Wind-aided flame spread over an unsteadily vaporizing solid

Abstract

A simple but realistic model for the flame length and the flame front speed during windaided flame spread on a thick vaporizing solid like PMMA is presented. The model predictions compare favorably with the experiments conducted in the ceiling configuration. This model is based on the following experimental observations: (i) During wind-aided flame spread, the solid-phase undergoes transient pyrolysis while the gas-phase remains quasi-steady, (ii) In the ceiling configuration, the flame stand-off distance is much smaller than the thermal boundary layer thickness. To incorporate the first observation into the model, an expression was derived for the transient mass flux as a function of the net incident heat flux. This expression was verified by transient pyrolysis experiments conducted on PMMA for heat fluxes ranging from 1.6 to 5 W/cm2 in N2 atmosphere. The second observation was exploited to obtain considerable simplification in the gas-phase enabling an explicit expression for the convective heat flux from the flame to the solid. This heat flux was corrected for shielding due to blowing of fuel mass from the solid surface. A comparison of the model with the experimental results show that shielding of heat transfer due to blowing and the radiative heat loss from the sample surface have a large effect on the flame length and the flame front speed. It was also found that for cases where the flow is laminar and flame radiation is small, surface heat loss causes the pyrolysis front speed to eventually become zero for a thick sample. This may occur even before the pyrolyzing zone achieves steady state as assumed by previous theoretical models. Once the pyrolysis front speed becomes zero, the flame spread rate is controlled by the speed of the burnout front which propagates behind the pyrolysis front for thin or charring materials.