Upslope spread of a linear flame front over a pine needle fuel bed: The role of convection cooling

This paper presents an experimental analysis on the role of convection cooling in upslope spread of a linear flame front over a pine needle fuel bed. The slopes of the fuel bed were varied within 0-30° in experiments. The experimental data of heat flux, gas temperature and gas velocity are used to characterize the significant effect of convection cooling in fire spread. The natural convection (due to the temperature difference between the unburnt fuels and the ambient) and flame-induced convection (caused by the flame-induced inflow which produces pressure difference between the flame and the ambient) are investigated by experimental and computational analyses, in order to understand their respective contributions to pre-heating of unburnt fuels. The results indicate that both kinds of convection have effects of cooling on the unburnt fuels ahead of the flame front, but with different spatial influence ranges. Natural convection takes effect in a region from far field to near field (close to the flame), while flame-induced convection plays an important role within a region close to the flame. The convection cooling is dominated by natural convection under lower slope angles, with comparable intensity as compared to radiation loss, and is controlled by the mixed convection (the combination of natural convection and flame-induced convection) under higher slope angles. Additionally, the fire line contour has marked impact on the convection heating, and compared to a convex flame front, the convection heating induced by a linear flame front has a much shorter spatial range of influence on unburnt fuels.