Effect of Ambient Atmosphere on Flame Spread at Microgravity

Abstract

The effects of atmosphere composition on the rate of opposed-flow flame spread (S_f ) over thin solid fuel beds at microgravity (μg) were measured and compared to earth gravity (lg) results and theoretical predictions. Two modifications to standard atmospheres were considered. First, the effects of sub-flammability-limit concentrations of a gaseous fuel (CO or CH₄) were measured and compared to an existing theoretical model that was extended to ug conditions. The agreement between the model and experiment is reasonable considering the simplicity of the model. Notably, both model and experiment show that the effect of added gaseous fuel is greater at μg than lg. Secondly, the effect of diluent type on S_f -was studied by comparing results using He, N₂, Ar, CO₂ and SF₆ diluents. It was found that, in agreement with prior studies in N₂ diluent, for He, N₂ or Ar diluents, S_f was larger at 1g than μg. In contrast, for CO₂ diluent, S_f was slightly lower at 1g than at μg and for SF₆ diluent, S_f was much lower at 1g than μg. Moreover, unlike He, N₂ and Ar, for CO₂ and especially SF₆ diluents the minimum O₂ concentration required to support flame spread was lower at μg than 1g. For SF₆, the minimum O₂ concentration at μg was even lower than the upward (concurrent-flow) limit. This unusual behavior is proposed to be a result of reabsorption of radiation emitted from the gases, which is significant only for gases with small mean absorption lengths.

Keywords:

• Flame spread
• microgravity
• radiation
• extinction limits

Notes

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