Heat transfer during wind-aided flame spread on a ceiling mounted sample

Abstract

In this paper the effect of wind speed and ambient oxygen mass fraction on heat transfer to the surface underneath and ahead of the flame during wind-aided laminar flame spread has been investigated experimentally and analyzed according to a simple model. These experiments were performed in the ceiling configuration. High temperature ceramic solids in-strumented with surface and in-depth thermocouples were used downstream of the PMMA sample experiencing flame spread. Simultaneous transient measurements of the PMMA surface temperature, ceramic solid temperatures, gas-phase temperatures, and the flame tip location were made. Since there was little excess pyrolyzate in all the experiments, the flame length was only slightly larger than the pyrolysis length. Thus, heat transfer measurements underneath the flame were conducted with a steady state methane diffusion flame in the boundary layer over the ceramic detectors mounted in the ceiling configuration. Results of heat transfer measurements downstream of the flame tip for the steady state flame agree well with those of the transient flame spread experiments. This shows quasi-steady gas-phase behavior during wind-aided flame spread and implies that any transient behavior is primarily due to the solid-phase. It was also found that for these laminar flames, convection is the dominant mode of heat transfer to the sample surface both ahead and underneath the flame for ambient oxygen concentration conditions. However, flame radiation becomes more significant as Y ox increases. Models for surface temperature downstream of the pyrolysis front and the pyrolysis front spread rate were also developed from these heat transfer correlations. The model predictions agree well with the measurements.