Transition from point ignition to flame spread in porous fuel beds: Effects of packing ratio and wind

Spot fires initiated by embers are a critical mechanism of wildfire spread, particularly in wildland–urban interface (WUI) areas where porous medium structure and wind influence the transition from localised ignition to flame propagation. This work provides information on the initial growth from an ember ignition source through the evolution of the burning area, mass loss rate, rate of spread, and flame geometry. Experiments were performed on beds of Pinus radiata needles with two packing ratios, β = 3% and 6%, under wind velocities ranging from 0 to 2.4 m/s. Within the investigated range of packing ratios, results show that loosely packed porous fuel beds produce larger burning areas and higher rates of spread than denser fuel beds, while the latter exhibited higher mass loss rates and longer flames. Flame geometry analysis revealed that in compact fuel beds buoyant effects counteract forced flow effects, resulting in smaller inclination angles whereas more forward-leaning flames were observed in more porous fuel beds. A Froude number analysis offered a representation of the relative influence of forced wind and buoyancy effects, identifying a wind-driven and buoyancy-driven regime at low and high β , respectively. The results provide information on the relevance of fuel bed parameters and environmental conditions to be considered in future fire risk assessments.