IGNITION AND TRANSITION TO FLAME SPREAD OVER A THERMALLY THIN CELLULOSIC SHEET IN A MICROGRAVITY ENVIRONMENT

An axisymmetric, time-dependent model is developed describing auto-ign ition and subsequent transition to flame spread over a thermally-thin cellulosic sheet heated by external radiation in a quiescent micrograv ity environment. Due to the unique combination of a microgravity envir onment and low Reynolds number associated with the slow, thermally ind uced flow, the resulting velocity is taken as a potential flow. A one- step global gas phase oxidation reaction and three global degradation reactions for the condensed phase are used in the model. A maximum ext ernal radiant flux of 5 W/cm2 (Gaussian distribution) with 21%, 30%, a nd 50% oxygen concentrations is used in the calculations. The results indicate that autoignition is observed for 30% oxygen concentrations b ut the transition to the flame spread does not occur. For 50% oxygen t he transition is achieved. A detailed discussion of the transition fro m ignition to flame spread is given as an aid to understanding this pr ocess. Also, a comparison is made between the axisymmetric configurati on and a two-dimensional (line source) configuration.