Abstract
The future generation of inhabited spacecraft will have a significantly different cabin environment from the present ones, characterized by low pressure and elevated oxygen concentration. This new atmosphere and the low-velocity gas flows in microgravity provide distinct conditions for the combustion of the solid materials used, and their influence on material flammability is of particular interest in the fire safety of spacecraft. Experiments have been conducted to investigate the effects of sub-atmospheric pressure and low flow velocity on the opposed flame spread and extinction behaviors over a thick PMMA. A flammability map was constructed that delineates the uniform regime, the flamelet regime, and extinction limits for thick PMMA under sub-atmospheric pressures. The limiting oxygen concentration increases with the reduced ambient pressure at a fixed opposed flow, while the flamelet regime becomes wider. Under low ambient pressure, the flame spread rate increases with the flow velocity, but the increasing rate slows down. At a constant oxygen concentration, the flame spread rate increases with the ambient pressure and gas-phase conduction dominates flame spread. At a constant oxygen partial pressure, the higher ignition temperature and less gas-phase conduction reduce the flame spread rate synchronously with the increased pressure.
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