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~2 Combustion Research Program A major fire in space resulting in any type of loss of mission will not be excused by the public and will lead to decades of setback for all kinds of space research and space applications. (p. 34) INTRODUCTION The NASA microgravity combustion research program has always been driven by two objectives: a desire to understand the physical phenomena thought to be relevant for spacecraft fire safety and a wish to deepen our knowledge of fundamental combustion processes on Earth (Sacksteder, 1990; Ross, 1993; Urban and King, 1999; Faeth, 2001~. The absence of any safe refuge in space makes the prevention or containment of small fires a subject of critical importance to NASA, and fire safety will remain critical as long as a human presence in space remains part of NASA's activities. Two of the three requirements needed to initiate a fire an oxygen-containing environment and an energy source for ignition are basic elements of any life support system. Elimination of the third element, a combustible fuel, has been a focus of both microgravity research and space vehicle design for decades. However, it remains an elusive goal, because almost all materials are combustible in the presence of oxygen at sufficiently high temperatures (for example, an aluminum storage canister burning with pure oxygen initiated the Mir fire). Moreover, all manned spaceflights have embarked with large amounts of even more readily ignitable materials. Microgravity is useful for the study of fundamental combustion processes on Earth because the most basic combustion phenomenon is the release of heat. The resulting density changes induced in the reacting gases on Earth generate buoyancy forces that strongly influence the mixing and transport of the reactants at all but the smallest scales of motion. Much of what one wants to understand about combustion takes place precisely at these small scales, which are often inaccessible to measurements on Earth. Moreover, buoyancy makes combustion phenomena extremely complex. The study of buoy- ancy-driven flows is an important field in its own right. Fluid convection induced by mechanically controlled pressure differences and buoyancy forces introduced by the thermal energy released by combustion together with radiative transport of part of that energy dominate the transport of mass, 28
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COMBUSTION RESEARCH PROGRAM 29 momentum, and energy in most earthbound combustion scenarios (e.g., power generation, furnaces, fires). The chemical reactions usually occur at the smallest length scales. These length scales are typically set by a balance between the rate at which chemical reactions liberate energy and transform mass, and the molecular diffusion that transports the reactants into and energy away from the thin flame zones where combustion chemistry occurs. This transport is highly localized and is only effective if the larger-scale processes have provided the immediate vicinity of the flame with an adequate supply of reactants and an appropriate thermal environment. The large density changes in flames induced by the release of chemical energy cause significant buoyancy-induced motion, impeding studies of combustion phenomena on Earth (Faeth, 2001~. As a result, much of the basic theory in this field has been developed under the assumption that the effects of gravity can be ignored at the smallest scales relevant to combustion, even though most earthbound experiments do involve effects of larger-scale, buoyancy- induced motions. This has made the inevitable conflict between simplified theories and earthbound experiments extremely difficult to resolve. The most important feature of the current NASA combustion program, then, is access to a micro- gravity environment. The negligible buoyancy in such an environment means that many small-scale effects, which are often masked on Earth (Sacksteder, 1990), can be probed in space. In addition, there are many practical situations that occur on a scale too small for buoyancy to normally be a factor. However, when these are simulated on a larger scale on Earth, buoyancy then becomes important, thereby modifying the physics. The microgravity environment allows this scale-up without introducing buoyancy, so that similitude is maintained and the measurement and resolution are improved. If the microgravity time requirements are not too demanding, the NASA 2-second and 5-second drop towers are also available. Special-purpose aircraft flying trajectories designed to provide a reduced gravity environment can also be used if the level and the steadiness of the microgravity required for the experiment are not too demanding. A useful description of how these facilities achieve microgravity is given by Ross (2001~. Many of the combustion processes have been found to require long times to reach a steady state from the time of ignition under microgravity conditions (Ross, 2001~. Flames that are ignited in normal gravity and then subjected to microgravity behave very differently from those ignited under microgravity. The sooting tendency of flames in limited-duration microgravity experiments was found to be vastly different from that of flames stabilized in extended-duration microgravity on the space shuttle (Faeth, 2001~. Many of the combustion processes, such as flame balls, are extremely sensitive to fluctuations in the microgravity levels, termed "jitter. For example, the flame balls observed in space shuttle experi- ments became unstable when low-power steering rockets fired and changed acceleration levels from 10 - to 10-3 times Earth gravity (Ronney, 1998~. Finally, studies of turbulent combustion phenomena require that the experiment be repeated often enough to draw statistical inferences (Cheng et al., 1999~. The time required for accumulation of these data is simply not available in any Earth-based microgravity facilities. For all these reasons, progress in combustion studies critically depends on the availability of the Combustion Integrated Rack facilities on the ISS. -A History and Current Program The first microgravity experiments were conducted by Kumagai and coworkers (Kumagai and Isoda, 1957) in Japan; they were interested in experimentally confirming the theoretical burning rates predicted for the combustion of small spherical droplets. NASA conducted its first combustion experi- meets in space on Skylab in 1974 (Kimzey, 1974~; the experiments focused on the flammability of solid materials commonly used in spacecraft (Urban and King, 1999~. By 1990, enough progress had been
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30 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA made to warrant an invited minireview of the emerging field of microgravity research by the Combus- tion Institute, the international scientific organization devoted to the entire field of combustion science (Sacksteder, 1990~. There had been no follow-up to the early Skylab experiments, and virtually all of the 42 citations in this paper were to ground-based experiments, theoretical analyses, or plans for future experiments in space. However, enough progress had been made to identify several distinct categories of microgravity research premixed gases, diffusion flames, droplet combustion, flame spread over liquid and solid fuels, and smoldering combustion that have persisted to the present day. By 1992 the situation had changed dramatically. The first NASA Research Announcement (NRA) had been issued in 1989, resulting in 13 definition study awards (ground- and aircraft-based experiments and analyses intended to establish the need for and viability of space-based experimentation) and 6 flight experiment candidacy awards (Ross, 1993~. Moreover, 4 space experiments had been completed, and a total of 24 microgravity combustion and spacecraft fire safety projects were being supported. The visibility of microgravity combustion research had advanced by 1994 to the point that an entire session of the Twenty-Fifth International Symposium on Combustion was devoted to this subject (Combustion Institute, 1994~. The expansion of the NASA program was quite rapid over the next few years. At the time of the Fourth International Microgravity Combustion Workshop, in 1997, the Microgravity Com- bustion Science Program supported 48 ground-based studies, 7 flight definition studies, and 11 space- flight experiments, among the total of 73 supported programs (King, 1997~. Since the late 1990s, combustion in microgravity has been accepted as an important component of basic research in most branches of combustion science. The Twenty-Seventh International Symposium on Combustion, held in 1998, provided three distinct measures of the impact this research has had on the field. First, the invited Plenary Lecture (Ronney, 1998), the most prestigious forum offered to the international combustion community, was devoted to microgravity research. The growth of the field in less than a decade is evident from the 115 references in the written version of that plenary lecture, which covered the topics considered in the earlier review (Sacksteder, 1990), including several experiments performed on the space shuttle. Second, the sessions on microgravity combustion contained 23 of the 367 papers accepted for presentation at the symposium (Olson et al., 1998~. Such presentations are very competitive; fewer than half the papers submitted are published in the proceedings. Third, a special presentation by G. Linteris, "Fire in Space," recounted his experience as the combustion science payload specialist on space missions STS-83 and STS-94. A summary of that talk (Linteris et al., 1999) can be found in the special issue of Combustion and Flame (1999) devoted to microgravity combustion re- search supported by NASA. The years 1998-2000 probably represent a high-water mark for the program. The Twenty-Eighth Symposium, held in 2000 (Combustion Institute, 2000), did not break out microgravity combustion as a separate entity but integrated papers on the topic into their appropriate technical sessions. The total 1 1 1 ~ 1 ~ , ~ ~ ~ ~ _ . number of papers Involving microgravity research was similar to that In the previous symposium. Microgravity Combustion: Fire in Free Fall (Ross, 2001) represents the first attempt to produce a volume that could serve as an advanced textbook devoted to microgravity research. It is current through 1999, with occasional citations to later work. It is interesting to note that nearly 80 percent of the book is concerned with the topics discussed by Sacksteder in his 1990 review (Sacksteder, 1990~. The Fifth International Microgravity Workshop, held in 1999, was marked by optimism about the immediate future. The National Center for Microgravity Research in Fluids and Combustion had recently been opened, a record number of new microgravity combustion experiments had been funded following the most recent NRA, and 120 papers were presented either orally or as posters. NASA was focused on the forthcoming ISS and the Combustion Integrated Rack (CIR). "The CIR is scheduled for launch on UF- 3 in October, 2002 and will begin its scientific work immediately" (King, 1999~.
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COMBUSTION RESEARCH PROGRAM 3 The year 2001 brought major uncertainty to the future of the ISS in general and the microgravity combustion research program in particular. The Sixth International Microgravity Workshop took place "at a time when the role of combustion research supporting NASA's future missions and our ongoing contributions to fundamental science are being re-examined" (Sacksteder, 2001~. The workshop pro- ceedings describe 117 investigations, demonstrating again the vitality of the research program. Unfor- tunately, the workshop took place at a time when the future of the CIR, intended as the laboratory for future microgravity combustion experiments, was in doubt. Although some additional funding for the CIR has been secured, its final status remains uncertain. The original 2002 operational date has slipped by several years. Moreover, as the report of the International Space Station Management and Cost Evaluation Task Force noted, " The existing ISS Program Plan for executing the FY 02-06 budget is not credible" (Young, 2001~. While the final consequences of this report are unclear, the current NRA explicitly states: "Due to severe resource limitations, we do not plan to make flight definition awards in the combustion area from this NRA" (NASA, 2001~. Whether this is a temporary setback or the beginning of the end of the microgravity combustion program remains to be seen. IMPACT OF NASA COMBUSTION RESEARCH The topics that have received extensive coverage in NASA's microgravity combustion research program include the following: · Flame spread on thin and thick solids, · Smoldering combustion, · Jet flame lengths and shapes, · Turbulent flames, · Soot and radiation, · Flame balls, and · Droplet combustion and chemical kinetics. The NASA microgravity combustion research program supported approximately 70 investigators in FY 2001. Of these NASA investigators, 5 are members of the National Academy of Engineering, 10 are fellows of the American Institute of Aeronautics and Astronautics, 5 are fellows of the American Physical Society, and 4 are fellows of the American Society of Mechanical Engineers. Two of the NASA combustion principal investigators (PIN have received the distinction of being among the 100 most cited engineers in the world. The microgravity combustion research conducted over the last 10 to 12 years has contributed to our fundamental knowledge of some of the most basic combustion phenomena, to the improvement of fire safety on present and future space missions, and to the advancement of knowledge about some of the most important practical problems in combustion on Earth. Substantial progress has been made in spite of the very challenging environment, the limited time available for experimentation, and the serious limits on the types of measurements that can be made. Flame spread over thick and thin solids is a fundamental problem in combustion and is very relevant to fire safety in space and on Earth. Experiments conducted in microgravity facilities on Earth and particularly in space provided very-high-quality data for flame spread rates because there were no significant buoyancy-induced fluctuations. These data have led to the validation of computer models of the ignition and flame spread processes (Mell and Kashiwagi, 2000; Mell et al., 2000~. Such validation
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32 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA is helping researchers to advance with renewed confidence these models for flame spread in accidental fires on Earth. Experiments on the flammability of thin and thick solids and also of hydrocarbon fuel droplets at varying oxygen concentrations in microgravity and normal gravity showed that microgravity flames are stable at lower oxygen concentrations. For example, the flammability for a vertical thin cellulosic tissue increased under microgravity, and combustion was possible at 13 to 14 percent oxygen concentration instead of the 15.6 percent oxygen concentration required at normal gravity. Recent studies of flame spread over thermally thick solids (those for which the thermal penetration depth is much less than the thickness of the material for the duration of the experiment) in microgravity have shown that such flames are unstable and eventually extinguish themselves because of the excessive heat loss away from the combustion zone (Altenkirch et al., 1998~. If this finding can be verified for a large range of materials and if the damage that occurs during the unstable combustion portion is minimal, then use of thermally thick solids may turn out to be a part of the fire safety strategy. However, additional work is necessary before the practical benefits of this research can be realized. More recent work in flame spread over solids is considering the effects of sample widths and edge effects. Such work is necessary from the point of view of practical design as well as for assessing the generality of the findings achieved with finite-width samples with edges. Work has also been initiated on fire spread on more general shapes such as cylinders (as opposed to flat plates and paperlike objects). Experiments on smoldering in microgravity, on the other hand, show that the process is slower than in normal gravity. In addition, the microgravity smolder is even less bright and less detectable than that on Earth. This makes the fire safety problem in space even more challenging because the slow-prowinp O—- —--A smolder can go undetected for a longer time and when it breaks into a fire, the fire will propagate faster. Experiments on hot surface ignition in microgravity show shorter ignition delays, reinforcing the need for improved fire safety. In experiments of very practical significance to spacecraft fire safety, it was found that flames over electric wire insulations spread 30 to 50 percent faster in microgravity than in normal gravity (Kikuchi et al., 1998~. Overheated electric wire insulation is one of the more common sources of accidental fires on Earth and will continue to be one in space. This hazard arises when the loss of energy from the flame region to the ambient is reduced by the lack of natural convection. An understanding of this effect is critical to maintaining spacecraft fire safety. The experimental finding from the research program concerning enhanced spread rates needs to be accommodated in material screening and approval tests. Such test apparatus should mimic a microgravity environment in Earth-based material-testing facilities. The microgravity combustion research program has already had a significant impact on space shuttle and ISS fire safety and fire-fighting procedures (Pedley, 2001~. Based on the microgravity combustion research finding that even weak ventilation flows lead to rapid fire spread in microgravity, it has now become standard practice to shut off ventilation flows if a fire starts in a module, rather than to deploy an extinguisher. The findings on the effects of oxygen enrichment on flammability limits have led to the minimization of the enrichment levels as much as possible. The engineers responsible for fire safety on the space shuttle and the ISS at the Johnson Space Center recognize that the microgravity combustion research program has shown many counterintuitive fire behaviors. They also recognize that the margin of safety based on normal gravity material testing is significantly greater than that in microgravity. However, significant further work is necessary to develop a real understanding of fire and fire suppression in space habitats, as discussed in the next section. Experiments with both nonluminous and luminous jet flames have shown that flame lengths in- crease significantly in microgravity. Significant improvements in the models were needed to achieve agreement with the experimental data for a range of fuels. The Froude number (the ratio of buoyancy to
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COMBUSTION RESEARCH PROGRAM 33 pressure forces) was established as the correct parameter for correlating the lengths of buoyant flames. This is an interesting development, particularly because of the seminal contribution of Burke and Schumann, who were able to predict flame lengths with an analytical expression derived by ignoring buoyancy that coincidentally provided good agreement with limited data. In fundamental science, correction of a theory that seems to have been accepted for a few decades is challenging and is a particularly significant accomplishment of the microgravity program. The results of the NASA microgravity research program concerning flame length variation with flow rate have been incorporated into newer undergraduate-level textbooks (Turns, 2000~. The older textbooks erroneously stated that the flame length increased with increasing flow rates and then became constant for high Reynolds number (ratio of inertial to viscous forces) turbulent flames. The data and discussion presented in Turns (2000) clearly show that the reality is more complicated. Flame lengths increase at different rates in microgravity and normal gravity, and the microgravity flames are much longer than the normal gravity flames at low and intermediate Reynolds numbers. However, the surprising feature of the data is that even at the highest Reynolds numbers at which extinction is observed, the flame length in microgravity continues to be much larger than that for flames in normal gravity. Furthermore, this holds for all known fuels. Theoretically, the two flame lengths should reach a common asymptotic value, but conditions appropriate for this asymptote have never been realized experimentally. Soot production and emission by combustors continues to be a major environmental problem for Earth-based applications such as diesel engines. Soot consists of primary 20- to 100-nanometer- diameter, mostly carbon particles that generally agglomerate into larger fractal structures. Soot produc- tion and oxidation mechanisms have been developed and validated using microgravity combustion data. Soot contains carbon particles that are a significant health risk to many urban populations in the United States and the world. Methods of controlling soot emissions developed based on a knowledge of their kinetics would be of significant value to human health here on Earth. The microgravity studies of sooty flames provided many surprises. The primary carbon particles grown in microgravity are much larger than those grown in normal gravity under similar conditions. Further, the microgravity agglomerates were also larger than those in normal gravity, possibly because of the longer residence times resulting from the long flame lengths. The agglomeration process is enhanced in microgravity, leading to very large blobs of soot. This result provided data on sooting tendencies that are being used to improve understanding of chemical kinetic mechanisms. The soot behavior in microgravity suggested numerous strategies for soot control (for example, retarded fuel velocity) and also improved experimental design for soot studies. For example, a flat flame premixed burner with a stabilization disk and interrogation along the axis is providing data for formation and oxidation rates of soot in very controlled environments, leading to significant improvements in the chemical kinetic rate models (Sunderland and Faeth, 2001; Sunderland et al., 1995~. ~ ~ ~ . ~ . .. . ~ . ~ ~ . ~ . . ^. . ~ ~ ~ . .. ~ . . ~ Many of the microgravity stuccoes nave shown the s~gn~cant role of rac`~at~on neat transfer in combustion phenomena (Bedir et al., 1997; Ju et al., 1998~. This role was usually ignored in the past in applications other than fire research. An improved awareness of the radiation coupling with chemistry 1 1 1 1 ~7 ~ and iluld mechanics in many combustion appllcahons IS a slgmilcant outcome of the microgravity combustion program. Other fundamental contributions have been made as a result of experimental studies involving the first-ever stabilization of flame balls, which are relatively stable spherical surfaces across which reac- tants become products (Ronney, 1998~. This phenomenon, which was only theoretically predicted before the microgravity experiments, is novel in that unlike propagating spherical flames, the flame ball surface remains stationary, separating reactants and products for as long as the experiment could be
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34 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA conducted on the space shuttle. This work was the highlight of Ronney's plenary lecture (Ronney, 1998~. This confirmation of simple and elegant theories by experiment is something NASA strives for. The existence of such a confirmation from a practical science such as combustion is very satisfying. Modern combustion equipment is designed using computational methods that require chemical kinetics information for liquid hydrocarbons. The industries that design aircraft combustors, such as General Electric Aircraft Engines (Mongia, 2002), are utilizing the chemical kinetics mechanisms developed by NASA microgravity combustion PIs like Dryer and coworkers (Marchese et al., 1999; Manzello et al., 2000~. These mechanisms are also being used by diesel engine manufacturers working in collaboration with the Department of Energy in a Cooperative Research and Development Agreement (CRADA) team. The chemical kinetics mechanisms being continuously upgraded using the NASA microgravity combustion data are used in the latest combustion textbooks (Turns, 2000~. The high-quality droplet combustion data not only contributed to the evaluation of the ]2 law (the regression rate of the burning droplet surface area is constant) but also led to the validation of detailed chemistry mechanisms for practical hydrocarbon fuels. The lack of understanding of the detailed chemistry mechanisms for practical fuels is one of the critical problems hampering the computer-aided optimization of combustion devices on Earth. The chemical mechanisms validated in the microgravity program by Dryer and coworkers (Marchese et al., 1999; Manzello et al., 2000) are of significant value in this effort. More recent work in droplet combustion is looking at fuel mixtures and the resulting partial distillation and its effects on vaporization and combustion. This topic is important for the use of alternative fuels in automobile engines, which could enhance cold start capabilities and reduce hydro- carbon pollutant emissions. ~ by, ~ am. . . . . . .. . . . . .. . FUTURE DIRECTIONS IN COMBUSTION RESEARCH Fire Safety Clearly, combustion research aimed at answering the very practical questions for which NASA engineers are currently seeking answers is needed soon. To remedy the paucity of knowledge about fire safety on the ISS, the combustion research necessary for completing the risk assessment project to satisfaction must receive the highest priority at all levels. A major fire in space resulting in any type of loss of mission will not be excused by the public and will lead to decades of setback for all kinds of space research and space applications. The fire danger and research need are very real based on the cited communications (Pedley, 2001) between engineers responsible for fire safety of the ISS. Fire safety onboard the ISS needs to be and will be one of the most important cornerstones of the future combustion program. It is important because fires in space are different in many respects from fires on Earth. The first difference is that they can be more challenging to extinguish and harder to detect, as revealed by the ongoing combustion research experiments discussed above. The second difference is that they can be disastrous to a very-high-value, high-visibility project. The third and most important difference is that there are almost no escape routes. Therefore, fire safety should be the highest priority in the short-term (perhaps 3 to 5 years) future. The program should strive for establish- ing special screening procedures and tests for space materials, and should account for the differences in detection, fire spread, and oxygen environments. Further, it should recognize that all the catastrophic and near-catastrophic fire events in the history of the U.S. and international space programs (Apollo 1, Apollo 13, and Mir) involved oxygen-supported fires. Finally, a physics-based computer simulation of fire development and suppression in space should form a backbone for future fire-safety-related re- search and design. ~ 1 ~ . · . · . hi,
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COMBUSTION RESEARCH PROGRAM 35 Much of current spacecraft fire safety design is based on risk assessment methodologies developed by the insurance industry over many decades of experience in successfully protecting and occasionally paying for high-value real estate. Data on expected fire hazards, fire behavior, and fire safety equipment function and effectiveness in space are, however, unavailable. Further, Earth-based fire protection techniques have evolved using thousands of years of fire-fighting experience. Obviously, there is no such experience base for space fires. Physics-based computer simulations are the only alternative. Indeed, even for Earth-based fires, such simulations have been of great value in assessing fire safety and control strategies. The recommended directions for fire safety research for the ISS and other space habitats are summarized below. Development of Computer Simulation of Fire Dynamics on Spacecraft The development of fire simulation computer codes based on computational fluid dynamics (CFD) techniques has been a milestone for fire safety research on Earth. In addition to documenting the progress made in understanding fire phenomena, such codes have been used to investigate specific fires (Madryzkowski and Vettori, 2000; Simcox et al., 1992~. They are also routinely used to evaluate smoke movement in buildings and are beginning to be used to evaluate the performance of fire protection systems (McGrattan et al., 2000~. One measure of the impact of these codes on fire research on Earth is the fact that the latest version of the Fire Dynamics Simulator (FDS), developed at the National Institute of Standards and Technology (McGrattan et al., 2001), has been downloaded over 4,000 times since its release and, together with its predecessor, is currently in worldwide use. The success of FDS is based on years of submodel development and validation with a variety of Earth-based basic experiments and applications to fire scenarios. The submodels involve processes that are affected significantly by gravity. Neither FDS nor any other CFD-based computer program written for fire research can currently be used to simulate fire scenarios on the ISS or any other vehicle designed to operate in a microgravity environment. The effects of buoyancy on combustion and convective transport on Earth so dominate fire spread, smoke transport, and suppression techniques that a research and development effort on modeling microgravity fires is needed. Experience with CFD code development in general and FDS development in particular has demonstrated that the creation of a code that simulates microgravity fire will involve many researchers for several years. Given that NASA is not soliciting flight experiments in combustion in its latest NRA, a modeling initiative would be the primary way to advance our knowledge of fire safety in space. A simulation code would also guide the choice of the most effective future space experiments. Research on Ignition, Flame Spread, and Screening Techniques for Engineering Materials in a Microgravity Environment The goal of research on ignition, flame spread, and screening techniques for engineering materials in a microgravity environment is to develop a science-based method for determining the fire perfor- mance of materials that are candidates for use in space. At present, NASA is supporting studies aimed at the development of screening tests in both Earth gravity (Olson et al., 2001) and microgravity (Fernandez-Pello et al., 2001~. However, this research needs to be supplemented with further studies that consider nonplanar material configurations and a wider variety of test materials. The results of an enhanced research program would also be directly usable in the space fire simulation codes described
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36 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA above. Since the flame spread phenomena take place on length scales much smaller than those associ- ated with the spacecraft geometry, having a body of knowledge that relates mass generation rates of combustible materials to the history of the heat flux incident upon the material and the local oxygen environment is a necessary ingredient of a macroscopic fire spread model. This information must be obtained as a function of the material studied and its local shape. The two programs, taken together, would provide a major advance in the understanding of fires in space and in the ability to ameliorate their consequences. Oxygen Systems Fire Safety One of the critical systems on the ISS and other space, lunar, and planetary habitats is the oxygen generation and handling system. Combustion with pure oxygen reaches temperatures capable of turning most materials into fuels, with the aluminum canister consumed in the Mir fire being a case in point. A systematic study of ignition and spreading of fires involving an oxygen source such as a jet formed by a leak or initiation of an internal reaction involving failure of an inert coating needs to be undertaken to better protect against such disasters. Ignition of an initial oxygen fire, flame spread by radiation from the ultrahigh-temperature initial ignition source, and special considerations for the extinguishment of fires involving oxygen are research topics requiring experimental and computational attention. Smoldering Combustion in many accident scenarios, such as chaffed electric wire insulation, combustible materials are heated to the point of generating toxic and combustible vapors. The generation of these vapors at temperatures too low for flaming combustion is called smoldering. Smoldering often occurs in areas hidden from view, making detection more difficult. The availability of oxygen and an ignition source can often force a transition of smoldering into flaming. Smoldering in Earth's gravity involves removal of the fuel vapors and the energy released during their generation by buoyancy. This means that smoldering and transition to flaming combustion in microgravity will be significantly different and must be studied further. Basic Combustion Research Soot and Radiation While combustion has been studied for hundreds of years, the limits of clean combustion technol- ogy are illustrated by large black billows of carbon particles leaving smokestacks and rigs pulling out of truck stops. The inhalation of such particles has been shown to significantly increase levels of morbidity in polluted areas. The basic processes that lead to the formation and emission of small carbon particles in high-temperature combustors remain to be understood. For example, although the transformation of small hydrocarbon molecules into larger polyaromatic hydrocarbons (PAHs) has recently been formu- lated, the detailed mechanisms of the phase change of such PAHs into nascent solid particles are unknown. Similarly, why the solid particles once formed are not fully burnt by the available oxygen remains an unanswered question. Microgravity flames were found to produce significantly larger quantities of soot particles than Earth-based flames with the same operating conditions. This is because of the longer residence times
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COMBUSTION RESEARCH PROGRAM 37 available for the inception and growth processes (which, incidentally, also provide an opportunity for more detailed studies of the processes). Soot particles in flames radiate energy to the surroundings, dominating the heat transfer from large fires and in furnaces. The formation and oxidation processes of soot particles are intimately tied to their temperature, which is affected by the radiation. This means that studies of radiation cannot be decoupled from those of soot formation and oxidation. Radiation heat fluxes from microgravity flames have been measured to be many times higher than those from Earth-based flames of the same power. In short, radiation studies in space environments are critical. Radiation heat transfer has many implications for the fire safety problem discussed above, including the effects of a highly absorbing carbon dioxide fire suppressant deployed on the ISS, flame spread by preheating, and transition from smoldering to flaming. Turbulent Combustion Most industrial combustion devices and natural fires involve turbulent combustion. Turbulent flows generally involve a balance between high inertial forces, pressure gradients, and gravity forces. In most propulsion devices, the pressure gradient forces are much larger than the gravity forces. However, in power plants, material processing, and home heating furnaces and in natural fires, the pressure gradients and gravity forces are comparable. In this regime of turbulent combustion, a large number of applications are affected by gravity and deserve consideration in the program. Some of these applica- tions, such as furnaces and fires, involve turbulent non-premixed flames while others, such as low- emission, lean-burning furnaces, involve turbulent premixed combustion. Turbulence in general and turbulence in the presence of combustion are exceedingly difficult phenomena that have defied generations of researchers. However, even incremental progress in these difficult areas could have a significant impact on industry, NASA, and the basic sciences. As noted above, the expected similarity between earthbound and microgravity turbulent non-premixed jet flames at high Reynolds numbers was not observed. The reasons for this discrepancy are unknown. The importance of flame size to many earthbound applications underscores the need for improved under- standing. Some recent studies on premixed turbulent flames indicate that the length scales that are affected by the instabilities are significantly different for normal flames and microgravity flames. Length scales in turbulent premixed flames determine the burning rate enhancement, so their improved characterization, including that of the effect of buoyancy, is necessary. Chemical Kinetics Chemical kinetics and reaction mechanisms for practical fuels and fuel blends of interest to industry remain unknown. The chemical rates are determined by basic molecular collision rates, velocities, and stearic effects (orientations of the molecules at the time of collision). In normal gravity these processes are often studied in a complex flow environment. Microgravity allows the design of a reacting system (for example, the droplet flame) to be probed in a simpler, one-dimensional flow. Accordingly, microgravity studies of chemical kinetics using droplet flames helped to establish a chemical mecha- nism for the oxidation of simple hydrocarbon fuels, as discussed earlier. However, similar progress for realistic fuels and fuel blends is necessary.
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38 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Nanomaterial Synthesis in Flames Flames provide an inexpensive means of producing nanoparticles for mass use. Indeed, the soot particles discussed above are carbon nanoparticles. More recent experiments have shown the possibili- ties for manufacturing more exotic forms of carbon, such as diamonds, buckey balls, and nanotubes, as well as other nanomaterials. The work to date has generally been empirical, and opportunities exist for understanding the chemical composition and thermal structure of the flow that is conducive to synthesis of the desired forms of materials. This is discussed further in Chapter 7. REFERENCES Altenkirch, R.A., Tang, L., Sacksteder, K., Bhattacharjee, S., and Delichatsios, M.A. 1998. Inherently unsteady flame spread to extinction over thick fuels in microgravity. Pp. 2515-2524 in Twenty-Seventh Symposium (International) on Combus- tion. Combustion Institute, Pittsburgh, Pa. Bedir, H., T'ien, J.S., and Lee, H.S. 1997. Comparison of different radiation treatments for a one-dimensional diffusion flame. Combustion Theory and Modeling 1: 395-404. Cheng, R.K., Bedat, B., and Kostiuk, L.W. 1999. Effects of buoyancy on lean premixed v-flames part I: Laminar and turbulent flame structures. Combust. Flame 116: 360-375. Combustion Institute. 1994. Twenty-Fifth Symposium (International) on Combustion. University of California, Irvine, July 31-August 5, 1994. Combustion Institute, Pittsburgh, Pa. Combustion Institute. 2000. Twenty-Eighth Symposium (International) on Combustion. University of Edinburgh, Scotland, July 30-August 4, 2000. Combustion Institute, Pittsburgh, Pa. Faeth, G.M. 2001. Laminar and turbulent gaseous diffusion flames. Pp. 83-182 in Microgravity Combustion: Fire in Free Fall (H.D. Ross, end. Academic Press, San Diego. Fernandez-Pello, A.C., Torero, J.L., Zhou, Y.Y., Walther, D., and Ross, H.D. 2001. Theoretical prediction of microgravity ignition delay of polymeric fuels in low velocity flows. Pp. 85-88 in Sixth International Microgravity Combustion Workshop. NASA/CP-2001-210826. National Aeronautics and Space Administration, Washington, D.C. Ju, Y., Masuya, G., Liu, F., Guo, H., Maruta, K., and Niioka, T. 1998. Further examinations on extinction and bifurcations of radiative CH4/air and C3Hg/air premixed flames. Pp. 2551 -2557 in Twenty-Seventh Symposium (International) on Com- bustion. Combustion Institute, Pittsburgh, Pa. Kikuchi, M., Fujita, O., Ito, K., Sato, A., and Sakuraya, T. 1998. Experimental study on flame spread over wire insulation in microgravity. Pp. 2507-2514 in Twenty-Seventh Symposium (International) on Combustion. Combustion Institute, Pitts- burgh, Pa. Kimzey, H. 1974. Skylab experiment M479 zero gravity flammability. Pp. 115-130 in Proceedings of the Third Space Processing Symposium on Skylab Results, Vol. 1. NASA TM X-70252. National Aeronautics and Space Administration, Washington, D.C. King, M.K. 1997. NASA microgravity combustion science program. Pp. 3-10 in Fourth International Microgravity Combus- tion Workshop: Proceedings of a Workshop. NASA Conference Publication No. 10194. National Aeronautics and Space Administration, Washington, D.C. King, M.K. 1999. NASA microgravity combustion science program. Fifth International Microgravity Combustion Workshop: Proceedings of a Workshop. NASA/CP-1999-208917. National Aeronautics and Space Administration, Washington, D.C. Kumagai, S., and Isoda, H. 1957. Combustion of fuel droplets in a falling chamber. Pp. 726-731 in Sixth Symposium (International) on Combustion. Combustion Institute, Pittsburgh, Pa. Linteris, G.T., Voss, J., and Crouch, R. 1999. Combustion experiments on STS-83 and STS-94: The crew's perspective. Combust. Flame 116~3~: 321-322. Madryzkowski, D., and Vettori, R.I. 2000. Simulation of the dynamics of the fire at 3146 Cherry Road NE, Washington D.C., May 30, 1999. NIST Report NISTIR 6510. National Institute of Standards and Technology, Gaithersburg, Md. Manzello, S.L., Choi, M.Y., Kazakov, A., Dryer, F.L., Dobashi, R., and Hirano, T. 2000. The burning of large n-heptane droplets in microgravity. Proceedings of the Combustion Institute 28: 1079-1086. Combustion Institute, Pittsburgh, Pa. Marchese, A.J., Dryer, F.L., and Nayagam, V. 1999. Numerical modeling of isolated n-alkane droplet flames: Initial compari- son with ground and space-based microgravity experiments. Combust. Flame 116: 432-459.
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COMBUSTION RESEARCH PROGRAM 39
Representative terms from entire chapter: