The effect of interstitial flow on the burning dynamics of porous fuel beds

Abstract

The effect of fuel bed structure on the processes controlling flame spread in porous media is investigated. The fuel loading and bulk density of Pine needle (Pinus taeda x rigida) fuel beds was varied resulting in changes in the fuel bed porosity. Experiments were conducted in no slope, quiescent conditions on a 1.5 m long flame spread table, which was instrumented at three locations to determine gas phase temperature and airflow. Flame height, heat release rate and burning rate were also measured. Increases in overall fuel loading or decreases in bulk density resulted in increasing flame spread rate, flame height and HRR. At the highest fuel loading (0.8 kg/m2) the spread rate for the lowest bulk density case (10 kg/m3) was 180 mm/min compared to 88 mm/min in the highest bulk density case (40 kg/m3), while a linear increase (R2 = 0.97) in flame spread rate as a function of fuel bed porosity was also observed. The magnitude of the entrainment flow towards the approaching flame front is shown to increase with porosity, from 0.4 m/s (94.5 % porosity) to 0.9 m/s (98.6 % porosity), at 0.8 kg/m2 fuel loading. An increase in the flow magnitude was also observed with increases in fuel loading for bulk densities of 10 kg/m3, but not for bulk densities of 20 kg/m3. At the higher bulk density, the effect of the increased fuel loading on the buoyancy-induced entrainment appears to be limited by porosity, resulting in reduced convective cooling ahead of the flame front, alongside the expected changes in radiation attenuation as the fuel bed depth varies.