On radiative extinction of microgravity diffusion flames

Radiative energy losses induce instability and extinction of low-momentum microgravity diffusion flames. In this work, the critical condition of the steady flame to exist and a correlation for the pre-extinction size of self-extinguished flames are derived using the critical flame temperature concept and considering the steady flame energy balance. With the critical temperature values of 1100 K for ethylene, 1200 K for propane, and 1300 K for methane flames, favorable performance of this correlation is demonstrated in predicting the pre-extinction radius of spherical microgravity flames (including those observed in the drop tower tests and Flame Design orbital experiment) and in evaluating the pre-extinction effective radius of flames produced by the flat circular burner in recently completed BRE (Burning Rate Emulator) space experiment. In the latter case, the radiative fraction at extinction is shown to agree with the estimates based on the radiometer data. Flame stability diagram is developed using the BRE experimental database. Analytical dependence of the nominal heat release on oxygen concentration in the ambient is shown to delineate the boundary between the experimentally observed steady (at least until fuel termination) and self-extinguished ethylene-fueled flames.