Quiescent flame spread over thick fuels in microgravity

Experimental results for flame spread over thick PMMA in microgravity are reviewed. The results were obtained abouard three different space shuttle missions, STS-54, STS-63, and STS-64. For the three conditions, 50% O 2 in N 2 at 1 atm, 50% O 2 at 2 atm, and 70% O 2 at 1 atm, the flame-spread rate slowly decreases with time, which varied from about 50 s to over 300 s. Computational modeling that includes the effects of radiation captures the essential features of the flame position versus time trajectory. When computations are carried out past the experimental time, the flames eventually retreat and then extinguish after spread times of about 450–600 s. With respects to the flame, the flow velocity into the flame is the spread rate. Absent any additional flow to press the flame close to the surface to provide a heat flux that allows the heated layer in the solid to develop., the process remains unsteady. The thermal and mass diffusion scales each are approximately the thermal diffusivity of the gas divided by the spread rate. The computed temperature and oxygen fields show that the distances over which temperature changes take place are small compared to those over which oxygen diffuses. This effect is due to the radiation causing a reduction in the length scale characteristic of the temperature field compared to the mass diffusion scale. The mismatch in the actual thermal scale and the mass diffusion scale grows with time until the oxygen diffusion rate to the flame is unable to sustaint it. For fuels with thickness below some critical value, the fuel thickness is heated fast enough and the spread rate is high enough that the mismatch in the thermal and the mass diffusion scales is unimportant, and the spread rate is steady.

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