Upward flame spread over thin solids in partial gravity

Abstract

Experiments to observe upward and downward flame spread and extinction over a thin solid fuel in partial-gravity environments were conducted in an aircraft flying parabolic trajectories. In the upward spreading case, flames with constant lengths and steady spread rates were observed using narrow fuel samples in reduced pressures. The upward flame spread rates and the flame and pyrolysis lengths increased linearly with the gravity level. The proportionality constants, however, increased with pressure and sample width. For comparison, downward spreading tests were also conducted using the same reduced-pressure atmospheres needed to obtain steady flames in the upward spreading case. In downward spreading, the steady spread rates and the flammability boundary exhibited a non-monotonic dependence on gravity. This behavior is attributed to competition between finite-gas-phase residence times in the flame stabilization zone and radiative heat losses from the flame. Throughout the accessible range of partial gravity, the upward spreading flames propagated at higher speeds than the downward spreading flames and the fuel is more flammable in the upward spread direction. A three-dimensional concurrent-flow flame-spreading model, originally developed for forced flows in a duct at microgravity, was reformulated and numerically solved for buoyant flow. The numerical flame spread simulation provides detailed flame structure including gas flow and temperature fields, oxygen and fuel transport, and solid temperature and thickness distributions and predicts the essential three-dimensional features observed for the narrow, reduced pressure flames