Microgravity Combustion Science: 1995 Program Update (original) (raw)

Microgravity greatly benefits the study of fundamental combustion processes. In this environment, buoyancy-induced flow is nearly eliminated, weak or normally obscured forces and flows can be isolated, gravitational settling or sedimentation is nearly eliminated, and temporal and spatial scales can be expanded. This document reviews the state of knowledge in microgravity combustion science, with the emphasis on NASA-sponsored developments in the current period of 1992 to early 1995. The scope includes basic research in gaseous premixed and diffusion-flame systems, flame structure and sooting, liquid droplets and pools, and solid-surface ignition and flame spread. Other subjects include applied research in combustion synthesis of ceramic-metal composites, advanced diagnostic instrumentation, and on-orbit fire safety. The review also describes the opportunities for Principal Investigator participation through the NASA Research Announcement program and the NASA Lewis Research Center ground-based facilities available to researchers. This review is compiled by the members and associates of the NASA Lewis Microgravity Combustion Branch. Highlights of Changes and Key Accomplishments in Microgravity Combustion Science Since the Last Review (NASA TM-105410, April 1992) Ground-Based (Drop Tower and Airplane) Experiments and Analyses • Dilution-enhanced flammability ("flame-balls") was observed in certain premixed gaseous systems in microgravity at high-dilution levels where normally flames do not exist. successful flight in a "Glovebox" facility on the Shuttle in less than three years. The experiments not only yielded interesting results but also demonstrated the practicality of the Glovebox accommodation for small-scale experiments on space laboratories. • Three large-scale experiments (Droplet Combustion Experiment, Laminar Soot Processes, and Structure of Flame Balls at Low Lewis Number), two moderate-scale experiments (Diffusive and Radiative Transport in Fires and Microgravity Smoldering Combustion), and five Glovebox experiments (Candle Flames in Microgravity, Comparative Soot Diagnostics, Fiber-Supported Droplet Combustion, Forced Flow Flamespreading Test, and Radiative Ignition and Transition to Spread) were authorized to proceed to flight development for near-future missions. • An archive for imaging and digital data and publications was established at the NASA Lewis Research Center to provide a central repository of spaceflight combustion-science information. Microgravity Combustion Diagnostics and Technology • A new method, based on light extinction of a single laser beam, was developed for soot volume-fraction measurements over a two-dimensional field. • A variety of advanced diagnostic techniques, including infrared and ultraviolet imaging, two-dimensional light extinction, particle-image velocimetry, two-wavelength soot pyrometry, and spectral and broadband radiometry, performed successfully on sounding rockets and demonstrated their feasibility for the Shuttle. • A reliable technology to deploy nearly-motionless fuel droplets without tethered supports in microgravity is now in practice in ground-based drop towers. Spacecraft Fire Safety An experiment that evaluates, for the first time, the response of the Shuttle and Space Station smoke detectors to model fires in microgravity is approved for a Shuttle flight in 1996. Ground-Based Facilities • The NASA Lewis Research Center drop-tower facilities are at an all-time high utilization. For experimenters, the 2.2-Sec Drop Tower is available on an extended-hour operation basis, and the 5.2-Sec Zero Gravity Facility is available for two drops per day. • The NASA Lewis 2.2-Sec Drop Tower has been extensively renovated. The major improvement is an air-bag decelerator, which reduces the landing loads to levels of 15 to 25 g (previous loads could approach deceleration levels of 100 g) and permits the use of more delicate instrumentation and equipment. • The new Space Experiments Laboratory at NASA Lewis provides eight advanced combustion-science laboratories, several spaceflight hardware assembly and integration laboratories, and high-bay and clean-room areas for facility-level hardware. • NASA Lewis also has a state-of-the-art image-processing and object-tracking workstation available to on-site and visiting investigators for analysis of microgravity-experiment data. • A new airplane laboratory, a DC-9 aircraft based at NASA Lewis, will be available for low-gravity experiments early in 1995. Program Management and International Collaboration • The NASA Lewis Research Center was designated the Agency Center of Excellence for Microgravity Combustion Science. • Open, competitive solicitations for microgravity combustion-science investigations are now planned by NASA Headquarters every two years. Upon award, approval is granted for nominal four-year investigations. • The number of NASA-supported projects in microgravity combustion science has more than doubled to over 40. • New subjects for microgravity combustion research (turbulent combustion, metal combustion, combustion synthesis of materials, and advanced diagnostics) were established as a result of successful proposals to a NASA Research Announcement in 1993. • An agreement on the mutual use of low-gravity facilities by investigators from each country was established between the NASA Microgravity Science and Applications Division and the Japan NEDO; a similar agreement for use of the NASA low-gravity aircraft and the Canadian vibration-isolation technology was established with the Canadian Space Agency. • An agreement to conduct collaborative science investigations was established between NASA and the Russian Space Agency. thering theunderstanding oflow-gravity behavior and, by comparison, related normal-gravity processes. • Normally obscured forces and flows may be isolated. Buoyancy-induced flows frequently obscure those of weaker forces, such as electrostatics, thermocapillarity, diffusion, and low-velocity forced flows, that may be particularly important in weak flames. By removing buoyancy, one may observe and analyze the roles of these forces and flows. • Gravitational settling is nearly eliminated. Free suspensions of fuel droplets or particles may be created and sustained in a quiescent environment, eliminating the need for mechanical supports, levitators, or stirring devices and enabling a high degree of symmetry to be achieved in a quiescent environment. • Expanded experimental time or length scales become feasible. The size or duration of tests in normal gravity is often constrained by the development of buoyancy-driven disturbances. Microgravity permits larger-scale experiments, which can allow more detailed diagnostic observations and new tests of similitude. Low-gravity combustion experiments are conducted in ground-based drop towers and aircraft and in space on sounding rockets and the Shuttle. These microgravity facilities provide opportunities for scientists and engineers to pursue fresh insights into the physics and chemistry of combustion. Unexpected phenomena have been observed-with surprising frequency-in these experiments, spawning the reexamination of classical theories and the vigorous pursuit of new hypotheses. Microgravity combustion research also has direct practical applications, The most obvious is in spacecraft fire safety. Fire protection for the Shuttle and its international laboratories is based on adaptation of terrestrial systems. Experience demonstrates that current fire safety is adequate to respond to expected fire-threatening incidents. Nevertheless, there is a recognized need for fundamental research and technology with the long-term objective of improving the safety and efficiency of spacecraft fire protection. Other microgravity combustion applications are in the improvement of combustion synthesis of ceramic-metal composites and in innovations in combustion diagnostics, effluent sampling, and combustion-system design. This document updates two previous overviews of microgravity combustion science (NASA TM-101424 and TM-105410, which appeared six and three years ago, respectively). The present review describes NASA sponsored basic research on microgravity flame structure and combustion in gaseous, liquid, solid, and mixed phases and applied research in combustion synthesis of ceramic-metal composites, advanced diagnostic instrumentation, and on-orbit fire safety. While recent findings are emphasized, some background information is retained from the prior overviews to make this review a "stand-alone" reference. An additional purpose of this document is the encouragement of the participation of new Principal Investigators in analytical and experimental microgravity combustion research through the frequent opportunities offered by open competitive solicitations, i.e., the NASA Research Announcements. A description of the NASA Lewis low-gravity test facilities and the spacecraft experiment hardware and accommodations available to researchers is in Appendix A. Current NRAawarded participants are listed in Appendix B, and a selected bibliography of recent publications is in Appendix C. This review has been compiled by members and associates of the NASA Lewis Research Center Microgravity Combustion Branch. Lewis is the recognized NASA Center of Excellence in this field. Note that color plates are in back and may not be in sequence. NASA Lewis Research Center Low-Gravity Facilities The low-gravity research facilities at the NASA Lewis Research Center are unduplicated in total anywhere in the world. These facilities are available to Principal Investigators collaborating with NASA in microgravity combustion-science projects. The new Space Experiments Laboratory at NASA Lewis provides eight advanced combustion-science laboratories, several spaceflight hardware assembly and integration laboratories, and high-bay and clean-room areas for facility-level hardware. A separate...