J. Chim. Phys. (1999) 96, 1031-1037 EDP Sciences, Les Ulis Effects of gravitational acceleration on high pressure combustion of methanol droplets C. Chauveau 1, B. Vieille 1, I. Gôkalp 1 *, D. Segawa 2, T. Kadota 2 and A. Nakainkyo 2 ' Laboratoire de Combustion et Systèmes Réactifs, Centre National de la Recherche 45071 Orléans cedex 2, France 2 Department of Mechanical Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Scientifique, * Correspondence and reprints. RÉSUMÉ Un programme de collaboration franco-japonais sur la combustion en microgravité a été initié. Une des études est consacrée à l'investigation expérimentale de la combustion de gouttelettes de methanol sous haute pression, dans des conditions de gravité terrestre et aussi de gravité réduite obtenues par l'utilisation d'une tour de chute libre et par les vols paraboliques d'un avion-laboratoire. Une goutte de methanol est suspendue au centre d'une chambre à haute pression et est enflammée par un filament électrique porté à haute température. Une caméra vidéo rapide permet le suivi temporel de l'évolution du diamètre de la goutte qui est utilisé pour déterminer le taux de combustion. Les résultats montrent que la loi en D 2 est valide pour toutes les conditions explorées dans la présente étude. Le taux de combustion augmente avec l'augmentation de la pression ambiante et ne démontre pas l'existence d'un maximum à la pression critique du methanol. Le taux de combustion décroît de manière évidente avec la diminution de l'accélération gravitationnelle. Mots-clés : combustion de gouttes, hautes pressions, microgravité. ABSTRACT A Franco-Japanese cooperative research program on microgravity combustion has been initiated. One of the studies is devoted to the experimental investigation of high pressure combustion of methanol droplets, under normal and reduced gravity
1032 C. Chauveau et a1 conditions, which are obtained in a drop tower and during the parabolic flight of an airplane. A methanol droplet is suspended at the center of a high-pressure combustion chamber, and is ignited with an electrically heated kanthal wire. A high-speed video camera is used to obtain the time histories of the squared droplet diameter, which are used to determine the droplet burning rate. The results show that the d-square law is valid for all the conditions of the present experiments. The droplet burning rate increases with ambient pressure and does not show a maximum around the critical pressure of methanol. Experiments also show that the droplet burning rate decreases with the reduction of gravitational acceleration. Key words: droplet combustion, high pressure, microgravity. INTRODUCTION A Franco-Japanese cooperative research program on microgravity combustion have been initiated since 1996; part of this program is devoted to understand the combustion processes of methanol droplets under high pressure conditions. Methanol is considered to be one of the most promising candidates as alternative he1 for several applications. Also, the operating pressures in the combustion chamber of liquid fueled internal combustion engines are increasing for higher thermal efficiency. Liquid he1 droplets bum therefore in high pressure gaseous environments and the ambient pressure often exceeds the critical pressure of the fuel. A review of the literature indicates that the combustion characteristics of he1 droplets burning in high-pressure gaseous environments remain largely unexplored [l]. For example, it was found experimentally under microgravity conditions [2, 31 and theoretically [4, 51 that the burning rate of various fuels shows a maximum around the critical pressure. However, the burning rates obtained in a recent experiment under microgravity conditions did not show a peak around the critical pressure [6]. The difficulty in the experiments on droplet combustion arises from the combustion generated buoyant flow in normal gravity, which is enhanced in high-pressure gaseous environments. The reduced surface tension of the droplet burning in highpressure gaseous environments does not allow the use of the well-known suspended
Effects of gravity levels on droplet combustion 1033 droplet technique in normal gravity. Microgravity conditions offer therefore the opportunity to perform droplet combustion experiments in an environment free from the effect of gravity induced natural convection. The primary objective of our study is to obtain the detailed information needed for the understanding of the combustion process of a single methanol droplet in high-pressure gaseous environments. The present paper describes recent results on the combustion of a methanol droplet under variable ambient pressures and gravitational accelerations. The experiments were conducted under normal gravity, and under microgravity with the use of the parabolic flights of the CNES A300 airplane in France and the drop shaft at JAMIC in Japan. EXPERIMENTAL APPARATUS AND PROCEDURE Different experimental facilities are used in Japan and in France. The global concept of the apparatus is the same, however. The two different apparatus have been fully described previously [7, 81. The well-known suspended droplet technique is adopted. A methanol droplet is suspended at the center of a high-pressure combustion chamber and is ignited with an electrically heated kanthal coil. The initial diameters of the droplet di range from 1 to 2 mm. The test chamber is filled with dried air at 298 K. Experiments are conducted in normal gravity, and microgravity with the use of the parabolic flights of the CNES A300 airplane (where the gravitational acceleration is of the order of 10.* go) in France and the drop shaft at JAMIC (where the gravitational acceleration is of the order of 10.~ go) in Japan. The backlighted images of the droplet is recorded on a high-speed video camera (500 fps). The droplet surface area is measured by using image processing. The droplet diameter d is defined as the diameter of the sphere that has the same surface area. The obtained time histories of the squared droplet diameter are used to determine the droplet burning rate. Degassed and dehydrated methanol is used in all experiments. The critical pressure (P,,) and the critical temperature of methanol are 8.09 MPa and 513K, respectively.
1034 C. Chauveau et al. RESULTS AND DISCUSSION NormaI Gravity The time variation of the burning methanol droplet diameter d is determined from backlighted images. Figure 1 shows the time histories of the squared droplet diameter at different ambient pressures. The abscissa is the time after ignition divided by the square of the initial droplet diameter di. The ordinate is the square of the instantaneous diameter normalized by the square of the initial droplet diameter. It is evident that the squared droplet diameter decreases quasi-linearly with time at all the ambient pressures; the d2 law is therefore valid for methanol droplet burning in high pressure air under normal gravity conditions. The slope of the fitted straight line to the data, i.e., the burning rate constant Kb = -d(d2)/dt increases with increasing ambient pressure. Microgravity The combustion experiments of methanol droplets on board the CNES A300 airplane were carried out at ambient pressures up to 11.1 MPa. Weak natural convection remains around the burning droplet at high ambient pressures. The droplet surface appears to be smooth even at 11.1 MPa, which is higher than the critical pressure of methanol. This indicates that the droplet has not reached its critical state. In the combustion experiments of methanol droplets in the JAMIC drop shaft, the ambient pressure was increased up to 14.0 MPa; at these highest pressures, the droplet image became blurred and the determination.of the droplet diameter more difficult. Figure 2 shows the time histories of the squared droplet diameter under microgravity in the drop shaft. As in the parabolic flight experiments, the d2 law is valid and the slope of the fitted straight line to the data increases with an increase in the ambient pressure.
Effects of gravity levels on droplet combustion Figure I : Time histories of the squared droplet diameter under normal gravity. Figure 2: Time histories of the squared droplet diameter under microgravily in the Jamic drop shaft. Figure 3 shows the dependence of the burning rate on the ambient pressure in normal gravity and in microgravity. The abscissa is the ambient pressure P, normalized by the critical pressure of methanol, P,,. The normal gravity curves show a good agreement between results obtained in France and in Japan by using different facilities. It is evident that the burning rate increases monotonically with the ambient pressure up to 1.25 times the critical pressure of methanol. The two other curves show the dependence of the burning rate on the ambient pressure in microgravity. The results obtained during the parabolic flights indicate that the burning rate constant increases monotonically with the ambient pressure up to 1.4 times the critical pressure of methanol. We can observe for all these experiments, that the burning rate is unlikely to show a peak near the ambient pressure equal to the critical pressure of methanol. It is in fact likely that the actual critical pressure of the methanol droplet exceeds that of pure methanol due to absorption of inert gases into the droplet. It can be noted however that the dependence of the burning rate constant on the ambient pressure becomes weaker with increasing ambient pressure above the critical pressure. The burning rates derived from the drop shaft experiments increase
1036 C. Chauveau et a1 monotonically with the ambient pressure up to 0.6 times the critical pressure. At each pressure, the buming rate constant of methanol droplets obtained in drop shaft experiments are lower than those obtained in parabolic flight experiments. Figure 3 : Dependence of the burning rate constant on ambientpressure Figure 4 : Estimation of the dependence of the burning rate versus pressure for dlferent gravitational acceleration. The data on methanol droplet burning rate constant vs. ambient pressure are fitted with curves as shown in Figure 3, and the burning rate constants at eight arbitrary pressures are estimated for normal gravity and the two microgravity conditions. The estimated burning rate constants at each ambient pressure are plotted against gravitational acceleration in Figure 4. The buming rate constant increases with an increase in gravitational acceleration as expected. The increase in gravitational acceleration enhances the influence of the combustion generated natural convection, resulting in the increasing burning rate constant, especially for the highest pressures. When all the experimental data for the buming rate constant, for all pressures and gravitational accelerations, are plotted versus the Grashof number estimated for each experiment, a power lower in the form of Kb oc ~ r 1s ' obtained ~. as ~ in Ref. ~ 6.
Effects of gravity levels on droplet combustion 1037 CONCLUSIONS An experimental study was canied out on the burning of methanol droplets in high-pressure air under microgravity conditions by using the parabolic flights of an airplane and a drop shaft, and under normal gravity. The primary conclusions reached in the present study are as follows. 1. The d2 law is valid for different ambient pressures and gravitational accelerations within the range of experimental conditions of the present study. 2. The burning rate constant increases monotonically with the ambient pressure up to 1.4 times the critical pressure of methanol. 3. In reduced gravity, dependence of the burning rate constant on the ambient pressure becomes weak above the critical pressure. 4. The burning rate constant decreases with a decrease in gravitational acceleration. 5. The variation of the burning rate constant as ~ r is confirmed ~. for ~ the ~ first time by varying the Grashof number both by varying the gravitaional accelaration and the ambient pressure. ACKNOWLEDGMENTS The Memorandum of Understanding signed between CNRS, CNES and NEDO has made this international collaboration possible. The authors wish to thank Novespace, Sogerma, JSUP and JAMIC for their cooperation. REFERENCES 1. Williams, A., (1973) Combust. Flame, 21, 1-31. 2. Faeth, G. M., Dominicis, D. P., Tulpinsky, J. F. and Olson, D. R., (1969) 12th Int. Symp. on Combustion, 9-18. 3. Sato, J., Tsue, M., Niwa, M. and Kono, M., (1990) Combust. Flame, 82, 142-150. 4. Spalding, D. B., (1959) ARS J., 29, 828-835. 5. Tsukamoto, T. and Niioka, T., (1993) Microgravity Sci. Technol., 4,219-222. 6. Vieille, B., Chauveau, C., Chesneau, X., Odei'de, A. and Gokalp, I., (1996) 26th Int. Symp. on Combustion, 1259-1265. 7. Chauveau, C.; Chesneau, X. and Gokalp, I., (1993) AIAA Paper No. 93-0824. 8. Segawa, D., Kadota, T., Nakainkyo, A., Chauveau, C., Gokalp, I., and Vieille, B., (1998) 27th Int. Symp. on Combustion, poster presentation.