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Toward a Microgravity Research Strategy (Appendix B) Toward a Microgravity Research Strategy B Combustion Science STATUS Combustion is a vital subject. For the foreseeable future, well over 90 percent of the U.S. energy supply will come from energy conversions that involve the high-temperature oxidation of fossil fuels. The world economy is affected by the efficiency of combustion processes in converting chemical energy to work or heat; the health of people and a balanced ecology depend on the reduction of pollutants emitted during combustion; and the viability of many transportation systems requires that combustion engines have ii favorable power-to-weight (or thrust-to-weight) ratio. The general subdiscipline of combustion science is very active. Three journals deal solely with combustion topics. Other journals in the fields of fluid REPORT MENU mechanics, heat and mass transfer, chemical physics, thermophysics, propulsion NOTICE and power, spectroscopy, applied mathematics, and environmental science MEMBERSHIP regularly contain articles relating to combustion. Several international research SUMMARY conferences are dedicated to combustion topics. The Combustion Institute, the CHAPTER 1 leading international research organization in the field, holds the premier CHAPTER 2 international conference and supports scores of national and regional meetings. CHAPTER 3 Combustion is the topic of many sessions at major annual conferences of the CHAPTER 4 American Institute of Aeronautics and Astronautics, the American Society of APPENDIX A Mechanical Engineers, and the Society of Automotive Engineers. Other societies APPENDIX B address combustion topics occasionally. APPENDIX C APPENDIX D Microgravity experimentation in combustion offers a variety of APPENDIX E simplifications compared to experiments done at Earth's gravity. They include a APPENDIX F reduction of dimensionality (the tendency toward spherical symmetry), the inhihition of settling effects for multiphase phenomena, and a substantial reduction of buoyancy forces. These simplifications provide the impetus for microgravity combustion research. Some challenges arise in this research, such as the emergence of surface tension as a major driving mechanism in certain two- phase experiments and the difficulty of achieving compact arrangements for forefront optical diagnostics. These challenges appear surmountable if given appropriate attention. file:///C|/SSB_old_web/cmgr92appendb.htm (1 of 6) [6/18/2004 11:09:50 AM]
Toward a Microgravity Research Strategy (Appendix B) Several experimental programs are under way to study combustion in microgravity. For the most part. these programs have been successful, and some interesting and useful results have been obtained. Until recently, the microgravity program has not been visible to a wide community of combustion researchers. However, recent efforts by the Combustion Science Disciplinary Working Group have attracted an unusually large number of interesting research proposals to NASA's headquarters, and more research funds are now being committed in this area. Opportunities in microgravity science have not been utilized at more than a fraction of their potential by the combustion research community. Gravity affects many combustion processes, especially in a laboratory simulation in which certain length scales are increased in order to improve resolution. Control of the acceleration environment offers the prospect of scientific breakthroughs in several areas of combustion science. MAJOR RESEARCH ACCOMPLISHMENTS Generally, the acceleration environment is found to have not only observable but also profound effects on material flammability characteristics, flame spread rates, and burning rates. This environment offers the possibility of addressing gravity-dependent variations to gain insights about fundamental processes. There is also the obvious research needed to develop new fire safety standards for spacecraft; the practice of applying in space standards developed and tested at Earth's gravity should not be continued. Flame spread rates over solid fuels have been found to decrease as the ambient oxygen concentration decreases.1,2 Interestingly, dependence on the magnitude of gravity increases as the concentration of oxygen decreases. A flammability limit for oxygen concentration has been found, below which flames cannot propagate or exist. Furthermore, the magnitude of this oxygen concentration limit does depend on the level of gravity. The existence of this lower limit is a fascinating scientific finding that has a potential for extraordinary technological impact on fire safety issues, including spacecraft fire safety. Forced air velocities have also been shown to affect spread rates. At low spread rates, radiative losses dominate the effects of transport rates that influence spread rates. Therefore, quenching occurs. Flame spread rates increase with increasing velocity to a point. Beyond that point, high velocity causes a blow-off of the flame. In the case of ignition and flame spread in a gaseous combustible mixture above a pool of liquid fuel, the effects of buoyancy and of surface tension as a driving mechanism have been shown to be important.' file:///C|/SSB_old_web/cmgr92appendb.htm (2 of 6) [6/18/2004 11:09:50 AM]
Toward a Microgravity Research Strategy (Appendix B) Flammability limits for gaseous combustible mixtures also have been found to depend on the magnitude and orientation of gravity.4,5 Natural convection tends to "stretch" flame fronts, that is, flame curvature and flow divergence are affected by natural convection. This stretch modifies the rate of transport across the front. It has been demonstrated that flame propagation through particle clouds is sensitive to the motion of the particles.6 Microgravity environments are useful in this case to avoid settling and to maintain strict control of the density of the particles. The buoyancy effect is predicted to have a stabilizing influence on flames. At reduced gravity, flames become unstable and cellular structures are formed, according to the theory. Cellular structures have been found experimentally at reduced gravity levels for lean hydrogen-air mixtures. Experiments indicate that jet diffusion flames become larger, more spherical, and sootier as gravity is reduced, but the mechanism is not yet fully understood.7 Preliminary data on spherically symmetric fuel droplet vaporization and burning support the theoretical prediction of the importance of certain unsteady effects.8 Buoyancy tends to destroy spherical symmetry, making the microgravity environment particularly appealing. Experiments in drop towers have indicated a new aspect of formation of a soot layer between the droplet and the oxidation flame, It has been suggested that thermophoresis is important in stabilizing this layer in the Stefan flow. RESEARCH PROSPECTS AND OPPORTUNITIES There are many opportunities for using microgravity facilities to do frontier research in combustion, Turbulent combustion, for example, is the area most in need of further fundamental characterization. The mixing that precedes a chemical reaction and that often is rate controlling occurs on very-small-length scales in practical situations. The resolution needed is not attainable in those cases. Theoretically, similitude can be maintained by decreasing velocities and increasing length and time scales within the proper relationship. However, at the larger-length scales, buoyancy can become important, destroying the similitude. Therefore, the microgravity environment offers an exciting opportunity for the study of turbulent-reacting flows with large scales, yet negligible gravity effects. Elimination of settling effects enhances conditions for the study of soot formation and agglomeration, The absence of settling also allows for useful studies of spray combustion and particle cloud combustion, as well as investigation of propagation rates and flammability limits. Extinction limits and details of the transient characteristics for an individual burning droplet and for an file:///C|/SSB_old_web/cmgr92appendb.htm (3 of 6) [6/18/2004 11:09:50 AM]
Toward a Microgravity Research Strategy (Appendix B) array of droplets burning together are also topics of interest. More experiments on the spread of flames adjacent to liquid or solid fuels are required. In particular, it is necessary to understand the individual influences of buoyant, forced, and Stefan flows on spreading rates and on detailed flow field characteristics, Experiments with separate controls for each of the three influential types of flow are desirable. A series of experiments on flame propagation through gaseous combustible mixtures is needed in order to evaluate the relative effects of various mechanisms on flammability limits. The consequences of chemical kinetics, radiative heat losses, fluid dynamical strain, Lewis number variation, and buoyancy should be determined, The consequences of these same parameters on flame front instabilities and cellular flame formation are also of interest. Access to a microgravity environment to separate the buoyancy effect from other potentially important effects is obviously useful. The stabilization mechanism for a jet diffusion flame calls for further study. The roles of buoyant flows and diffusion should be determined, especially near the burner rim where the flame-holding action occurs. The Combustion Science DWG has given the highest scientific priority to turbulent-reacting flows and the next highest to heterogeneous combustion.9 Laminar homogeneous combustion receives the third priority. With regard to applications, spacecraft fire safety receives the highest priority, Combustion experiments will require a range of sophisticated instruments; more study is needed to obtain appropriately compact optical diagnostic equipment. Further examination of the allowable levels of artificial gravity is required. REFERENCES 1. Olson, S., P. Ferkul, and J. T'ien. 1989. "Near-Limit Flame Spread Over a Thin Solid Fuel in Microgravity." Pp. 1213-1222 in Proceedings of the Twenty-Second Symposium (International) on Combustion. Combustion Institute, Pittsburgh. 2. Bhattacharjee. S., R.A. Altenkrich, S.L. Olson, and R.G. Sotos. 1988. "Heat Transfer to a Thin Solid Combustible in Flame Spreading at Microgravity." Proceedings of the 1988 Fall Technical Meeting on Chemical and Physical Processes in Combustion. Eastern Section, Combustion Institute, Clearwater Beach, Fla., December 1988. 3. Abramzon, B.. D.K. Edwards, and W.A. Sirignano. 1987. "Transit, file:///C|/SSB_old_web/cmgr92appendb.htm (4 of 6) [6/18/2004 11:09:50 AM]
Toward a Microgravity Research Strategy (Appendix B) Stratified, Enclosed Gas and Liquid Behavior with Concentrated Heating from Above." Journal of Thermophysics and Heat Transfer 1:355-364. 4. Ronney, P.D, and H.Y. Wachman. 1987. "Effect of Gravity on Laminar Premixed Gas Combustion I: Flammability Limits and Burning Velocities." Combustion and Flame 62:107-119. 5. Strehlow, R.A., K.A. Noe, and B.L. Wherley. 1987. "The Effect of Gravity on Premixed Flame Propagation and Extinction in a Vertical Standard Flammability Tube." Pp. 1899-1908 in Proceedings of the Twenty-First Symposium (International) on Combustion. Combustion Institute, Pittsburgh. 6. Berlad, A.L., H.D, Ross, L. Facca, and V. Tangirala. 1990. "Particle Cloud Flames in Acoustic Fields." Combustion and Flame 82:448-450. 7. Edelman, R.B. and M.Y. Bahadori. 1987. "Effects of Buoyancy on Gas Jet Diffusion Flames, Experiment and Theory." Acta Astronautica 13:681-688. 8. Shaw, B.D., F.L. Dryer, F.A. Williams, and J.B. Haggard. 1988. "Sooting and Disruption in Spherical Symmetrical Combustion of Decane Droplets in Air." Acta Astronautica 17:1195. 9. Office of Space Science and Applications, Microgravity Science and Applications Division. 1990. Microgravity Program Strategic Plan 1990. National Aeronautics and Space Administration, April. file:///C|/SSB_old_web/cmgr92appendb.htm (5 of 6) [6/18/2004 11:09:50 AM]
