ignition process of fuel spray injected into high pressure high temperature atmosphere

Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 695-702

I G N I T I O N P R O C E S S OF FUEL S P R A Y INJECTED I N T O H I G H PRESSURE H I G H T E M P E R A T U R E A T M O S P H E R E

JUN'ICHI SATO, KATSUYUKI KONISHI

Research Institute, Ishikawajima-HmJma Heavy Industries Co., Ltd. Toyosu, Koto-ku, Tokyo 135, Japan

HIROSHI OKADA

Department of Marine Engineering, Tokyo University of Mercantile Marine

Etsuchujima, Koto-ku, Tokyo 135, Japan

TAKASHI NIIOKA

National Aerospace LaboratoD', Kakuda Branch Ohgawara, Miyagi 989-12, Japan

The ignition process of fuel sprays injected into high pressure high temperature atmosphere has been studied experimentally and theoretically, and a new concept for the ignition process has been proposed. Experiments were conducted using a large high pressure combustion chamber and a high pressure fuel injection system. Ignition and fuel spray behavior were observed with high speed photography. From these experiments, it may be inferred that the ignition of fuel spray occurs at the stagnation region of the fuel spray tip. The color of the first flame observed was blue. The stagnation velocity gradient at the fuel spray tip, proportional to the fuel spray tip speed and inversely proportional to the fuel spray tip width, decreases rapidly with time from the start of fuel injection or with distance from the fuel nozzle tip.

Based on experimental results, a new concept for fuel spray ignition, derived from the knowledge of ignition in a stagnation flow field, has been proposed. By using the equations governing ignition phenomena in the stagnation flow field solved by the asymptotic method, the ignitable limit and ignition time in the stagnation region at the fuel spray tip have been analyzed.

These studies show that the ignition behavior of the fuel spray can be well explained by considering the effects of the stagnation velocity gradient at the fuel spray tip on the ignition time of the fuel-air system. The ignition delay of a fuel spray is divided into two parts: one is the time spent for reducing the velocity gradient at the spray tip below the critical velocity gradient for ignition; the other part is the time for an ignition reaction at the given velocity gradient. Since the latter is much smaller than the former, most of tbe ignition delay is time for reducing the velocity gradient at the fuel spray tip below the critical velocity gradient for ignition. Experimentally obtained ignition delay data can be predicted fairly well by using the above concept.

1. Introduction

Igni t ion o f a fuel spray injected into high t e m p e r a t u r e air is a practically impor tan t prob- lem not only for diesel eng ines but also for j e t engines. Many studies have been done in this area, both practical and fundamenta l . 1-12 It has been though t that the injected fuel evaporates and mixes with hot air and follows the chemical igni t ion reaction. T h e igni t ion delay has been d iv ided into the physical delay r equ i red for evapora t ion and mixing, and the chemical

delay r equ i r ed for the igni t ion reaction. These concepts for fuel spray igni t ion could be de- r ived f rom the knowledge o f ignit ion o f homogeneous p r emixed fuel-a i r mixtures . T h e r e f o r e until now, ignit ion delay data have been plot ted against 1/Tg (Tg: ambien t air t empera tu re ) , and the effects o f the fuel type, air t empera tu re , and air pressure, etc. on the igni t ion delay have been considered.

However , many fuel sprays used with various del ivery methods , such as p ressure atomizat ion, conta in mainly fuel d rople t s and fuel vapor. In

695

696 SPRAY COMBUSTION

some experiments , ignition has been observed to occur at the per iphery of the fuel spray. 6-8 For ignition in un-mixed fuel-air systems, some experimental and theoretical studies have been conducted, and the ignition process has been explained using the DamkOhler similality group. 13-2~ Thus, the explanation for ignition of a fuel spray based on a homogeneous fuel-air mixing system may not be feasible. The ignition process based on the un-mixed fuel-air system has to be taken into account in the stretched flow field due to the transient fuel spray behavior.

In this study, in o rder to explore the process of fuel spray ignition, the location of ignition in the lhel spray and the variation of stretch rate in these areas were studied experimentally. A theoretical analysis of the ignition region was conducted using an asymptotic method. Combining these studies, a new concept for fuel spray ignition has been proposed. Finally, the ignition delay values predicted theoretically and measured experimental ly have been compared.

2. Experimental

The appara tus used in this study is shown in Fig. 1.11 The combustion chamber is a cylinder with an inner d iameter of 500 mm and a length of 600 mm (volume: 0.118 m3). It has a removable electric heater for ignition and combustion tests. Its capacity is large enough to conduct ignition and combustion tests at a constant pressure and temperature , if the quantity of fuel is 6 • 105 mm ~ per injection. Because of the large size of the chamber, wall surfaces have little or no effect on fuel spray behavior. Fuel spray and combustion in the chamber can be observed through the nine windows (diameter: 50mm) on the chamber

0,ameter: 5OO He~g"t 6 0 0 ~

Fro. 1. Schematic of experimental apparatus.

walls. Experiments can be conducted at ambient gas pressures up to 20 MPa and temperatures up to 900 K. The gas in the combustion chamber was air.

A single component fuel is best for analyzing ignition and combustion. N-decane was the fuel used in these experiments. The fuel spray system consists of a high pressure pump, accumulator, fuel injection rate control unit, and solenoid valves. The accumulator injects fuel at a constant pressure, easily control led up to 200 MPa. With the fuel injection rate control unit, the fuel injection rate can be controlled precisely up to 12 • 103 mm 3 dur ing each injection. With this system, when fuel is first supplied, the fuel manifold pressure at the fuel injection valve inlet increases sharply. About 0.01 sec later, its pressure becomes constant. Details of the system are described in Ref. 11. Various diesel engine fuel injection valves can be used. A medium speed marine diesel engine fuel injection valve, IHI-S.E.M.T.-Pielstick PC2-6 was used after the nozzle tip was modi- fied to fit a single hole. In these experiments, the fuel injection rate was 3 x 103 mm B dur ing each injection. The fuel injection pressure P,, the maximum fuel pressure at the fuel injection valve inlet, was 50 MPa. The valve opening pressure Po, the pressure when fuel is first injected, was 20 MPa.

3. Results

3.1 Fuel spray ignition and combustion behavior

Direct pho tography using a 16 mm high speed cine-camera and high sensitivity color films was used to observe the ignition and combustion behavior of the n-decane spray in a high pressure, high tempera ture atmosphere. The framing rate was lO00f/s. Figure 2 shows the process from ignition to flame propagat ion. Blue flamelets were always observed initially near the axis of the fuel spray (Fig. 2 (a)). Sometimes, at a low ambient gas pressure, a few flying burning droplets were observed outside of the spray per iphery before the blue flamelets appeared. Our previous paper suggested that these burning droplets might ignite the main fuel-air mixture. 1" However, recent experiments have shown that they are not responsible for fuel spray ignition. The blue flames increased in size, and then a yellow flame appeared inside the blue flame (Fig. 2 (b)). The flame spread rapidly to the fuel spray down- stream area and slowly to the upstream area (Fig. 2 (c)).

The position of the first blue flame along the

IGNITION OF FUEL SPRAYS 697

0 8

~_" 50

100

150

200

(o) (b) (c)

IGNITION OF SPRAY FLAME SPREAD FLAME SPREAD t = 4,0 x 10 -3 s t = 5,0 x 10 -3 S t = 6,0 x 10 .3 S

Fro. 2. High speed photograph of the ignition and combustion: Pg = 2.1 MPa, Tg = 773 K,P, = 80 MPa, Po = 20 MPa.

fuel spray axis varied with the ambient gas pressure and temperature. In these experiments, the area observed by the high speed cine-camera through a window was between 34.5 mm to 179 mm from the fuel nozzle tip. Figure 3 shows the location of the first blue flame, li is the distance measured along the fuel spray axis from the nozzle tip to the point where the first visible blue flame appears. The values of li decreases as the ambient gas pressure Pg or temperature Tg increases.

3.2 Ignition delay

Normally, the time interval between the start of fuel injection and ignition of the fuel spray is referred to as ignition delay. Since ignition delay is a very important parameter governing

200

5 u_ 150

~' 100

50

Pg

0 1,6 MPo A 2,1 "

D 3.0 ,, <)' 5 .0 ,,

0 i i I 700 750 800 850 900

AMBIENT GAS TEMPERATURE, Tg, K

Fro. 3. Location of the first blue flame.

diesel engine performance, it has been measured by many researchers. 1,2 ,4-9 ,11,12 A photo- transistor or pressure transducer has mainly been used to detect ignition. In this experi- ment, ignition delay was defined as the interval between the fuel injection starting time and the time when the photo-transistor first detects a flame. The photo-transistor used could not detect a very weak or small blue flame. There- fore, the measured ignition delay time included the time required for the flame to become large enough to be detected by the photo-tran'sistor.

The relationship between the ignition delay td and the ambient gas temperature T e and pressure Pg is shown in Fig. 4. Ignition delay decreases as the ambient gas temperature increases. The ambient gas pressure level, also, has marked effect on the ignition delay, as the ambient gas pressure increases, the ignition delay again decreases.

3.3 Fuel spray behavior

The n-decane spray tip behavior was observed by shadow-photography using a 16 mm high speed cine camera and monochrome film. The framing rate was 4000 f/s. Figure 5 shows the spray tip behavior and the variations of spray tip speed V, with the distance l from the fuel nozzle tip, measured at room temperature. The lines showing the spray profile indicate changes every 1/4000 second. As l is increased, Vs decreases and the spray tip head widens. The decreasing and increasing rates with the distance I becomes large as the ambient gas density

100

l ,

Pg

O 1,1 MPo A 1,5 o 2.1 O 3,0 �9 5.0 �9 7 . 0

�9 9.9

0,1 i i I

700 750 800 850 900

AMBIENT GAS TEMPERATURE, Tg, K

FIG. 4. Variations of" the ignition delay with the ambient gas temperature and pressure.

(598 SPRAY COMBUSTION

150

>-

ioo

DO

Pg ~ . I MPo ' Pg@~ MPo '

0 2.1 /'IPo 5.0-

t I I i

50 i00 150 200

DISTANCE FROM FUEL NOZZLE TIP, l , mm

FIG. 5. Spray tip behavior and spray tip speed.

increases. At high temperatures, no good pho- tographs of fuel spray behavior could be obtained. From the previous studies of fuel sprays, it was found that the fuel spray tip configuration and its speed do not change with the ambient gas temperature , if the ambient gas densities remain the same. 7'21 Therefore , the spray tip behavior described above is the same as for high temperatures .

,2" 10

I ' - "

N 20

J 30

a_ 40

z :

cq 50

60

70

r<-> M_/

~ 0

( d

Blue flame e

3.4 Point oj ignition on the fuel spray

The ignition point on the fuel spray is very important in discussing ignition phenomena theoreticallv. Some efforts were made to locate this point on the fuel spray. It was concluded that ignition occurred at the per iphery, near the middle of the fuel spray. 6-~ Since shadow- photography with a high speed cine-camera of 4000f/s was used in the previous studies, only a yellow flame could be detected. We also tried shadow-photography with a high speed cine- camera of 2000 f/s and high sensitivity color film. The results were the same as those of previous researchers.

In the present experiments, therefore, high speed color movie film of 1000 f/s, which recorded blue flame, was compared with high speed monochrome movie film of 4000 f/s, which recorded spray tip behavior. The timing mark of the fuel injection was recorded on the films as a reference. The spray tip behavior was photographed at a density (room temperature) identical to the condit ion for the ignition test. Figure 6 shows a double image of the ignition behavior and spray tip behavior. The lines of the spray profile indicate changes every 1/4000 second. The blue flame first observed appears on consecutive frames between spray contours c and d. It can be inferred that ignition occurs in the stagnation region of the fuel spray tip.

FIG. 6. Double image of the ignition behavior and spray tip behavior.

4. Discuss ion

From the time of the first diesel engine operation, ignition of the fuel spray has been explained using the premixed ignition concept. Ignition delay has been divided into physical delay and chemical delay. The present experiments showed that ignition of the fuel spray occurs in the stagnation region of the fuel spray tip. These results indicated that previous concepts of fuel spray ignition are not correct. New concepts may be required from a practical and fundamental point of view.

Figure 7 shows a model proposed for fuel spray ignition. Fuel spray penetrates into the hot air at a velocity V~. Assuming the coordi- nates fixed on the fuel spray tip, the hot air flows against the spray at a velocity V,. The present flow model shows a steady axisymmet- ric stagnation flow model ahead of the fuel. The state of the fuel, liquid or gas, is impor tant for this analysis. In these experiments, the initial speed of the fuel spray tip is more than 100 m/s. Calculation of the fuel drople t evapo- ration time, assuming a droplet d iameter of 0.03 mm, shows that the evaporat ion time is very small compared to the measured ignition


SPRAY TIP WIDTH

d s

..-'i,'--: :....'. ~ .::,

ii!i iiiiiiiiiiiii iiiiiiill .: FUEL SPRAY :.:

�9 " . ' 7 . . : ' ; : ' - . ' : "

T

T,Y

HOT AIR STREAM

FIG, 7. Model for the ignition analysis.

delay. The recent study of fuel sprays, also, has found that the fuel spray tip consists of fuel vapor in a high t empera tu re high pressure ambient gas condition. 21 Therefore , the fuel spray tip is considered to be gaseous.

Ignition analysis of oppos ing gas fuel and gas oxidizer systems was done in our previous paper . 16 Ignition time and ignitable limit were studied. In that paper , the following assumptions were used for simplifications; incompres- sible and inviscid gases, one-step overall chemical reaction of fuel + n oxygen ~ products expressed by Arrhenius kinetics, and the unity Lewis number. Using these assumptions, the ignition time tig of the fuel spray was obtained using Eq. (27) of Ref. 16 with the same nomenclature of Ref. 16 as follows:

, 1,nil "1 e2(1--t3)2Ai (1)

Here, tig is the ignition time at a given velocity gradient for the configurat ion in Fig. 7. The ignition time tig is the time required for an ignition reaction in the stagnation region and is not the same as the ignition delay t~ of the fuel spray. The velocity gradient at the spray tip "a", which is inversely propor t ional to the first Damk61er number A1, is the most important parameter for ignition. In the present experiments, the velocity gradient a is expressed as Vide, where ds is the fuel spray tip width and V~ is the fuel spray tip speed (Fig. 7).

Calculating Eq. (1) numerically, ignition time

variations with the velocity gradient can be obtained. For the numerical calculation, the activation energy of the decane-air reaction was chosen as 35 Kcal/mol. 22 T h e pre-exponential factor of the Arrhenius kinetics was determined to be 9.12 x l0 is f rom the exper imental fit atpg = 5 MPa and Tg = 773 K. Variation oft,g with a are shown in Fig. 8, at an ambient gas pressure of 5 MPa and an ambient gas temperature of 773 K. Values of rig are very small and change little, except when a = ac. As a approaches ac, tig increases rapidly. Finally tig becomes infinite at a = a , Beyond ac, there is no ignition under this condition.

As shown in Fig. 3, the velocity gradient V,/d, at the front of the fuel spray tip varies with the distance 1 from the fuel nozzle tip, The velocity gradient VJds is very large when the spray is initiated, and rapidly becomes smaller as 1 increases. Compar ing Fig. 3 and Fig. 8, it is found that fuel spray ignition occurs when the velocity gradient at the fuel spray tip decreases with the increase of 1 and goes below the critical velocity gradient at. Therefore , the ignition delay of the fuel spray can be divided into two time periods. One is the time it takes for the velocity gradient at the spray tip to decrease below ac. The o ther is the time it takes for the ignition reaction to occur at the given velocity gradient, expressed by tig in Fig. 8. The comparison of the calculated ignition time rig, except near ac with a measured ignition delay ta of the fuel spray indicates that the measured ignition delays are about ten times larger than the calculated ignition ones. It shows that if the velocity gradient at the fuel spray tip is below ac, a very short time compared to the ignition delay of the fuel spray is requi red for the ignition reaction. In other words, most of the ignition

Pg = 5 MPo

Tg = 773 K

YO = 0,21

0,2 I I I I I I I [ I i

0.i 0.2 0.4 0.6 0.8 i 2 4 6 8

VELOCITY GRADIENT, a, xlO 3 i/s

FIG. 8. Variation of the ignition time with the stagnation velocity gradient.


delay is the time required for reducing the velocity gradient at the fuel spray to a point below a<. Therefore , by calculating the critical velocity gradient < for ignition, the ignition delay of a fuel spray can be estimated.

From the above discussion, if the value of ac and the variation of Vdd, of the spray tip is known, the ignition delay and ignition point of the fuel spray can be estimated. The value of the critical velocity gradient a~, at which tig becomes infinite, can be obtained from Eq. (1) as follows,

e'2AMgQ*pYoooYF_oo(1--13) ~ ( E * ) ac= 87/~(2 - r (O=,-- 1) exp -~-~- (2)

This equation shows the dependency of ac on the ambient gas pressure, temperature , oxygen concentration, and fuel propert ies for chemical reaction. Compar ison of the critical velocity gradient a~ predicted by Eq. (2) with the experimentally obtained velocity gradient at ignition is shown in Fig. 9 at various ambient gas pressures and temperatures . The experimentally obtained critical velocity gradient (V]&)~ is calculated with values of V~ and d~ at the location of the first blue flame (see Fig. 3 and 5). They are in fairly good agreement . Therefore, tlae ignition delay of the fuel spray may be estimated using Eq. (2) and the variation of V]& of the spray tip.

Figure 10 shows the comparison of the predicted ignition delay with the measured values in Fig. 4 using a Arrhenius plot. The predicted values are smaller than the measured values. This may be because the measured values include the time required for the first weak flame to grow enough to be detected with the photo-transistor. The predicted lines are not straight, but slightly concave upward. Some previous researchers have obtained the activation energy of a fuel-air reaction from the plot of td vs I/T~. It is noted that an essential difference exists between the two approaches. However, since a< in Eq. (2) has a strong dependency on exp (-E/RTg), the value obtained from the plot of td vs l/Tg might give a value for the activation energy not too different than that used here.

5. Conclus ion

Ignition of a fuel spray injected into a high pressure high t empera tu re a tmosphere has been studied exper imental ly and theoretically. Experiments were conducted using a large high pressure combustion chamber and high pres-

10 5 - Tg

L 0 723 K

A 773 K

n 823 K

LJ

203

I/o

lo 2 0 2 4 6

AMBIENT GAS PRESSURE, Pg, MPo

FIG. 9. Comparison of the predicted critical velocity gradient with the experimentally obtained velocity gradient at ignition.

sure fuel injection system. The ignition process and fuel spray behavior were observed with high speed photography. Based on the experimental results, a new concept for fuel spray ignition is proposed, and the resulting equations have been analyzed. Some conclusions obtained from these studies are that:

100.0 Pg

o 1,1 MPo . . . . . MEASURED A 1.6 , , - - PREDICTED 1.1 MPo ,-i 2.1 ,, / O 3.0 , , SA:I'.. 1.6 MPo

�9 5.0 - A ~ , ~ 2.I Mp a 1oo �9 7o , ,

�9 ,, MPo

I-- f l i.,= i / / ~ i " / ' 5.0 MPo

�9 "4~ " ~ / A ~ / / / 9 . 9 MPa , . o y / / , / , , / / /

0.1 1.1 1.2 1.3 1.4 1.5

AMBIENT GAS TEMPERATURE, i/Tg, xl0 -3 1/K

FIG. 10. Comparison of the predicted ignition delay of the fuel spray with the measured values.


1). Igni t ion o f the fuel spray seems to occur in the s tagnation reg ion of the moving fuel spray tip. T h e color o f the first flame observed is blue.

2). T h e s tagnat ion velocity grad ien t at the fuel spray tip decreases rapidly as the t ime f rom the start o f fuel inject ion or the distance f rom the fuel nozzle tip increases.

3). Fuel spray igni t ion behavior can be expla ined by cons ider ing the effects of the s tagnat ion velocity g rad ien t at the fuel spray tip on the ignition t ime of the fuel-air system.

4). T h e ignit ion delay o f the fuel spray is d iv ided into two parts: one is the t ime r equ i r ed to reduce the velocity g rad ien t at the spray tip below the critical velocity g rad ien t for ignition; the o ther is the t ime r e q u i r e d for an ignit ion react ion to occur at the g iven velocity gradient . Since the lat ter is m u c h smaller than the fo rmer , most o f the igni t ion delay is the t ime for r educ ing the velocity g rad ien t at the fuel spray tip below the critical velocity grad ien t for ignit ion.

5). T h e ignit ion delay o f the fuel spray can be es t imated using the critical velocity grad ien t expressed by Eq. (2) and the change in the velocity grad ien t at the fuel spray tip with time.

REFERENCE

1. WOLFER, H. H.: VDI-Forsch 392, 15 (1938-39). 2. S-rARKMAN, E.: Trans. A.I.Ch.E. 42, 107 (1946). 3. MULLINS, B. P.: Fuel 32, 221 (1953). 4. HENE1N, N. A. AND BOLV, J.: "Ignition Delay in

Diesel Engines," SAE Paper 670007, 1967. 5. HENEIN, N. A. AND BOLT, J. A.: "Correlation of

Air Charge Temperature and Ignition Delay for Several Fuel in a Diesel Engine," SAE Paper 690252, 1969.

6. IIKU1L~,, S. KADOTA, T. AND HIROYASU, H.: Trans. of JSME 41-345, 1559 (1975).

7. FUJIMOTO, H., TANABE, H., SA'ro, G. T. AND

KUNIYOSH[, H.: Thirteenth International Con- gress on Combustion Engines, D 25, The Inter- national Council of Combustion Engines, 1979.

8. KOBAYASHI, H., KAMIMOTO, T. AND MATSUOKA, S.: Trans. JSME 48-426, 389 (1982).

9. ARAI, M. HIGUCHI, G. AND HIROYASU, H.: Trans. of JSME 50-453, 1345 (1984).

10. AGGARWAL, S. K. ANn S1RIG.','ANO, W. A.: Twenti- eth Symposium (International) on Combustion, P. 1773, The Combustion Institute, 1984.

11. SATO, J., KoNism, K. AND OKADA, H.: Sixteenth International Congress on Combustion Engines, D 76, The International Council of Combustion Engines, 1985.

12. HIROVASU, H.: International Symposium on Di- agnostics and Modeling of Combustion in Recip- rocating Engines, P. 53, The Japan Society of Mechanical Engineers, 1985.

13. ANNAMALA1, K. AND DURBETAKI, P.: Comb. and Flame 27, 253 (1976).

14. KRISHNAMURTHY, L.: Acta Astronautica 3, 935 (1976).

15. NnOKA, T.: Comb. Sci. and Tech. 18,207 (1978). 16. NIIOKA, T.: Eighteenth Symposima~ (Interna-

tional) on Combustion, P. 1807, The Combustion Institute, 1981.

17. NIIOKA, T., TAKAHASHI, M. AND IZUMIKAWA, M.: Eighteenth Symposium (International) on Com- bustion, P. 741, The Combustion Institute, 1981.

18. SAYrOH, T., ISHIGURO, S. AND NnOKA, T.: Comb. and Flame 48, 27 (1982).

19. NIIOKA, T, MITANI, T. AND SATO, J: Twentieth Symposium (International) on Combustion, P. 1877, The Combustion Institute, 1984.

20. CHAO, B. H., MATALON, M. AND LAW, C. K.: Comb. and Flame 59, 43 (1985).

21. NISHIDA, K., MURAKAMI, N. AND HIROYASU, H.: International Symposium on Diagnostics and Modeling of Combustion in Reciprocating En- gines, p. 141, The Japan Society of Mechanical Engineers, 1985.

22. CULLIS, C. F. AND FOSTER, C. D.,: Fourteenth Symposium (International) on Combustion, p. 423, The Combustion Institute, 1973.

COMMENTS

A. Cavaliere, University of Naples, Italy. Do you think that your results can be applied to any fuel rate?

Author's Reply. Yes. The ignition process should not be affected by the fuel rate per one injection. But the values of ignition delay may vary with the fuel rate if the variation of stretch rate, V/d, with time is the function of fuel rate.

O. L. Gulder, National Research Council of Canada. I have two questions related to ignition delay of the fuel sprays:

1. In one of your slides, you showed a figure of ignition delay vs. inverse of temperature with data points falling on straight lines, meaning that ignition


delay can be expressed by an Arrhenius type expres- sion with an overall constant activation energy. How- ever, previous and most recent studies [1-3] show that activation energy of spray ignition is dependent on temperature and pressure as well as fuel type.

2. You conclude that the contribution of the time spent for reducing the velocity gradient at the spray tip below critical velocity gradient for the ignition to the total ignition delay is much larger than the contribution of the time spent tor ignition reactions. Could you explain, then, the fact that the two hydrocarbons (e.g., n-octane and iso-octane), with almost identical physical properties, yield signifi- cantly different ignition delays when injected into a CFR cetane testing engine under exactly the same operating conditions?

REFERENCES

1. HuRx, R.W., AND SMITH, H.M.: Ind. Eng. Chem. 43, 2788 (1951).

2. BAr:v, V.K., ET AL.: Dynamics (?/Flames and Reactive Systems, (J.R. Bower, et al., Eds.), Vol. 95 p. 554, AIAA Progress in Astronautics and Aeronautics, 1984.

3. GULDER, O.L., AND GLAVINCESKI, B.: Combust. Flame 63, 231 (1986).

Author's Reply. 1. Dependences of ignition delay on inverse of

temperature may not be expressed by straight lines. In fact, the lines obtained theoretically are slightly concave upward (Fig. 10). In our theoretical analysis, the values of physical parameter were assumed constant, but, actually, these values are functions of pressure and temperature. The variations of stretch

rate, VJd, with time are also functions of pressure and temperature. These may be responsible for the dependency of the relations between ignition delay and 1/Tg on pressure and temperature.

2. The values of critical velocity gradient, which is expressed as Eq. (2), depend strongly on the fuel properties for chemical reaction, such as activation energy. This is the reason for the difference of ignition delay between two fuels with almost identical physical properties and with different reaction properties.

S. R. Gallahalli, University of Oklahoma, USA. Have you been able to observe whether the droplets were present in the liquid form when ignition occurred? If the critical point conditions are attained, the factors governing the delay period may be different from those you have considered, and hence it is vital to know that.

Author's Reply. Our measuring system of fuel spray behavior, shadow photography, could not make clear whether the fuel droplets exist or not in the sprays. Recent study on the diesel sprays by Nishida et al. 2t has shown that the fuel spray tip consists of fuel vapor in a high temperature high pressure condition.

The critical pressure and temperature of n-decane are 2.1 MPa and 619 K. Figure 2 is just at the critical pressure condition, and Fig. 6 is the supercritical condition (Pg = 5.0 MPa, Tg = 773 K). Ignition phenomena observed under and over the critical pressure conditions are the same each other. There- fore, we think that the ignition process and the factors governing the delay period are the same at both side of critical pressure.

ignition process of fuel spray injected into high pressure high temperature atmosphere

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