse Sprays", Paper No. 81-21, Eastern States Section, The

Combustion Institute.

Chigier, N., 1983,

"Group Combustion Models and Laser Diagnostic Methods in Sprays: A

Review", Combustion and Flame 51, p. 127.

Chiu, H.H., Kim, H.Y. and Croke, E.J., 1983,

"Internal Group Combustion of Liquid Droplets", Nineteenth Symposium

(International) on Combustion, p. 671, The Combustion Institute,

Pittsburgh, PA.

Faeth, G.M., 1979,

"Spray Combustion Models-A Review", AIAA Seventeenth Aerospace Sciences

Meeting, Paper No, 79-0293, New Orleans, LA.

12
llnyashi, S. and Kunnfi , S., 1974,

"Flnmc Preparation In Fuel Droplet-Vnpor-Alr Mixtures" ,

Fifteent h Syrposlim .(Intcrnn tional) on Combustion, p. 445-452, The

Combustion Institute , Pittsburgh , PA.

Hayashl, S., Kur.ar.ai, S. and Sakai, T., 1976,

"Propagation Velocity and Structure of Planes In Droplet-Vapor-Air

Mixtures" , 'Co rib ustion Science and Technology "15, p. 169.

Koshland, C.P. and Bowman, C.T., 1984,

Combustion of Monodisperse Droplet Clouds In a Reactive Environment",

Twentieth Synposlutn (Int'l) on Combustion/ The Combustion Institute,

Pittsburgh, PA.

Lahowsky, M., 1976, ,

"The Effects of Nearest Neighbor Interactions on the Evaporation Rates of

Cloud Particles", Chemical Engineering Science 31, p. 803.

Labowsky, M., 1980,

"Calculation of the Burning Rates of Interacting Fuel Droplets",

Combustion Science and Technology 22, p. 217.

Labowsky, M., 1980,

"Transfer Rate Calculations for Compos!tionally Dissimilar Interacting

Particles", Chemical Engineering Science 35, p. 1041.

Marberry, M., Ray, A.K. and Leung, K., 1984

"Effect of Multiple Particle Interactions on Burning Droplets",

Combustion and Flame 57, p. 237-245.

Miyasaka, K. and Law, C.K. 1981,

"Combustion of Strongly-Interacting Linear Droplet Arrays",

Eighteenth Symposium (Int'l) on Combustion, p. 283, The Combustion

Institute, Pittsburgh, PA.

Nizaml, A.A. and Cernansky, N.P., 1978,

"A Monodisperse Spray Combustion System and the Measurement of Oxides of

Nitrogen", Paper No. 78-37, Western States Section, The Combustion

Institute.

Nizami, A.A. and Cernansky, N.P., 1979,

"NOX Formation in Monodisperse Fuel Spray Combustion",

Seventeenth Symposium (International) on Combustion, p. 475, The

Combustion Institute, Pittsburgh, PA.

Nizami, A.A., Singh, S. and Cernansky, N.P., 1982,

"Formation of Oxides of "Nitrogen in Monodisperse Spray Combustion of

Hydrocarbon Fuels", Combustion Science and Technology 28, p. 97.

Peters, J.E., 1981,

"An Ignition Model for Gas Turbine Engines", Ph.D. Thesis, Purdue

University, W. Lafayette, Indiana.

13
Peters, J.F., Krior, H. nnd Kin, K. , 19S4,

"Fuel Spray Evaporation and Combustion" , Presented at AFOSR/ONR

Contractors Mootin g on Alrbronthtn ^ Propulsion, 20-21 June, Pittsburgh,

PA.

Plndera, M. and Brzustowski, T.A., 1984,

"A Model for the Ignition Predictions of Liquid Fuel Droplets",

Cp n h 11 ff t Jon an < \__ F1 n rc • .5 *>, No.. 1, p. 79. ...... ....... ..... ...... .... -,..-

Polyrriropotilos, C.E. and Pas, S., 1975,

//"The Effect of Droplet Size on the Rumln Burninjg Velocity of Kerosene-Air

(JSprays", Conbustion and Flane 25, p. 247.

Polyneropoulos, C.F,.,

"Flane Propagation in Aerosols of Fuel Droplets, Fuel Vapor and Air",

Conh. Sci. and Tech. 40, p. 217-232.

Polymeropoulos, C.E., 1985,

Private Communication. -

Rao, H.N.S. and Lefebvre, A.H., 1973,

"Ignition of Kerosene Fuel Sprays in a Flowing Air Stream", Combustion

Science and Technology J5, p. 95.

Rao, K.V.L. and Lefebvre, A.H., 1976,

"Minimum Ignition Energies in Flowing Kerosine-Air Mixtures", Combustion

and Flame 27, p. 1.

Ray, A.K. and Davis, E.J., 1980,

"Heat and Mass Transfer With Multiple Particle Interactions Part I.

Droplet Evaporation", Chem. Eng. Commun. 6, p. 61-79.

Ray, A.K. and Davis, E.J., 1981,

"Heat and Mass Transfer with Multiple Particle Interactions Part II.

Surface Reactions", Ghem. Eng. Commun. 10, p. 81-102.

Refael, S. and Sher, E., 1985,

"A Theoretical Study of the Ignition of a Reactive Medium by Means of an

Electrical Discharge", Combustion and Flane 59, p. 17.

Sangiovanni, J.J. and Kesten, A.S., 1977,

"Effect of Droplet Interaction on Ignition in Monodispersed Droplet

Streams", Sixteenth Symposium (International) on Combustion, p. 577, The

Combustion Institute, Pittsburgh, PA.

Sarv, H., Nizami, A.A. and Cernansky, N.P., 1983,

"Droplet Size Effects on NOX Formation in a One Dimensional Monodisperse

.Spray Combustion System", Trans, of the ASME, J. of Engineering for Power

103, p. 791-796.

Slrignano, W.A., 1984,

"The Effect of Droplet Spacing on Spray and Group Combustion", Paper No.

84-WA/HT-26, ASME Winter Annual Meeting, New Orleans, LA.

14
Slripnano, W.A. and Somner, II.T. , 1984, .

-"Irnltion Studies of Liquid nnd Gaseous Hydrocarbon Fuels", Presented at

AFOSR/ONR Contractors Meetinp, on Airbrea thinr, Propulsion, 20-21 June,

Pittsburgh, PA.

Tal (Thau), R., Lee, D.N. and Sirijrnano, W.A., 1983,

"Hydrodynamics and l!ca t Transfer in Sphere Assemblages-Cylindrical Cell

Mo'dels"', Int. J. lieat Mass Transfer 26, No. 9, p." 1265-1273. '

Williams, A., 1973,

"Combustion of Droplets of Liquid Fuels -A Review", Combustion and Flame

21, p. 1. .

15
E 8S

OF POOR QUALITY

Mounting Plate For 3-D Traversing Mechanism \ "" •"" "" "" ^ " - "•-

Drain Plug ,

Liquid Syringe

Pump pk

AC Signal Generator

Dropiet Generator

Dilution Air

Disoersion Air

Perforated Plate

Reducing

Section

Quartz Combustion Tunnel—-

Electrode Traversing Assembly

Figure I. Schematic of the Spray Burner Facility

16
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1)

o:

o

~ ^ ^

r? m

o

CO

T

V)

u

(O

CO

I

OJ

s<->

ca

>^

in

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o

G

OK

H

(1)

o

o

•H

4-)

at

a

U)

cs

a>

>j

oc

17
V

o

/

t

q

3

e

Figure 3. Typical Traces of Voltages

Measured by Probes 1 and 2-

K-H

I i

^jTf9& .

JL'

> .^^^

Grot/nd

S'de. ^^^^

^~~

\ <T

•

i*t»^»

_ ^^ /

Figure 4. Design of Electrode-Tips.

18
13

a

LJ

u

2

O

e

e 50 100 150

SPARK DURATION Cue«c5

200

Figure 5. Effect of Spark Duration on the Energy at which 90%

Frequency of Ignition is Achieved for: K -Heptane

Sprays; Do = 68 vm; $ «= 1.0; Spark Gap - 2.0 mm.

U

u

O

LJ

z

O

M

tM

O

M

•88

78

68

58

1.5 2.8 2.5 3.8 3.5

SPARK GAP CniO

4.5

Figure 6. Effect of Spark Gap on the Frequency of Ignition for

N-Keptane Sprays: D 70 un:

Duration = 76 us; Spark Energy = 0.8 irJ.

1.0; Spark

19
o

UJ

o

UJ

oc

tee

08

86

78

68

8.5 1.8 1.5 2.0

SPARK ENERGY C«J)

2.5

Figure 7. Effect of Equivalence Ratio, $ , and Spark Enerpy on the

Frequency of Ipnition of N-Heptane Sprays; D 60

Spark gap = 3.0 m; Spark duration = 77 ysec. O - <J> = 1.0;

Q - 4 = 0.8; A - <£ = 0.6.

o

r>

E

>

19

a. 2

UJ *•

UJ

z

o

M

H

0

M 1

M

a

I

•5

5

{ ' 5 1

i

X

5

 '5 -5

5

 - 5 - 5 . '

38 40 58 68 78

INITIAL DROPLET DIAMETER CurO

88

Figure 8. Effect of Initial Droplet Diameter and Equivalence Ratio,

<J, on the >!inirun Ignition Enerpy of N-Heptane SpraysSpark pap = 3.0 rr: Spark duration =.77 visec.

O - < • 1-0; D - <• = 0.8; A - C = 0.6.

20
2

LJ

o

H

13

M

r

a

o

e

"0.5 0.6 0.7 0.8 0.9

EQUIVALENCE RATIO

I .8 1 . I

Figure 9. Effect of Fuel Flow Rate and Equivalence Ratio, $, on

the Minimum Ignition Energy of 60 urn N-Heptane Sprays

Spark gap » 3.0 mm; Spark duration «= 77 usec;

Fuel flow =O ~ 0.18 cc/min, Q - 0.24 cc/min.

cc.

UI

IU

z

o

z

o

M

e

D

O

o

D

o

8.5 8.6 0.7 0.8 0.9

EQUIVALENCE RATIO

I .8 I. I

Figure 10. Effect of Fuel Flow Rate and Equivalence Ratio, 0, on

the Minimum Ignition Energy of 70 un N-Heptane Sprays;

Spark gap - 3.0 mo: Spark duration « 77 usec;

Fuel flow -O- 0.18 cc/raln, D- 0.24 cc/rain.

21