Flame intermittency and its influence on fire spread across a porous vegetative fuel bed

Recent years have seen a significant rise in the frequency of high-intensity and wind-driven wildfires, encouraging fundamental research to develop reliable prediction models. Intermittency is an inherent fire phenomenon leading to flame breakup. The coupling between intermittency and heat transfer makes the flame spread analysis of spreading flames complex. This study investigates the role of flame intermittency and vortical structures on various aspects of heat transfer and flame spread. Experiments were performed by heterogeneously distributing longleaf pine needles across a testbed of 0.6x1.65m² placed inside a wind tunnel. The unsteady structures generated due to flame intermittency were acquired using time-resolved measurements captured with a high-speed camera at 500fps. Flame puffing frequency, along with the size of vortical structures, was measured for a range of fuel and crossflow conditions. Additionally, total and radiative heat flux was measured at various locations along the bed, and their correlation against the flame intermittency was analyzed. The mean flame length was observed to increase with intermittency, leading to a higher flame spread rate. Moreover, the flame puffing frequency increased with increasing wind velocity, suggesting a faster flame breakup.