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

Abstract

This paper describes experimental observations of downward, opposed-flow flame spreading made underpartial-gravity conditions aboard NASA research aircraft. Special apparatus and techniques for these tests are described, including schlieren imaging of dim near-limit flames. Flame-spreading and flammability limit behaviors of a thin cellulosic fuel, 1×10 −3 g/cm 2 tested at 1 atm of pressure in oxygen/nitrogen mixtures of 13–21% oxygen by volume, are described for effective acceleration levels ranging from 0.05 to 0.6 times normal earth gravity (1 g ). Downward-burning flammability increases in partial gravity, with the limiting oxygen fraction falling from 15.6% oxygen in 1 g to 13–14% oxygen in 0.05–0.1 g . Flame-spread rates are shown to peak in partial gravity, increasing by 20% over the 1- g value in air (21% oxygen). Partial-gravity flame-spreading results, corrected for fuel density and thickness, are consistent with results obtained at acceleration levels above 1 g in a centrifuge. The results compare qualitatively with predictions of flame spreading in buoyant flow by models that include finite-rate chemical kinetics and surface and gas-phase radiative loss mechanisms. A correlation of experimental buoyant downward flame-spread results is introduced that accounts for radiative heat losses using a dimensionless spread rate, V f o , a radiation/conduction number. S R , and the Damkohler number, Da , as parameters. The correlation includes data from 0.05 g to 4.25 g and oxygen/nitrogen mixtures from 14% to 50% oxygen.