Description

Wildfire spread requires fuel particles heated to ignition but the roles of radiation and convection heat transfer have not before been examined in detail. This paper reports on laboratory experiments and numerical modeling of wood particle response when subjected to a fixed radiant flux. The experiments used a propane-fueled radiant panel with an elliptical mirror system to focus a constant radiant flux on fuel particles of different sizes and cross-sectional shapes. Particle temperature was measured with fine-wire thermocouples embedded on the surface. Particle heating during free and forced convection with ambient air was found to depend on particle size, specifically the convective length of the irradiated surface. Ambient convection cooled 1 mm square particles sufficiently to prevent ignition but not 3 mm particles and larger. Surface area-to-volume ratio did not govern particle surface heating but did determine a particle’s thermal response rate. A finite difference, numerical method was developed to solve a two-dimensional, transient conduction model with radiative and convective boundary conditions. Model results were reliable when compared to experimental results and confirmed particle surface length (convection) as a principal determining factor governing fuel particle heating and fuel particle ignition.