Predictions of wind-opposed flame spread rates and energy feedback analysis for charring solids in a microgravity environment

Abstract

The changes in the mechanisms controlling microgravity flame spread over cellulosic fuels are studied, as the forced gas flow opposing the flame spread and the solid thickness are varied. The analysis is based on the numerical simulation of the problem by means of a two-dimensional, nonconstant spread rate, quasi steady mathematical model. It consists of gas-phase momentum, energy and chemical species mass equations including density variations, finite rate combustion kinetics and surface and flame radiative heat transfer. For the two-dimensional, unsteady solid-phase processes, thermal degradation of the porous fuel to volatiles and chars, variation of thermal properties and convective and conductive heat transfer are described. A quenching limit at very low opposed flows is predicted for widely varying solid thicknesses. However, the mechanisms responsible for such a behavior are different. For very thin fuels (τ < 0.004 × 10 −2 m), where the flame spread rate increases with the solid thickness, surface radiative heat losses are controlling. As the solid becomes thicker and the flame spread rate starts to decrease, flame radiative heat transfer plays a role of increasing importance. Indeed, for thin fuels (0.004 × 10 −2 < τ < 0.12 × 10 −2 m), a reduction in the spread rate with the opposed flow velocity is predicted only if this contribution is taken into account. For thicker solids (τ > 0.12 × 10 −2 m), flame radiation is reduced whereas surface radiative heat losses are again at a high level. However, a low speed flow quenching limit is predicted even if the mechanisms of radiative heat transfer are completely disregarded. The increase of the spread rate with the opposed flow velocity is the result of enhanced heat flux from the flame to the solid which favors heat transfer to the unburned region through solid phase heat conduction.