Dynamics of concurrent flame spread over a thin charring solid in microgravity

The dynamics of microgravity concurrent flame spread over thin cellulosic sheets are theoretically investigated. The mathematical model is based on the laminar, reactive Navier-Stokes equations coupled to solid-phase enthalpy and mass conservation equations. Simulations have been made for forced flow velocities in the range 0.25-15 cm/s, by decreasing the oxygen mass fraction of the concurrent Bow below the ambient value and by increasing the solid charring rate (fire-retarded cellulose). For air, non-retarded cellulose and Bow velocites larger than 5 cm/s, the dynamics of concurrent flame spread are qualitatively similar to those of normal gravity. As the concurrent Bow is decreased below 5 cm/s, after short transients, a transition from fast flame spread to slow solid burning and then to flame quenching is predicted. Flame quenching is also observed, for relatively high Bow velocities, in vitiated air or for fire-retarded cellulose. Finally, blow-off at the highest velocity considered (15 cm/s) is predicted only for sufficiently low oxygen concentrations. (C) 1998 John Wiley & Sons, Ltd.