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Microgravity Research Opportunities for the 1990s: Chapter 4 Microgravity Research Opportunities for the 1990s PART II—SCIENTIFIC ISSUES 4 Combustion INTRODUCTION AND OVERALL GOALS The science and engineering of combustion processes involve many disciplines of physics and chemistry. Combustion is a complex process and one that is influenced by gravity in many ways. Gravitational forces are instrumental in the burning of a candle, where buoyant forces lift the hot gases of combustion and allow the convection of fresh oxygen into the flame region. As illustrated below, however, in the scale-up of a combustion experiment for observational purposes, gravity often obscures the phenomenon of interest by introducing buoyancy that is absent in the experiment at the original scale. As much as the atmosphere has been an impediment to optical astronomy on Earth, so has gravity been REPORT MENU an impediment to understanding combustion in certain cases. The near absence of gravity NOTICE in spaceflight situations ensures different combustion behavior compared to that on Earth. MEMBERSHIP Not only would experimentation under microgravity conditions be useful in improving PREFACE scientific understanding of that behavior, but it is also imperative from the standpoint of EXECUTIVE SUMMARY spacecraft fire safety. PART I CHAPTER 1 CHAPTER 2 The uniqueness of the microgravity environment includes a reduction of buoyancy PART II forces, an inhibition of particle or droplet settling, and in some cases, a reduction of CHAPTER 3 dimensionality (e.g., droplet burning tends to become spherically symmetric). Some CHAPTER 4 surmountable challenges do arise, such as the emergence of surface tension as a major CHAPTER 5 mechanism in certain two-phase experiments and the need to design compact optical CHAPTER 6 diagnostic equipment. CHAPTER 7 PART III CHAPTER 8 The great reduction of buoyancy forces in the microgravity environment allows the APPENDIX A study of certain phenomena with much higher resolution because length scales can be APPENDIX B increased. For example, similitude with the Reynolds number or the Peclet number for turbulent flows can be maintained in microgravity conditions by increasing the length scale and appropriately decreasing the velocity scale. At Earth gravity, such a change would result in a large increase in the Grashof number (a nondimensional magnitude of buoyancy). That is, buoyancy might be insignificant in the phenomenon of original interest but could inadvertently be introduced in the attempt to improve resolution. Under microgravity conditions, however, the magnitude of the Grashof number remains insignificant. The other type of experiment is one in which buoyancy plays a role in the file:///C|/SSB_old_web/mgoppch4.htm (1 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 phenomenon of original interest at Earth gravity. Here, the microgravity environment presents an opportunity to treat gravity as a variable input parameter by designing an experiment to be operated in both microgravity and Earth-gravity environments. In this context, it is noteworthy that conduct of the experiment again in partial-gravity environments (Moon, Mars, or the space station centrifuge) could be helpful. There are a number of important issues in combustion, including flammability, stability, ignition, extinction, flame speed and spread rate, radiation, multiphase effects, critical point behavior, and smoldering. These are all affected by gravity in any experiment on Earth with normal gravity (1 g). A further complication is the wide disparity of relevant characteristic times in various subprocesses in combustion. For example, chemical kinetic times can be as short as l0-6 s, droplet burning times can be of the order of milliseconds to seconds, and smoldering times can be of the order of minutes to hours. Thus, both time of observation and gravity are variables that must be taken into account in experimentation. This is discussed below under facilities requirements. The overall goal of a microgravity program in combustion should be to enhance the science base of combustion phenomena where gravity is an important parameter in the problem. Secondarily, the science base should be expanded in areas where meaningful experiments cannot be performed on Earth because gravity interferes with scaled-up experiments. (See Plates 4.1 and 4.2 for examples of low-gravity combustion experiments.) PROBLEMS UNIQUE TO COMBUSTION There have been several recent reviews of the status of research and technology of gravitational effects on combustion.1-3 The problem that 1 g poses in studies of combustion is best elucidated by considering some dimensionless groupings that naturally arise. One of these is the Grashof number, which indicates the ratio of buoyant to viscous transport. Since density differences of nine to one (cold to hot) easily occur in combustion, gravity acts differently on hot gases than on cold gases. This extreme density difference is common in combustion. In situations at 1 g where the size scales of interest are less than the order of 100 m (droplet burning, for example), buoyancy effects are negligible compared to those of molecular transport. These small scales, however, present severe problems in measurement and experiment design. Note that the Grashof number is proportional to the cube of the length scale and is therefore extremely sensitive to experimental size. If forced convection is present, the appropriate dimensionless group is the Richardson number, which indicates the relative effect of buoyancy-induced velocity to the forced velocity. In situations where a laminar burning velocity of a premixed mixture is the appropriate velocity, the buoyancy-induced velocity can be neglected only for laminar speeds above about 1 m/s. This is near the upper limit of observed flame speeds for any mixture and is orders of magnitude greater than flame speeds observed near flammability limits, with these limits intensely relevant to the spacecraft fire issue. In nonpremixed flames (e.g., a jet flame), the Reynolds number based on forced convection velocity must be large compared to the square root of the Grashof number so that buoyant velocities are negligible relative to forced convection velocities. With flow speeds of the order of meters per second or more and size scales of the order of centimeters, this condition on the Reynolds number is met easily. It is not possible to meet file:///C|/SSB_old_web/mgoppch4.htm (2 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 this condition for small Reynolds numbers (<1) and length scales of the order of a millimeter or greater at Earth gravity. Small Reynolds numbers at a reasonable laboratory size scale introduce gravitational effects on Earth. The implication is that investigation of the highly important Stokes regime (Reynolds number less than 1) is excluded from observation at 1 g. This regime can be of interest, for example, in droplet burning. In turbulent flows, characterized by high Reynolds number, there can be a large range of size scales from the large eddies to the smallest structures. For buoyancy-induced stresses to be small compared to the turbulent stresses under investigation, flow velocities must be large (>50 m/s) in a reasonably sized apparatus (say 1 cm). While this may be achieved locally, there are always low-speed regions in a turbulent combustion experiment for which the buoyancy-free condition will be violated. On the other hand, in practical devices, buoyancy is not present at the speeds, size scales, and geometries of interest. The turbulence prediction problem is difficult enough without gravitational effects, and the complications presented by gravity obscure comparison of theory and experiment. At the University of Sydney, Sandia Laboratories, and the General Electric Company, turbulent jet diffusion flame studies at 1 g show clear buoyancy effects that either destroy symmetry or induce axial disturbances that make agreement between theory and experiment difficult far enough downstream in the flame.4 The downstream region is, however, one of the important flame regions for the study of turbulence decay. A database is needed in this region to study closure methods or computational methods for turbulence prediction. As another example, consider droplet burning, a concern in all spray combustion devices. It is usually true that buoyancy forces are small in practical spray combustion devices. However, the droplets are usually small (of the order of 100 m or less) in a real combustion chamber, and experimental access on this scale is impossible at the current time. Scaling up the experimental size to the order of several millimeters or centimeters allows experimental access, but then the buoyancy problem appears and the experimental data are not relevant to the real industrial process. While 1 g often presents a hindrance to gaining scientific understanding of combustion phenomena, experiments in the microgravity environment can present challenges in the form of new phenomena that become dominant. These challenges can be viewed as opportunities to address new scientific issues in combustion through microgravity experiments and supporting theory. Although some problem areas relate to the gravity level, as discussed above, other problems can arise in microgravity experiments of combustion. For example, the fuel/oxidizer ratio, relevant to the issue of spacecraft fire safety, is another variable that must be investigated. The oxygen content of spacecraft atmospheres can vary over a reasonable range and still be satisfactory to humans. Consequently, data for oxygen at only 21% by volume, common to the 1-g problem, may be unsatisfactory. Another problem concerns the ability to acquire enough data for statistical confidence in results. The data acquired in a typical microgravity experiment to date consist of data at one point. This problem is compounded in combustion because of the large number of variables, including the gravity level. Exhaust gas presents yet another problem area in microgravity testing of combustion. Because the exhaust of combustion products is restricted on spacecraft, special means must be employed in experiment design. Experimental requirements are especially severe in combustion because of the many variables of interest: pressure, temperature, velocity, concentration of major and minor species, and radiation (in the visible, ultraviolet, and infrared). This taxes the field of file:///C|/SSB_old_web/mgoppch4.htm (3 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 experimental diagnostics to the limits of technology, especially in view of the compactness requirements of equipment in flight configurations. STATUS OF GRAVITATIONAL-BASED CURRENT KNOWLEDGE The flammability limits of premixed gases are crucial to spacecraft fires and have already been shown to be wider at microgravity than at 1 g.5,6 Both magnitude and orientation of gravity affect flammability limits. The mechanisms operating at flammability limits are currently uncertain; it is not known whether heat loss, chemical kinetics, or various instabilities are the influential phenomena. A series of experiments on flame propagation through gaseous combustible mixtures is needed to evaluate the relative importance of various mechanisms to the flammability limits. The effects of chemical kinetics, radiative heat losses, fluid dynamic strain, mixture ratio, Lewis number, and buoyancy should be determined. The effects of these same parameters on flame front instability and cellular flame formation are also of interest. The use of a microgravity environment to separate the buoyancy effect from other potentially important effects would be highly useful for the comparison of theory and experiment and in the practical arena of spacecraft fire safety. A number of instabilities arise in various gas mixtures that are used to study turbulent flames. The Rayleigh-Taylor7 instability requires the presence of accelerations- gravity or otherwise. The density differences that are inherent in flames are the root cause of the Landau-Darrieus instability,8 which is not gravitationally dependent. Other instabilities that may depend on gravitational effects are thermodiffusive in nature.9 The study of flame instability behavior could be carried out fruitfully in microgravity to gain unambiguous information about the causes of instability. Some unique behavior at microgravity has been discovered in the area of self-extinguishing flames10 and stationary spherical flames, which involve flame stretch (strain), radiation losses, and various instability mechanisms. Microgravity studies of gaseous diffusion flames have resulted in the discoveries of new extinction behavior and excessive sooting.11 In droplet burning studies, soot problems occur in microgravity in the form of nonremoval of soot by buoyant forces and thermophoretic transport of soot toward the droplet surface.12 Because of the elimination of settling effects, important opportunities exist for the study of soot formation and agglomeration. The absence of settling also allows for useful studies of spray combustion and particle cloud combustion. A primary area of interest in spacecraft fire safety concerns how flame spread over solid and liquid fuels (with and without forced convection environments) is affected by gravity. Buoyancy and surface tension have been shown to be important in ignition and flame spread above liquid fuel pools. Flame spread rates over solid fuels have been found to decrease as the ambient oxygen concentration decreases, and the dependence on the magnitude of gravity increases as the oxygen level decreases.13,14 Forced air velocities affect spread rates; spread rates increase with increasing velocity until blow-off occurs. Curious optima on flame spread rate have also been discovered with regard to the convective flow rate in such flames. USML-1 results15 indicate that radiation heat transfer is dominant in the flame spread above solid fuels. Smoldering of solid fuels, often a precursor to fire, will be especially susceptible to gravitation, because of the nonremoval of flammable products of pyrolysis by buoyancy. file:///C|/SSB_old_web/mgoppch4.htm (4 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 Furthermore, USML-1 results16 indicate that carbon monoxide production from smoldering at reduced gravity is an order of magnitude greater than on Earth. Fire detection and extinguishment methods will also have to be different at microgravity as opposed to those at 1 g. For example, smoke detectors on Earth are located in ceilings because of buoyancy, as are detectors for hydrogen. Progress is being made in direct numerical simulation of turbulent flames, albeit at fairly low Reynolds numbers (quasi turbulence). These simulations cannot be tested on Earth, however, because of gravitational influences on these low-velocity configurations (i.e., because simulation at low Reynolds numbers introduces buoyancy). The same problem exists with large eddy simulations and second-order closure methods (two methods of approximating turbulent transport). The microgravity environment may be useful to verify theoretical methods because flows may be constructed at low velocities and large size scales free of buoyant effects. SPECIAL INSTRUMENTATION AND FACILITIES REQUIREMENTS As mentioned above, the duration of observation is central to the facilities required if combustion experiments at microgravity are to be conducted. NASA facilities include drop towers offering 2 to 5 seconds of test time, airplanes offering tens of seconds in parabolic flight and also allowing various gravity levels, the shuttle offering hours to days, and the future space station with days to years, each with its unique gravity-level capabilities.17 Most of the information that has been gained in the facilities used to date has been photographic. It has been found, however, that even in drop tower experiments with a sudden impact at the end, lasers can survive. It appears that more sophisticated interrogation equipment may be employed in the future. Advanced laser diagnostics18 have been used in ground applications to generate much fundamental combustion and aerodynamic data and to study the operational behavior of many kinds of practical devices in a nonperturbing manner. The capabilities in this field are driven by advances in the equipment used: (1) improved laser power, higher repetition rates, smaller physical size, and increased electrical-to-optical efficiency; (2) greater multielement detector sensitivity, faster gating capability, smaller pixel size, and larger formats; (3) faster computer speed, smaller physical size, increased data storage capability, and lower power consumption; (4) more choices in fiber-optic characteristics; and (5) advances in spectrograph designs. Based in part on these advances, spectroscopic diagnostics have progressed from laboratory demonstrations to measurements in full-scale test stands. Hardened, mobile apparatus have been successfully developed that, with further evolution, are suitable for flight applications. Available laser powers and detector sensitivities now permit many nonintrusive diagnostics to be performed using one- or two-dimensional imaging configurations. The availability of specific wavelengths, especially those in the ultraviolet, has caused dramatic revivals in the importance of some diagnostics (e.g., excimer-pumped spontaneous Raman scattering). Some of the currently used technology derives from related areas: flight-certified lasers (especially solid-state) from satellite communications; intensified detectors and spectrograph designs from satellite, astronomical, and military imaging; and diode laser/fiber-optic technology from communications and photonics. Much recent work has been directed toward instantaneous imaging in short-duration facilities such as shock tunnels. Recent advances in diagnostic strategies have permitted measurements to file:///C|/SSB_old_web/mgoppch4.htm (5 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 progress from semiquantitative visualization to more quantitative results. These developments have greatly increased the utility of the data obtained for the validation of computational fluid dynamic (CFD) codes. In the absence of buoyancy, the structures of many microgravity flames are exceedingly fragile; the use of physical probes is unacceptably perturbing. Many of the general improvements in equipment choices described above support current and future microgravity applications. The need to work with hardened, mobile apparatus in short- duration, hostile, test cells has paved the way for the drop tower arena. Flight-certified equipment with reasonable size and power requirements is compatible with reduced-gravity aircraft flights. Space-qualified equipment that has been used in satellites is, of course, especially relevant to the space shuttle. To be useful in short-duration, reduced-gravity, ground-based experiments, diagnostic approaches must be at least line imaging. Most microgravity combustion- diagnostic work to date has involved either physical probes or simple and relatively conventional backlighted schemes with standard, high-speed, photographic recording. A diagnostic technique as simple as recording luminosity has provided significant insight into flame structure (e.g., flamefront location). Microgravity research should be increasingly impacted by many of the techniques becoming standard in combustion research, such as the direct, digital recording of fluorescence from flame radicals (e.g., planar laser-induced fluorescence, PLIF). Laser sheet illumination will allow resolution of spatial structures not possible with line-of-sight approaches (e.g., schlieren) that use backlighting. Laser light scattering from soot should help characterize soot formation and from added seeds permit velocity field imaging using methods such as particle image velocimetry (PIV). Multiphase flow phenomena should be especially interesting in microgravity, and many of the droplet/spray diagnostics developed over the last decade should be relevant. RECOMMENDATIONS AND CONCLUSIONS In the field of microgravity research on combustion phenomena, there are several important areas. The following conclusions and recommendations are in order of priority: 1. Microgravity research in combustion is needed because of fire concerns on spacecraft and potential lunar and Mars bases. Implicit here is that variable gravity would be helpful. 2. The needed scientific information concerning the problem of fire involves the subfields of ignition, flammability limits, smoldering, extinguishment, and flame spread. 3. Turbulent combustion research is needed because of its importance on Earth. Reduced gravity would allow a size scale-up without major buoyancy effects, permitting scientific access to the small scales of turbulence that are important to the problem. 4. Laminar premixed and diffusion flames and spray-flow interactions are important on Earth. Experiments should be performed at reduced gravity to allow scale-up in size without complications due to buoyancy. Other recommendations concerning equipment and facilities follow: file:///C|/SSB_old_web/mgoppch4.htm (6 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 An extended orbiting platform capability is important because some combustion phenomena are long-duration events and may require a survey of a wide range of parameters. The demonstration of reproducibility is required and adds to the need for an extended platform. The space available on spacecraft for combustion experiments is generally limited, and many different types of measurements are required. For these reasons, the development of miniaturized diagnostic equipment should be undertaken. REFERENCES 1. Faeth, G.M. 1991. Homogeneous premixed and nonpremixed flames in microgravity: a review. Pp. 281-293 in Proceedings of AIAA/IKI Microgravity Sciences Symposium, American Institute of Aeronautics and Astronautics, Washington, D.C. 2. Law, C.K. 1990. Combustion in Microgravity: Opportunities, Challenges, and Progress. AIAA Paper No. 90-0120. 3. Strahle, W.C., and S.G. Lekoudis. 1985. Evaluation of Data on Simple Turbulent Flames. Technical Report , Air Force Office of Scientific Research. 4. Strahle, W.C., and S.G. Lekoudis. 1985. Evaluation of Data on Simple Turbulent Flames. Technical Report, Air Force Office of Scienctific Research. 5. 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. 6. Strehlow, R.A., K.A. Roe, and B.L. Wherley. 1987. The effect of gravity on premixed flame propagation and extinction in a vertical standard flammability tube. 21st Symposium (International) on Combustion. The Combustion Institute, Pittsburgh. 7. Taylor, G.I. 1950. The instability of liquid surfaces when accelerated in a direction perpendicular to their planes. Proc. R. Soc. London, 201A:192-196. 8. Landau, L.D., and E.M. Lifshitz. 1959. Fluid Mechanics. Pergamon, London. 9. Williams, F.A. 1985. Combustion Theory, 2nd Ed. Benjamin/Cummings, Menlo Park, Calif. 10. Ronney, P.D. 1988. Effect of chemistry and transport properties on near-limit flames in microgravity. Combust. Sci. Technol., 59:123. 11. Edelman, R.B., and M.Y. Bahadori. 1987. Effects of buoyancy on gas jet diffusion flames, experiment and theory. Acta Astronautica, 13:681. file:///C|/SSB_old_web/mgoppch4.htm (7 of 10) [6/18/2004 11:16:56 AM]
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Microgravity Research Opportunities for the 1990s: Chapter 4 12. 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. 13. Olson, S.L., P. Ferkul, and J. T'ien. 1989. Near-limit flame spread over a thin solid fuel in microgravity. 22nd Symposium (International) on Combustion. The Combustion Institute, Pittsburgh. 14. Bhattacharjee, S., R.A. Altenkirch, S.L. Olson, and R.G. Sotos. 1988. Heat transfer to a thin solid combustible in flame spreading at microgravity. Eastern States Section of the Combustion Institute, Pittsburgh. 15. Ramachandra, P.A., et al. 1994. The behavior of flames spreading over thin solids in microgravity. Twenty-Fifth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh. 16. Stocker, D.P., S.L. Olson, J.L. Torero, and A.C. Fernandez-Pello. 1993. Microgravity smoldering combustion on the USML-1 Space Shuttle mission. Heat Transfer in Microgravity, Vol. 269, C.T. Avedisian and V.A. Arpaci, eds. ASME Heat Transfer Division. 17. NASA. 1989. Microgravity Combustion Science: A Program Overview. NASA TM 101424. 18. The following four paragraphs were provided by Gregory Dobbs of United Technologies Research Center. PLATE 4.1 Combustion of a gas-jet (propane) flame in quiescent atmosphere of air under normal gravity and microgravity conditions (the latter in a drop-tower test at NASA's Lewis Research Center). Due to the buoyancy effects under normal gravity conditions, the flame file:///C|/SSB_old_web/mgoppch4.htm (8 of 10) [6/18/2004 11:16:56 AM]