Bouyant downward diffusion flame spread and extinction in partial-gravity accelerations

This investigation is an experimental study of flame spread over thin fuel in partial-gravity environments that was conducted aboard test aircraft of the National Aeronautics and Space Administration. The experimental data obtained include the first observations of flame spreading in gravity-level environments between normal Earth gravity and free-fall or near-zero gravity. In partial gravity, flame spreading was observed in purely-buoyant flow that varied with the local gravity level. The test results included data obtained at gravity levels simulating Lunar and Martian environments under which conditions the fuel tested is more flammable and burns faster than in the normal Earth gravity environment.;Experimentally determined flame spread rates were shown to be non-monotonic with variations in gravity, with a peak value at partial-gravity levels that vary with atmospheric oxygen content. At gravity levels above the peak, spread rates decrease with gravity level because of limited residence time of the reactants in the flame. Below the peak, spread rates increase with gravity level because of radiative heat-loss effects. Residence time limitations also render the fuel more flammable as gravity is reduced from normal Earth levels.;The experimental data are compared with the results of two published numerical simulations of opposed-flow flame spreading that consider purely-buoyant flow in variable gravity and include radiative heat losses. The numerical models reproduce existing experimental data obtained at enhanced gravity levels in a cenufuge, but do not yet accurately predict the partial-gravity results.;The experimental data are used to develop a new scheme for correlating flame spread results with residence time limitations using a Damkohler number. An independent term characterizing radiative heat loss is needed for the correlation to be successful. The correlation has the property that a single Damkohler number predicts a flammability limit for observed combinations of atmospheric oxygen content and gravity levels that vary from Lunar gravity to four-times normal Earth gravity.