- Email: [email protected]
The droplet group microexplosions in water-in-oil emulsion sprays and their effects on diesel engine combustion
Twenty-Fifth Symposium (International) on CombustionYrhe Combustion Institute, 1994/pp. 175-181
T H E D R O P L E T G R O U P MICROEXPLOSIONS IN WATER-IN-OIL E M U L S I O N SPRAYS A N D T H E I R EFFECTS O N D I E S E L E N G I N E C O M B U S T I O N H. Z. SHENG, L. CHEN,* Z. P. ZHANG* AND C. K. WU
Institute of Mechanics Chinese Academy of Sciences Beijing, China 100080 AND
C. AN AND C. Q. CHENG
Department of Vehicle Engineering Beijing Institute of Technology Beijing, China 100081 To clarify the combustion mechanism of water-in-diesel fuel emulsion sprays and to evaluate the possible benefit of emulsions in practical usage, combustion bomb experiments, dynamic engine tests, and computer simulation were carried out, and some useful conclusions have been reached. The droplet group (lump-fashioned) microexplosions in water-in-diesel fuel emulsion sprays on an eddysize scale during the atomization, evaporation, and combustion processes in a high-pressure, high-temperature bomb were observed with a multipulsed, off-axis, image-plane, ruby laser holocamera and a highspeed camera. The explosions eject droplet fragments from the spray region to several millimeters away, improving the fuel-air mixing process and speeding up the flame propagation. A no-water layer formed by a Hill vortex was also observed in emulsion droplets. The ambient temperature has the most important influence on the occurrence and violence of the microexplosion. Road-load simulation engine tests were carried out on a dynamic engine test bed. The experimental results show that emulsion fuels have no significant influence on fuel consumption and reduce engine torque if no adjustment is made for the injection system, but that smoke emission is much improved when emulsion fuel is used. The combustion characteristics and the rate of heat release are also analyzed to reveal the difference between emulsion and diesel fuel. The relationships between the optimum water percentages and fuel consumption under various operating conditions were analyzed by numerical combustion modeling.
Introduction Eighty years ago, water was first introduced into engine cylinders to increase engine output [1]. Since then, nmch attention has been paid to the preparation and use of water-in-oiI (w/o) emulsified fuels with/without stabilizer, to improve the engine efficiency and exhaust emissions, or as safety fuels [2-7]. Microexplosion was first observed in 1965 [8]. Many questions cannot be well answered by the existing knowledge, since previous fundamental research on emulsions mainly emphasized droplet microexplosions at room pressure and the results from engine tests were often contradictory. Do microexplosions take place in diesel engine cylinders? Which is the key factor controlling the emulsified fnel combustion, microexplosion or water-gas *Formerly Graduate Student at the Institute of Mechanics, Chinese Academy of Sciences.
reaction? What are the detailed features of the explosion in spray combustion? Is the e~cplosion strong enough to improve the combustion process? How can the mieroexplosion of emulsified fuels be effectively used in engines? Why have positive and negative energy saving been observed in engine tests? The interaction of droplets in sprays, the droplet size, the ambient pressure, and the heating history have significant effects on the combustion process, so the microexplosion in dense emulsified sprays under diesel engine combustion conditions needs further study for clarifying these issues. In order to evaluate the potential energy saving by the use of emulsified fuels on a large scale, research was started several years ago to understand the mechanism of the microexplosion in sprays and its application in engines. In order to link the broken chain between droplet combustion and engine application for emulsions, experiments in a combustion bomb, dynamical engine tests, combustion 175
176
INTERNAL COMBUSTION ENGINES
modeling, and theoretical work were carried out [9,10,11]. The "lump-fashioned" droplet group microexplosion (in earlier publications, droplet group microexplosion was called "lump-fashion microexplosion") and a nowater layer in emulsion droplets were observed in the bomb by a multipulsed ruby laser holocamera [9]. The explosions were strong enough to expand the spray region and speed up flame propagation, which was verified by high-speed photograplay [10]. The road-load-simulation engine tests show no significant effects on energy saving by using emulsion, but the smoke can be reduced by 30%. The modeling reveals the combustion features and the optimum water percentage in emulsion for different operating conditions. Tim questions mentioned above are mainly answered by this series of work.
TABLE 1 Testing conditions in combustion bomb Environment in bomb Pressure (MPa): 0.1, 2.4, 3.2, 4, 4.5, 5 Temperature: 293, 723, 733, 753, 773, 823, 873, 923 K W/O emulsions Water fraction: 0, 10%, 12% Internal phase: 2 5 ,urn Viscosity:5 centistoke at 293 K Holography (Nitrogen in bomb) Laser energy: 50 mJ/pulse Pulse width: <40 ns Pulse numbers: 1, 2, 3, 4 Pulse interval: 50/ts, 100 ffs High-speed photography (Fresh air in bomb) Focal length: 100 mm Aperture: f/2.3 Film: black/white, ISO 400 Camera speed: 3000 PPS
Mieroexplosion in Sprays under Diesel Engine Conditions The early fimdamental research was focused on the combustion of either isolated droplets or arrays of single droplets (h = 0.3-1.0 ram) suspended on quartz filaments or fine thermocouples under atmospheric pressure. Since the suspending filaments may be particularly detrimental to the study" of combustion characteristics of evaporating emulsions, a droplet generator was employed to generate small droplets floating in an upward heated gas stream [12]. The internal phase size (the size of the dispersed phase in emulsion) may affect mieroexplosion [13]; the higher cylinder pressure may suppress the occurrence of microexplosion [14]. Emulsion sprays were maeroscopieafly observed by high-speed sehlieren photography in a combustion bomb (Zhejiang University, China). The theoretical work was also carried out based on the experimental results [15-17], even for a multicompound liquid [J.8]. Previous work on fuel spray research [19] emphasized the spray shape, penetration, and entrainment using hot-wire anemometer, particle tracing, highspeed photography, single-flash photography, instantaneous microphotography, schlieren photography, shadowgraph, Fraunhofer diffraction particle analyzers, and single-pulsed Gabor laser holography. However, these methods cannot measure the droplet velocity and trajectory and the microexplosion of the micron-sized moving particles in sprays, since the measurement needs very high resolution in a large field of view. To observe the very weak diffraction from small moving droplets in a spray inside the high-pressure bomb, a multipulsed, off-axis, ruby laser holocamera was employed in this work. The holocamera can produce four single-mode laser pulses of 50 mJ at 30-ns pulse width. The pulse interval can be adjusted from
5 to 100 fs in 1-fs steps. The pressure drop in the injector was used as the synchronizing signal to trigger the laser beam after a 0.8-6.5-ms delay (controlled by a timer). The laser beam at first was split into an object wave and a reference wave by a splitter, and then the reference wave was expanded and collimated to 90 mm in diameter. The object wave was 50 mm. Two quartz windows of 48 mm in diaineter with high optical quality were used as the optical path of the bomb. Passing through the bomb, the beam was premagnified up to 3.5 times with an h = 22 mm high-quality convex lens located between the bomb and the image plane, and recorded on a holographic plate located at the image plane. To avoid speckle noise and obtain homogeneous illumination in the field of view, a convergent white light beam was employed to reconstruct the 2D images by a 35-mm single lens reflex (SLR) camera. To verify the effects of the microexplosion, a series of highspeed photographic tests were made in the bomb under diesel engine combustion conditions with a 16ram HYSPEED camera. The testing conditions simulate the cylinder pressure and temperature (Table 1, refer to Refs. 9 and 10). It is worth mentioning that a thermoisolating watercooling jacket covers the exposed part of the injector to keep the nozzle temperature under 353 K to prevent the evaporation of the water in emulsion before injection.
Droplet Group Microexplosions in Emulsion Sprays: The droplet group microexplosions take place in eddy scale near the outer layer of the emulsion sprays (Fig. 1). The pictures taken at different moments show various strengths of the microexplosions.
DROPLET GROUP MICROEXPLOSIONS IN EMULSION SPRAYS
177
T (K)
600
Limit of Superheat at 4 MPa
9
550
.
.I//
ex
~ t u r a t i o n Temp. at 4 MPa.
500 FIG. 1. Droplet group microexplosionof in w/o emulsion sprays; arrows point out the explosion centers (12% water, 773 K, 3.2 MPa, 3 • 100 fs, 2.3-ms delay)9
e~
No-Water L a y e r / 450
Lump-shaped flames were also observed by highspeed photography. The droplet group microexplosion is strong enough to eject fragments of torn droplets to a distance several millimeters away from the spray boundary at much higher speed, greatly expanding the spray head and spray angle, improving air-fuel mixing, and causing the flame edge to become unclear and irregular [10]. Perhaps the droplet group microexplosion occurs mainly in the turbulent eddy in which the droplets more or less have homogeneous size, similar heating history, and superheated state. If a droplet is exploding, the pressure wave may induce all the droplets in the same eddy to explode, an avalanche of mieroexplosions of droplet group. Different eddies have different sizes and different superheated states, so the explosions are also very different from one to another. The explosion region is near the spray head and outer layer, where the gas temperature is high enough and the droplets go through sufficient heating time to be superheated. Because of the explosion in the droplet group, the water fraction in the range of 8-12% has no significant influence on the violence of the explosion. No explosion was observed in the higher density, lower-temperature spray core. If the ambient temperature is not high enough (733 K in the test), the water dots (internal phase of w/o emulsion) in the droplets will not be superheated and will evaporate and vanish before being heated into superheated state, and no microexplosion takes place (Fig. 2). If the ambient temperature is proper (773 K), all the water dots in droplets rapidly go over the saturation temperature (523.5 K at 4.0 MPa) and into a superheated state, a pseudo-stable state, and vaporize and explode at the same time when the outer water dots achieve the limit of superheat (583 K at 4.0 MPa). This tears up the droplet and greatly expands the spray region. If the temperature is too high (823 K), the heat transfer between gas and droplets is strong enough, some water dots near the outer layer are over the limit of superheat, but central water dots are not in superheated state in this case, the
0
/
Droplet Radius
FIG. 2. The influence of different ambient temperature on droplet heating process and temperature gradient. explosion occurs earlier and closer to the injector, and the violence of explosion is not very strong since all water dots do not explode at the same moment. The torn droplets fly much faster in a lower gas density, causing a larger flame angle and stronger microexplosions. If the gas density is constant, gas pressure may have little effect on explosion violence. The injection pressures between 18 and 24 MPa have little effect on atomization and microexplosion of emulsions. Droplet group microexplosion phenomena can be supported by high-speed photography[10], by which the bright diesel flame is more homogeneous, more yellow in color, and lasts a longer period than the emulsions flame; the ignition delay of emulsions is longer than diesel, but the flame propagation is much faster after ignition, and ignition occurs at multiple points simultaneously. No-Water Layer in Emulsion Droplets:
When the gas in the bomb is heated up to 700 K, a comet-shaped vapor cloud surrounds the droplet, and water dots (1 fm) concentrate in the central part of the droplets. A no-water layer can also be recognized near the surface of the droplets from 10 times remagnified reconstructed holographs. The image is conceptually shown in Fig. 3. The velocity difference between gas and droplets causes a uniaxially symmetrical shear stress, which reduces droplet speed, induces a Hill vortex (internal circulation) in the droplets, rotates the droplet, and then forms a rapidmixing layer. In this layer, the water dots will be moved onto the droplet surface and vanish before the water dots are heated up to the saturation temperature, favoring the no-water layer formation because of the low evaporation and dif-
178
INTERNAL COMBUSTION ENGINES moving direction /k
water dot 'If
droplet boundary I inner core
,-water :as phase vapor cloud Fro. 3. The computer-treated image of evaporating w/o droplet and no-water layer model. fusion rates of oil vapor. The circulation is not strong enough in the inner core to homogenize the temperature field and the water dot distributions in the whole droplet since new no-water layer still exists if the droplet is still moving. The water dots in the eore are easily heated up to the superheat state if the ambient temperature is proper. When the water dots near the outer layer achieve the limit of superheat and evaporate, all the superheated water dots also evaporate and expand rapidly at the same moment, tearing up the fuel droplet--the mieroexplosion. The inhomogeneous situation is more easily kept and the microexplosion takes place more easily in big droplets than in small ones. The no-water layer eoncept is very important in explaining the microexplosion of emulsion droplets. A no-water layer model [11] based on the Hill vortex analysis by Prakash and Sirignano [20] agrees very well with Aggarwal's work for hexane, deeane, and hexadecane under the same operating conditions, and experimental data for emulsions. The system is treated as a sphere containing the inner core of the droplet as a homogeneous phase, with the no-water layer and gas phase around the droplet (Fig. 3). The model shows that the initial droplet diameter is another important factor in the microexplosion. If the initial droplet is too small, the residual water mass is too low or will even vanish before the droplet is heated up to the saturation point, and the explosion can hardly be observed in this condition. This result implies that the mieroexplosion effect may be helpful in the case of an old injection pump with lower injection pressure, where the atomization has deteriorated. It may be eonjeetured that, the more the water in the emulsion, the larger the explosive energy will be. However, if the initial diameter of the emulsion droplet is held constant, the emulsion with larger water fraction will lose more water as a result of the existence of the vortex and no-water layer, thus lowering the surface temperature. The additional water
gives the spray more initial momentum to improve the air-fuel mixing process. These factors may have opposite influences on the effects of the microexplosion on eombustion. In our experiments, the water fraction has a weak influence on the occurrence of the mieroexplosion. Engine Test under Simulated Road-Load Conditions The effect of microexplosions on engine performance is fnlly dependent on the application. Many well-performed steady-state engine tests show that the energy saving is less than 5% for most diesel engines. The steady-state tests may be very different from road running: 20-30% benefits during road rnnning were reported by many regional sourees in China. To evaluate the potential benefits for emulsion applieations, a large number of expensive road tests for different road conditions and different engines should be done because of the complicated influences. However, the results can hardly be repeatable and reliable. Therefore, a series of steady-state and road-load-simulation engine tests have been recently performed for this purpose on an AST-386 microcomputer-controlled dynamic engine test bed EDST-2 with a Siemens 200-kW DC electric dynamometer, which can run various road routines and measure engine parameters automatically. The engine test bed very aeeurately controls engine torque (<1%) and speed (<10 rpm). To analyze the rate of heat release (ROHR), the eyfinder pressures were recorded by AVL QC42D piezoelectrie transducer, Kistler 5004 charge-voltage convertor, PEI eneoder, and IBM microcomputer with PEI ECA911 software package. An improved compound band reject filtering (CBRF) filtering method [21] was employed to eliminate the effeets of measurement passage on pressure curves for ROHR analysis. For evaluating the effects of emulsions on exhaust emission, both liquid and solid particles were measured in dynamic engine tests by a Hartridge MK3 Smokemeter, which has very fast response. For comparison, a Bosch Smokemeter was employed in the steady-state engine tests. The mieroexplosion may have more effect on combustion in direct injection engines with a bowl combustion chamber, in which the mixing process occurs mainly in the bowl space, than on other types of diesel engine. The engine employed in this test is a V6 engine with cylinder bore/stroke of 150/150 ram, compression ratio 16.8, rated engine power 215 kW at 2600 rpm, and shallow bowl combustion chamber. The load characteristics of 1100-2100 rpm and the speed characteristics of 40 and 80% load were designed for steady-state engine tests. The ECE-15, SAE J35 SEP88, free acceleration for smoke evaluation, 6-1node cycle (Chinese Stan-
DROPLET GROUP MICROEXPLOSIONS IN EMULSION SPRAYS Ge (g/kW*h) 290 -
30001 n(r/min) Z000J ~
/-~
~
,
M(Nm)
L~'\J
looo ld
/~
HSU(~) -. ,
o
q ..
0
6~
,'~ " :ii,,
i00
t-..~
700
..... "'~
650
.-'-d--"
600
,
280 ~... -"-.-..~
ge
250 .
.
.
.
.
200
.
.
.
.
.
.
.
.
300
.
.
s (Time)
1O- P (MPa)/ROHR (100 J/'CA) - -
550
.
FiG. 4. Engine torque, speed, and smoke measurements under No. 1 Bus routine (Me-engine torque, HSU-Hartridge Smoke Unit).
p
-800
"-%..
,
mr,//
15~
+ ,~.., ......
Me (Nm)
Water Water Water
750
w/o
<-;,1
.....
C~ 6~ 10~
280 /
50 fl
-,~
flooo
2g0 100
.....
179
240 , 1000
~ . . . . . . . , .. 1400 1800 n (r/min)
......
. . . . .
500 2200
FIG. 6. Specific fuel consumption (water excluded) under engine speed characteristics test (Ge-break specific fuel consumption, Me-engine torque). Ge (g/kW*h)
Diesel
9
. ", ",,
500-
" \
,,~
O~ ._-_- ._. B~ . ._. ~. _ _ _ 1 0 ~
Water Water Water
*__*_*_.*__* 1 5 ~
Water
',\ \ 400
'~ \
',, ";5 oo.
o 16 ........
6 .........
,'o ........
'2'o.........
do "CA
FIG. 5. The cylinder pressure and the rate of heat release (1500 rpm, 750 Nm). dard), and Beijing No. 1 bus routines (Fig. 4) were all designed for engine dynamic tests. The No. 1 bus is the busiest bus running along the Chan'an Street across central Beijing, and the total mass of the bus is 19.2 tons with a typical 160-kW direct injection engine. The diesel fuel and emulsions with 6, 10, and 15% water in weight were chosen for all engine tests, since the optimum water percentage is about 10% according to our e~cperiences. The results are summarized as follows. The engine smoke is at least 30% HSU (Hartridge Smoke Unit) less than diesel fuel, and the water fraction (6-15%) has certain effects on HSU (Fig. 4 shows typical measurements). The water postpones the ignition about 0.2 ins, and the microexplosions change the combustion rate and ROHR (Fig. 5). Energy saving can be obtained under partial load conditions by emulsions, but negative effects may appear under high load conditions because of longer injection per-
200
0
. . . . . . . . . . . . . . . 50 I00
Ne(kW) 150
FIG. 7. Specific fuel consumption (water excluded) under engine load characteristics test (Ge-break specific fuel consumption, Me-engine torque). iods. The engine performances are a little bit better in the range of 1200-1800 rpm under 80% load (Fig. 6). Figure 7 and Table 2 show that about 3-5% more energy was consumed by emulsion applications in both steady-state and dynamic engine tests without injection-timing adjustment. According to our previous work (unpublished), the injection timing should be adjusted 2 x CA (crank angle) in advance to obtain 3% average energy saving for emulsions. The exhaust temperature is reduced by the addition of water, especially for high load conditions, and engine torque is also lower since the maximum fueling volume rate is the same as the diesel. It is suggested that injection timing should be adjusted to optimize the engine performance and emissions.
INTERNAL COMBUSTION ENGINES
180
TABLE 2 Fuel consumption in road-load test Emulsio~test (g)
Water content (%)
Energy sa~ng (%)
6-Mode
970 1065 1133 1178
O 6 10 15
0 -3.2 -5.1 -3.2
ECE-15
325 360 370 385
0 6 10 15
0 - 4.1 -2.5 -0.7
1380 1540 1570 1625
0 6 10 15
0 -4.9 - 2.4 -0.1
No. 1 Bus
The dynamic engine test shows that both fuel consumption and exhaust emissions have very good correlation with steady-state tests, which can qualitatively evaluate the emulsion applications if there is no dynamic test bed. For energy saving by emulsion applications, the positive effects are fully dependent upon the engine and operating conditions; 3-5% may be the maximum benefit according to our previous engine tests, this series of engine experiments, and other researchers' reports for well-performed engine tests. Sometimes the influence may be negative if no proper adjustment is made.
Computational Simulation of Engine Using Emulsions A combustion model, which can predict the engine performance under low load conditions [22], was modified for emulsions. This model divides the injected fuel into 30 small packages. Mr entrainment, droplet sizes, the evaporation process, and ignition delay in individual packages are calculated according to the transient injection pressure of the packages and then integrated by Duhamel Integral. The physical and chemical properties of the emulsions, ignition delay calculation, and the rates of air entrainment and evaporation were based on the water fraction to simulate the effect of the microexplosion. The simulated operating conditions are as follows: engine speed, 1500-2100 rpm BMEP, 0.13-0.6 MPa; water fraction, 0-20%. The combustion characteristics of emulsions are reasonable. The higher initial momentum of emulsion sprays and the micro explosions raise the local air-fuel ratio. The combustion rate and engine efficiency, which
s2 7
~i(~) o,
Pi~O. I S
48~j 46
44-
42-
....
d ....
1'0 ....
l's ....
z'0 '~/aier
(~)
FIG. 8. The relation between water fractions and thermal efficiency (1500 rpm) (/-thermal efficiency,Pi-break mean effective pressure). are more significant under low load conditions where the initial momentum of diesel sprays is not sufficient, also increased. The fuel injection system is designed to match high load conditions, and under such conditions, the emulsion injection stops later. The combustion period is hardly shortened by the microexplosion because of the very large spray region already existing. For an engine at 1500 rpm, the optimum water fraction is 12-16% for idling and 6% for high load conditions (Fig. 8). The highest flame temperature appears later and is lowered by 200-300 K as a result of more abundant air and evaporation of the water in the flame region, which greatly improves the soot and Nox formation but slightly reduces the thermal efficiency. The ignition timing is 2 • CA later than diesel fuel, but combustion fin-
DROPLET GROUP MICROEXPLOSIONS IN EMULSION SPRAYS ished earlier. Postponing the injection has less effect on engine efficiency than for diesel fuel, since the microexplosion improves the combustion process.
181
complex factors, and improvements are not always present under all conditions. Acknowledgements
Conclusions Experiments using holography and high-speed photography in a high-pressure and high-temperature bomb, dynamic engine tests, and computer modeling were carried out to link the broken chain from the droplet combustion to engine application for enmlsions. Microexplosion is a phenomenon in which the microwater dots in the emulsion droplet are heated into an unstable superheated state and vaporize rapidly, tearing the droplet violently. A no-water layer (rapidmixing layer) exists near the droplet surface because of the Hill vortex, and many mierowater dots exist in the droplet inner core. A mieroexplosion occurs in a certain temperature range and becomes stronger at a proper ambient temperature. Raising chamber pressure has little effect on the occurrence of the explosion, but the penetration of the torn droplet will be much shorter as a result of higher gas density, which weakens the effect of the explosion. Droplet group microexplosions in emulsion sprays occur on a turbulence eddy scale near the outer layer and the head of the spray. If the ambient temperaturn is proper, the explosion possesses enough energy to eject torn droplets to a distance several millimeters away from the spray boundary, improving atomization, evaporation, and air-fuel mixing processes and speeding up flame propagation. Since physical processes of liquid fuel combustion take a much longer time than chemical reaction, the mieroexplosion can speed up combustion rate and ROHR, raising engine thermal efficiency. Meanwhile, the injection timing must be adjusted, and the proper water fraction for different loads and the internal phase size of the emulsions must be carefully chosen. Emulsions may cause better efficiency under partial load conditions, especially for direet injection and old engines, but negative effects may occur for new engines under high load conditions. Much less soot is formed in emulsion spray combustion, and the engine smoke is at least 30% less than diesel fuel. The water fraction (6-15%) has no significant effects on engine smoke. Nox formation can be reduced because of the lower temperature by the addition of water. In general, the road-load-simulation engine tests agree with the steadystate tests and the fundamental research for emulsions. The experimental results show that the microexplosion in emulsion sprays may occur under diesel engine combustion conditions. Engine efficiency can be slightly improved by emulsions under certain conditions through the microexplosions improving the mixing and combustion processes. The system efficiency is influenced by many
This work was supported by the State Planning Commission of China. The authors are indebted to Prof. S. B. Yan, Institute of Aeoustics, Chinese Academy of Sciences, for supplying the emulsion.
REFERENCES 1. Hopkinson, B., Proc. Institution of Mechanical Engineers, 679-715 (1913). 2. Dryer F. L., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 279-295. 3. Viehinievsky,R., Murat, M., Parois, A., and Dujen, M., CIMAC Congress, I. Mech. E. Press, London, 1975,
4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15, 16. 17, 18. 19, 20,
21,
22.
pp. 615~352. Callanhan, T. J., Ryan, I. T. W., O'Neal, G. B., and Waytulouis, R. W., SAE 830553, 1983. Johnson, R. T., and Stoffer, J. O., SAE 830557, 1983. Dryer, F. L., Rambaeh, G. D., and Glassman, I., paper presented at the 1976 Spring Meeting of the Central States Seetion of the Combustion Institute, 1976. Jacques, M. T., Combust. Flame 29:7%85 (1977). Ivanov, V. M., and Nefedov, P. I., NASA Technical Transal. TIF-258, 1965. Sheng, H. Z., and Chen, L., The 4th ICLASS, The Fuel Society of Japan, Sendal, Japan, 1988, pp. 391 396. Sheng, H. Z., Chen, L., and Zhang, Z. P., The Seminar on Diesel Fuel Injection Systems, Institution of Mechanical Engineers, London, 1989, pp. 49-56. Sheng H.-Z., Zhang, Z.-P., and Wu, C.-K., International Symposium COMODIA 90, Kyoto, Japan, 1990, pp. 275-280. Lasheras, J. C., Fernandez-PeIlo, A. C., and Dryer, F. L., Combust. Sci. Technol. 21:1-14 (1979). Marrone, N. J., Kennedy, I. M., and Dryer, F. L., Co~r~ust. Sci. Technol. 33:299-307 (1983). Wang, C. H., and Law, C. K., Combust. Flame 59:5362(1985). Law, C. K., Lee, C. H., and Srinvasan, N., Combust. Flame 37:125-143 (1980). Birehley, J. C., and Riley, N., Combust. Flame 29:145165 (1977). Katsoulakos, P. S., C84/83, Institution of Mechanical Engineers, London, 1983, pp. 51-62. Law, C. K., Combust. Flame 26:219 233 (1976). Chigier, N., Combust. Flame 51:127-139 (1983). Prakash, S., and Sirignano, W. A., Int. J. Heat Mass Transfer 253-288 (1980). Shi, S.-X., and Sheng, H.-Z, International Conference on Computers in Engine Technology, Institution of Mechanical Engineers, London, 1989, pp. 325 332. Sheng, H., Trans. Chinese Soc. Intern. Combust. Engines 3:243 256 (1985) (in Chinese).
T H E D R O P L E T G R O U P MICROEXPLOSIONS IN WATER-IN-OIL E M U L S I O N SPRAYS A N D T H E I R EFFECTS O N D I E S E L E N G I N E C O M B U S T I O N H. Z. SHENG, L. CHEN,* Z. P. ZHANG* AND C. K. WU
Institute of Mechanics Chinese Academy of Sciences Beijing, China 100080 AND
C. AN AND C. Q. CHENG
Department of Vehicle Engineering Beijing Institute of Technology Beijing, China 100081 To clarify the combustion mechanism of water-in-diesel fuel emulsion sprays and to evaluate the possible benefit of emulsions in practical usage, combustion bomb experiments, dynamic engine tests, and computer simulation were carried out, and some useful conclusions have been reached. The droplet group (lump-fashioned) microexplosions in water-in-diesel fuel emulsion sprays on an eddysize scale during the atomization, evaporation, and combustion processes in a high-pressure, high-temperature bomb were observed with a multipulsed, off-axis, image-plane, ruby laser holocamera and a highspeed camera. The explosions eject droplet fragments from the spray region to several millimeters away, improving the fuel-air mixing process and speeding up the flame propagation. A no-water layer formed by a Hill vortex was also observed in emulsion droplets. The ambient temperature has the most important influence on the occurrence and violence of the microexplosion. Road-load simulation engine tests were carried out on a dynamic engine test bed. The experimental results show that emulsion fuels have no significant influence on fuel consumption and reduce engine torque if no adjustment is made for the injection system, but that smoke emission is much improved when emulsion fuel is used. The combustion characteristics and the rate of heat release are also analyzed to reveal the difference between emulsion and diesel fuel. The relationships between the optimum water percentages and fuel consumption under various operating conditions were analyzed by numerical combustion modeling.
Introduction Eighty years ago, water was first introduced into engine cylinders to increase engine output [1]. Since then, nmch attention has been paid to the preparation and use of water-in-oiI (w/o) emulsified fuels with/without stabilizer, to improve the engine efficiency and exhaust emissions, or as safety fuels [2-7]. Microexplosion was first observed in 1965 [8]. Many questions cannot be well answered by the existing knowledge, since previous fundamental research on emulsions mainly emphasized droplet microexplosions at room pressure and the results from engine tests were often contradictory. Do microexplosions take place in diesel engine cylinders? Which is the key factor controlling the emulsified fnel combustion, microexplosion or water-gas *Formerly Graduate Student at the Institute of Mechanics, Chinese Academy of Sciences.
reaction? What are the detailed features of the explosion in spray combustion? Is the e~cplosion strong enough to improve the combustion process? How can the mieroexplosion of emulsified fuels be effectively used in engines? Why have positive and negative energy saving been observed in engine tests? The interaction of droplets in sprays, the droplet size, the ambient pressure, and the heating history have significant effects on the combustion process, so the microexplosion in dense emulsified sprays under diesel engine combustion conditions needs further study for clarifying these issues. In order to evaluate the potential energy saving by the use of emulsified fuels on a large scale, research was started several years ago to understand the mechanism of the microexplosion in sprays and its application in engines. In order to link the broken chain between droplet combustion and engine application for emulsions, experiments in a combustion bomb, dynamical engine tests, combustion 175
176
INTERNAL COMBUSTION ENGINES
modeling, and theoretical work were carried out [9,10,11]. The "lump-fashioned" droplet group microexplosion (in earlier publications, droplet group microexplosion was called "lump-fashion microexplosion") and a nowater layer in emulsion droplets were observed in the bomb by a multipulsed ruby laser holocamera [9]. The explosions were strong enough to expand the spray region and speed up flame propagation, which was verified by high-speed photograplay [10]. The road-load-simulation engine tests show no significant effects on energy saving by using emulsion, but the smoke can be reduced by 30%. The modeling reveals the combustion features and the optimum water percentage in emulsion for different operating conditions. Tim questions mentioned above are mainly answered by this series of work.
TABLE 1 Testing conditions in combustion bomb Environment in bomb Pressure (MPa): 0.1, 2.4, 3.2, 4, 4.5, 5 Temperature: 293, 723, 733, 753, 773, 823, 873, 923 K W/O emulsions Water fraction: 0, 10%, 12% Internal phase: 2 5 ,urn Viscosity:5 centistoke at 293 K Holography (Nitrogen in bomb) Laser energy: 50 mJ/pulse Pulse width: <40 ns Pulse numbers: 1, 2, 3, 4 Pulse interval: 50/ts, 100 ffs High-speed photography (Fresh air in bomb) Focal length: 100 mm Aperture: f/2.3 Film: black/white, ISO 400 Camera speed: 3000 PPS
Mieroexplosion in Sprays under Diesel Engine Conditions The early fimdamental research was focused on the combustion of either isolated droplets or arrays of single droplets (h = 0.3-1.0 ram) suspended on quartz filaments or fine thermocouples under atmospheric pressure. Since the suspending filaments may be particularly detrimental to the study" of combustion characteristics of evaporating emulsions, a droplet generator was employed to generate small droplets floating in an upward heated gas stream [12]. The internal phase size (the size of the dispersed phase in emulsion) may affect mieroexplosion [13]; the higher cylinder pressure may suppress the occurrence of microexplosion [14]. Emulsion sprays were maeroscopieafly observed by high-speed sehlieren photography in a combustion bomb (Zhejiang University, China). The theoretical work was also carried out based on the experimental results [15-17], even for a multicompound liquid [J.8]. Previous work on fuel spray research [19] emphasized the spray shape, penetration, and entrainment using hot-wire anemometer, particle tracing, highspeed photography, single-flash photography, instantaneous microphotography, schlieren photography, shadowgraph, Fraunhofer diffraction particle analyzers, and single-pulsed Gabor laser holography. However, these methods cannot measure the droplet velocity and trajectory and the microexplosion of the micron-sized moving particles in sprays, since the measurement needs very high resolution in a large field of view. To observe the very weak diffraction from small moving droplets in a spray inside the high-pressure bomb, a multipulsed, off-axis, ruby laser holocamera was employed in this work. The holocamera can produce four single-mode laser pulses of 50 mJ at 30-ns pulse width. The pulse interval can be adjusted from
5 to 100 fs in 1-fs steps. The pressure drop in the injector was used as the synchronizing signal to trigger the laser beam after a 0.8-6.5-ms delay (controlled by a timer). The laser beam at first was split into an object wave and a reference wave by a splitter, and then the reference wave was expanded and collimated to 90 mm in diameter. The object wave was 50 mm. Two quartz windows of 48 mm in diaineter with high optical quality were used as the optical path of the bomb. Passing through the bomb, the beam was premagnified up to 3.5 times with an h = 22 mm high-quality convex lens located between the bomb and the image plane, and recorded on a holographic plate located at the image plane. To avoid speckle noise and obtain homogeneous illumination in the field of view, a convergent white light beam was employed to reconstruct the 2D images by a 35-mm single lens reflex (SLR) camera. To verify the effects of the microexplosion, a series of highspeed photographic tests were made in the bomb under diesel engine combustion conditions with a 16ram HYSPEED camera. The testing conditions simulate the cylinder pressure and temperature (Table 1, refer to Refs. 9 and 10). It is worth mentioning that a thermoisolating watercooling jacket covers the exposed part of the injector to keep the nozzle temperature under 353 K to prevent the evaporation of the water in emulsion before injection.
Droplet Group Microexplosions in Emulsion Sprays: The droplet group microexplosions take place in eddy scale near the outer layer of the emulsion sprays (Fig. 1). The pictures taken at different moments show various strengths of the microexplosions.
DROPLET GROUP MICROEXPLOSIONS IN EMULSION SPRAYS
177
T (K)
600
Limit of Superheat at 4 MPa
9
550
.
.I//
ex
~ t u r a t i o n Temp. at 4 MPa.
500 FIG. 1. Droplet group microexplosionof in w/o emulsion sprays; arrows point out the explosion centers (12% water, 773 K, 3.2 MPa, 3 • 100 fs, 2.3-ms delay)9
e~
No-Water L a y e r / 450
Lump-shaped flames were also observed by highspeed photography. The droplet group microexplosion is strong enough to eject fragments of torn droplets to a distance several millimeters away from the spray boundary at much higher speed, greatly expanding the spray head and spray angle, improving air-fuel mixing, and causing the flame edge to become unclear and irregular [10]. Perhaps the droplet group microexplosion occurs mainly in the turbulent eddy in which the droplets more or less have homogeneous size, similar heating history, and superheated state. If a droplet is exploding, the pressure wave may induce all the droplets in the same eddy to explode, an avalanche of mieroexplosions of droplet group. Different eddies have different sizes and different superheated states, so the explosions are also very different from one to another. The explosion region is near the spray head and outer layer, where the gas temperature is high enough and the droplets go through sufficient heating time to be superheated. Because of the explosion in the droplet group, the water fraction in the range of 8-12% has no significant influence on the violence of the explosion. No explosion was observed in the higher density, lower-temperature spray core. If the ambient temperature is not high enough (733 K in the test), the water dots (internal phase of w/o emulsion) in the droplets will not be superheated and will evaporate and vanish before being heated into superheated state, and no microexplosion takes place (Fig. 2). If the ambient temperature is proper (773 K), all the water dots in droplets rapidly go over the saturation temperature (523.5 K at 4.0 MPa) and into a superheated state, a pseudo-stable state, and vaporize and explode at the same time when the outer water dots achieve the limit of superheat (583 K at 4.0 MPa). This tears up the droplet and greatly expands the spray region. If the temperature is too high (823 K), the heat transfer between gas and droplets is strong enough, some water dots near the outer layer are over the limit of superheat, but central water dots are not in superheated state in this case, the
0
/
Droplet Radius
FIG. 2. The influence of different ambient temperature on droplet heating process and temperature gradient. explosion occurs earlier and closer to the injector, and the violence of explosion is not very strong since all water dots do not explode at the same moment. The torn droplets fly much faster in a lower gas density, causing a larger flame angle and stronger microexplosions. If the gas density is constant, gas pressure may have little effect on explosion violence. The injection pressures between 18 and 24 MPa have little effect on atomization and microexplosion of emulsions. Droplet group microexplosion phenomena can be supported by high-speed photography[10], by which the bright diesel flame is more homogeneous, more yellow in color, and lasts a longer period than the emulsions flame; the ignition delay of emulsions is longer than diesel, but the flame propagation is much faster after ignition, and ignition occurs at multiple points simultaneously. No-Water Layer in Emulsion Droplets:
When the gas in the bomb is heated up to 700 K, a comet-shaped vapor cloud surrounds the droplet, and water dots (1 fm) concentrate in the central part of the droplets. A no-water layer can also be recognized near the surface of the droplets from 10 times remagnified reconstructed holographs. The image is conceptually shown in Fig. 3. The velocity difference between gas and droplets causes a uniaxially symmetrical shear stress, which reduces droplet speed, induces a Hill vortex (internal circulation) in the droplets, rotates the droplet, and then forms a rapidmixing layer. In this layer, the water dots will be moved onto the droplet surface and vanish before the water dots are heated up to the saturation temperature, favoring the no-water layer formation because of the low evaporation and dif-
178
INTERNAL COMBUSTION ENGINES moving direction /k
water dot 'If
droplet boundary I inner core
,-water :as phase vapor cloud Fro. 3. The computer-treated image of evaporating w/o droplet and no-water layer model. fusion rates of oil vapor. The circulation is not strong enough in the inner core to homogenize the temperature field and the water dot distributions in the whole droplet since new no-water layer still exists if the droplet is still moving. The water dots in the eore are easily heated up to the superheat state if the ambient temperature is proper. When the water dots near the outer layer achieve the limit of superheat and evaporate, all the superheated water dots also evaporate and expand rapidly at the same moment, tearing up the fuel droplet--the mieroexplosion. The inhomogeneous situation is more easily kept and the microexplosion takes place more easily in big droplets than in small ones. The no-water layer eoncept is very important in explaining the microexplosion of emulsion droplets. A no-water layer model [11] based on the Hill vortex analysis by Prakash and Sirignano [20] agrees very well with Aggarwal's work for hexane, deeane, and hexadecane under the same operating conditions, and experimental data for emulsions. The system is treated as a sphere containing the inner core of the droplet as a homogeneous phase, with the no-water layer and gas phase around the droplet (Fig. 3). The model shows that the initial droplet diameter is another important factor in the microexplosion. If the initial droplet is too small, the residual water mass is too low or will even vanish before the droplet is heated up to the saturation point, and the explosion can hardly be observed in this condition. This result implies that the mieroexplosion effect may be helpful in the case of an old injection pump with lower injection pressure, where the atomization has deteriorated. It may be eonjeetured that, the more the water in the emulsion, the larger the explosive energy will be. However, if the initial diameter of the emulsion droplet is held constant, the emulsion with larger water fraction will lose more water as a result of the existence of the vortex and no-water layer, thus lowering the surface temperature. The additional water
gives the spray more initial momentum to improve the air-fuel mixing process. These factors may have opposite influences on the effects of the microexplosion on eombustion. In our experiments, the water fraction has a weak influence on the occurrence of the mieroexplosion. Engine Test under Simulated Road-Load Conditions The effect of microexplosions on engine performance is fnlly dependent on the application. Many well-performed steady-state engine tests show that the energy saving is less than 5% for most diesel engines. The steady-state tests may be very different from road running: 20-30% benefits during road rnnning were reported by many regional sourees in China. To evaluate the potential benefits for emulsion applieations, a large number of expensive road tests for different road conditions and different engines should be done because of the complicated influences. However, the results can hardly be repeatable and reliable. Therefore, a series of steady-state and road-load-simulation engine tests have been recently performed for this purpose on an AST-386 microcomputer-controlled dynamic engine test bed EDST-2 with a Siemens 200-kW DC electric dynamometer, which can run various road routines and measure engine parameters automatically. The engine test bed very aeeurately controls engine torque (<1%) and speed (<10 rpm). To analyze the rate of heat release (ROHR), the eyfinder pressures were recorded by AVL QC42D piezoelectrie transducer, Kistler 5004 charge-voltage convertor, PEI eneoder, and IBM microcomputer with PEI ECA911 software package. An improved compound band reject filtering (CBRF) filtering method [21] was employed to eliminate the effeets of measurement passage on pressure curves for ROHR analysis. For evaluating the effects of emulsions on exhaust emission, both liquid and solid particles were measured in dynamic engine tests by a Hartridge MK3 Smokemeter, which has very fast response. For comparison, a Bosch Smokemeter was employed in the steady-state engine tests. The mieroexplosion may have more effect on combustion in direct injection engines with a bowl combustion chamber, in which the mixing process occurs mainly in the bowl space, than on other types of diesel engine. The engine employed in this test is a V6 engine with cylinder bore/stroke of 150/150 ram, compression ratio 16.8, rated engine power 215 kW at 2600 rpm, and shallow bowl combustion chamber. The load characteristics of 1100-2100 rpm and the speed characteristics of 40 and 80% load were designed for steady-state engine tests. The ECE-15, SAE J35 SEP88, free acceleration for smoke evaluation, 6-1node cycle (Chinese Stan-
DROPLET GROUP MICROEXPLOSIONS IN EMULSION SPRAYS Ge (g/kW*h) 290 -
30001 n(r/min) Z000J ~
/-~
~
,
M(Nm)
L~'\J
looo ld
/~
HSU(~) -. ,
o
q ..
0
6~
,'~ " :ii,,
i00
t-..~
700
..... "'~
650
.-'-d--"
600
,
280 ~... -"-.-..~
ge
250 .
.
.
.
.
200
.
.
.
.
.
.
.
.
300
.
.
s (Time)
1O- P (MPa)/ROHR (100 J/'CA) - -
550
.
FiG. 4. Engine torque, speed, and smoke measurements under No. 1 Bus routine (Me-engine torque, HSU-Hartridge Smoke Unit).
p
-800
"-%..
,
mr,//
15~
+ ,~.., ......
Me (Nm)
Water Water Water
750
w/o
<-;,1
.....
C~ 6~ 10~
280 /
50 fl
-,~
flooo
2g0 100
.....
179
240 , 1000
~ . . . . . . . , .. 1400 1800 n (r/min)
......
. . . . .
500 2200
FIG. 6. Specific fuel consumption (water excluded) under engine speed characteristics test (Ge-break specific fuel consumption, Me-engine torque). Ge (g/kW*h)
Diesel
9
. ", ",,
500-
" \
,,~
O~ ._-_- ._. B~ . ._. ~. _ _ _ 1 0 ~
Water Water Water
*__*_*_.*__* 1 5 ~
Water
',\ \ 400
'~ \
',, ";5 oo.
o 16 ........
6 .........
,'o ........
'2'o.........
do "CA
FIG. 5. The cylinder pressure and the rate of heat release (1500 rpm, 750 Nm). dard), and Beijing No. 1 bus routines (Fig. 4) were all designed for engine dynamic tests. The No. 1 bus is the busiest bus running along the Chan'an Street across central Beijing, and the total mass of the bus is 19.2 tons with a typical 160-kW direct injection engine. The diesel fuel and emulsions with 6, 10, and 15% water in weight were chosen for all engine tests, since the optimum water percentage is about 10% according to our e~cperiences. The results are summarized as follows. The engine smoke is at least 30% HSU (Hartridge Smoke Unit) less than diesel fuel, and the water fraction (6-15%) has certain effects on HSU (Fig. 4 shows typical measurements). The water postpones the ignition about 0.2 ins, and the microexplosions change the combustion rate and ROHR (Fig. 5). Energy saving can be obtained under partial load conditions by emulsions, but negative effects may appear under high load conditions because of longer injection per-
200
0
. . . . . . . . . . . . . . . 50 I00
Ne(kW) 150
FIG. 7. Specific fuel consumption (water excluded) under engine load characteristics test (Ge-break specific fuel consumption, Me-engine torque). iods. The engine performances are a little bit better in the range of 1200-1800 rpm under 80% load (Fig. 6). Figure 7 and Table 2 show that about 3-5% more energy was consumed by emulsion applications in both steady-state and dynamic engine tests without injection-timing adjustment. According to our previous work (unpublished), the injection timing should be adjusted 2 x CA (crank angle) in advance to obtain 3% average energy saving for emulsions. The exhaust temperature is reduced by the addition of water, especially for high load conditions, and engine torque is also lower since the maximum fueling volume rate is the same as the diesel. It is suggested that injection timing should be adjusted to optimize the engine performance and emissions.
INTERNAL COMBUSTION ENGINES
180
TABLE 2 Fuel consumption in road-load test Emulsio~test (g)
Water content (%)
Energy sa~ng (%)
6-Mode
970 1065 1133 1178
O 6 10 15
0 -3.2 -5.1 -3.2
ECE-15
325 360 370 385
0 6 10 15
0 - 4.1 -2.5 -0.7
1380 1540 1570 1625
0 6 10 15
0 -4.9 - 2.4 -0.1
No. 1 Bus
The dynamic engine test shows that both fuel consumption and exhaust emissions have very good correlation with steady-state tests, which can qualitatively evaluate the emulsion applications if there is no dynamic test bed. For energy saving by emulsion applications, the positive effects are fully dependent upon the engine and operating conditions; 3-5% may be the maximum benefit according to our previous engine tests, this series of engine experiments, and other researchers' reports for well-performed engine tests. Sometimes the influence may be negative if no proper adjustment is made.
Computational Simulation of Engine Using Emulsions A combustion model, which can predict the engine performance under low load conditions [22], was modified for emulsions. This model divides the injected fuel into 30 small packages. Mr entrainment, droplet sizes, the evaporation process, and ignition delay in individual packages are calculated according to the transient injection pressure of the packages and then integrated by Duhamel Integral. The physical and chemical properties of the emulsions, ignition delay calculation, and the rates of air entrainment and evaporation were based on the water fraction to simulate the effect of the microexplosion. The simulated operating conditions are as follows: engine speed, 1500-2100 rpm BMEP, 0.13-0.6 MPa; water fraction, 0-20%. The combustion characteristics of emulsions are reasonable. The higher initial momentum of emulsion sprays and the micro explosions raise the local air-fuel ratio. The combustion rate and engine efficiency, which
s2 7
~i(~) o,
Pi~O. I S
48~j 46
44-
42-
....
d ....
1'0 ....
l's ....
z'0 '~/aier
(~)
FIG. 8. The relation between water fractions and thermal efficiency (1500 rpm) (/-thermal efficiency,Pi-break mean effective pressure). are more significant under low load conditions where the initial momentum of diesel sprays is not sufficient, also increased. The fuel injection system is designed to match high load conditions, and under such conditions, the emulsion injection stops later. The combustion period is hardly shortened by the microexplosion because of the very large spray region already existing. For an engine at 1500 rpm, the optimum water fraction is 12-16% for idling and 6% for high load conditions (Fig. 8). The highest flame temperature appears later and is lowered by 200-300 K as a result of more abundant air and evaporation of the water in the flame region, which greatly improves the soot and Nox formation but slightly reduces the thermal efficiency. The ignition timing is 2 • CA later than diesel fuel, but combustion fin-
DROPLET GROUP MICROEXPLOSIONS IN EMULSION SPRAYS ished earlier. Postponing the injection has less effect on engine efficiency than for diesel fuel, since the microexplosion improves the combustion process.
181
complex factors, and improvements are not always present under all conditions. Acknowledgements
Conclusions Experiments using holography and high-speed photography in a high-pressure and high-temperature bomb, dynamic engine tests, and computer modeling were carried out to link the broken chain from the droplet combustion to engine application for enmlsions. Microexplosion is a phenomenon in which the microwater dots in the emulsion droplet are heated into an unstable superheated state and vaporize rapidly, tearing the droplet violently. A no-water layer (rapidmixing layer) exists near the droplet surface because of the Hill vortex, and many mierowater dots exist in the droplet inner core. A mieroexplosion occurs in a certain temperature range and becomes stronger at a proper ambient temperature. Raising chamber pressure has little effect on the occurrence of the explosion, but the penetration of the torn droplet will be much shorter as a result of higher gas density, which weakens the effect of the explosion. Droplet group microexplosions in emulsion sprays occur on a turbulence eddy scale near the outer layer and the head of the spray. If the ambient temperaturn is proper, the explosion possesses enough energy to eject torn droplets to a distance several millimeters away from the spray boundary, improving atomization, evaporation, and air-fuel mixing processes and speeding up flame propagation. Since physical processes of liquid fuel combustion take a much longer time than chemical reaction, the mieroexplosion can speed up combustion rate and ROHR, raising engine thermal efficiency. Meanwhile, the injection timing must be adjusted, and the proper water fraction for different loads and the internal phase size of the emulsions must be carefully chosen. Emulsions may cause better efficiency under partial load conditions, especially for direet injection and old engines, but negative effects may occur for new engines under high load conditions. Much less soot is formed in emulsion spray combustion, and the engine smoke is at least 30% less than diesel fuel. The water fraction (6-15%) has no significant effects on engine smoke. Nox formation can be reduced because of the lower temperature by the addition of water. In general, the road-load-simulation engine tests agree with the steadystate tests and the fundamental research for emulsions. The experimental results show that the microexplosion in emulsion sprays may occur under diesel engine combustion conditions. Engine efficiency can be slightly improved by emulsions under certain conditions through the microexplosions improving the mixing and combustion processes. The system efficiency is influenced by many
This work was supported by the State Planning Commission of China. The authors are indebted to Prof. S. B. Yan, Institute of Aeoustics, Chinese Academy of Sciences, for supplying the emulsion.
REFERENCES 1. Hopkinson, B., Proc. Institution of Mechanical Engineers, 679-715 (1913). 2. Dryer F. L., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 279-295. 3. Viehinievsky,R., Murat, M., Parois, A., and Dujen, M., CIMAC Congress, I. Mech. E. Press, London, 1975,
4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15, 16. 17, 18. 19, 20,
21,
22.
pp. 615~352. Callanhan, T. J., Ryan, I. T. W., O'Neal, G. B., and Waytulouis, R. W., SAE 830553, 1983. Johnson, R. T., and Stoffer, J. O., SAE 830557, 1983. Dryer, F. L., Rambaeh, G. D., and Glassman, I., paper presented at the 1976 Spring Meeting of the Central States Seetion of the Combustion Institute, 1976. Jacques, M. T., Combust. Flame 29:7%85 (1977). Ivanov, V. M., and Nefedov, P. I., NASA Technical Transal. TIF-258, 1965. Sheng, H. Z., and Chen, L., The 4th ICLASS, The Fuel Society of Japan, Sendal, Japan, 1988, pp. 391 396. Sheng, H. Z., Chen, L., and Zhang, Z. P., The Seminar on Diesel Fuel Injection Systems, Institution of Mechanical Engineers, London, 1989, pp. 49-56. Sheng H.-Z., Zhang, Z.-P., and Wu, C.-K., International Symposium COMODIA 90, Kyoto, Japan, 1990, pp. 275-280. Lasheras, J. C., Fernandez-PeIlo, A. C., and Dryer, F. L., Combust. Sci. Technol. 21:1-14 (1979). Marrone, N. J., Kennedy, I. M., and Dryer, F. L., Co~r~ust. Sci. Technol. 33:299-307 (1983). Wang, C. H., and Law, C. K., Combust. Flame 59:5362(1985). Law, C. K., Lee, C. H., and Srinvasan, N., Combust. Flame 37:125-143 (1980). Birehley, J. C., and Riley, N., Combust. Flame 29:145165 (1977). Katsoulakos, P. S., C84/83, Institution of Mechanical Engineers, London, 1983, pp. 51-62. Law, C. K., Combust. Flame 26:219 233 (1976). Chigier, N., Combust. Flame 51:127-139 (1983). Prakash, S., and Sirignano, W. A., Int. J. Heat Mass Transfer 253-288 (1980). Shi, S.-X., and Sheng, H.-Z, International Conference on Computers in Engine Technology, Institution of Mechanical Engineers, London, 1989, pp. 325 332. Sheng, H., Trans. Chinese Soc. Intern. Combust. Engines 3:243 256 (1985) (in Chinese).