Convective heat transfer in pine forest litter beds

Abstract

To properly parameterize physics-based models of wildland fire behavior, it is necessary to understand the magnitude of convective heat transfer in various scenarios. In order to do so, we isolated the convective heating process in an idealized wildland fuel: pine needle litter layers. A heated wind tunnel was used to obtain a Reynolds number dependent correlation for the Nusselt number for: (1) a single representative fuel element, and (2) the bulk porous medium - both exposed to sudden pulses of high temperature air. The former was obtained through resistive thermometry and the latter through optimization of a 1-dimensional advection-diffusion model, trained by gas-phase measurements. It was determined that the Nusselt number for a single fuel element, when oriented perpendicular to the flow, did not significantly deviate from that of a single isolated cylinder. However, the correlation for the bulk medium showed a Nusselt number roughly 11% to 25% less than that of a single cylinder, over the range studied. This was attributed to the complex orientation of individual elements within the bed and has implications for modeling ignition and flame spread in porous fuels.