Microgravity Research Opportunities for the 1990s (1995) / Chapter Skim
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Microgravity Research Opportunities for the 1990s: Chapter 4
Microgravity Research Opportunities for the 1990s: Chapter 4
Pages 62-69
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From page 62... ...
PREFACE Not only would experimentation under microgravity conditions be useful in improving EXECUTIVE SUMMARY scientific understanding of that behavior, but it is also imperative from the standpoint of PART I spacecraft fire safety. 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)
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From page 63... ...
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.
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From page 64... ...
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.
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From page 65... ...
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.
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From page 66... ...
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.
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From page 67... ...
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)
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From page 68... ...
1987. Effect of gravity on laminar premixed gas combustion I: Flammability limits and burning velocities.
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From page 69... ...
1988. Heat transfer to a thin solid combustible in flame spreading at microgravity.
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