Background Ignition of natural fuels by hot porous particles originating from electrical faults and firebrands is a major cause of wildfire initiation and spread. Methods This study investigated the ignition behavior of pine needles with a moisture content of 13% induced by a hot hollow steel particle. Particle effective energy was varied by adjusting the void ratio (0–0.85), while particle temperature (700–1100°C) and diameter (8–16 mm) were independently controlled under a constant ambient wind speed of 2 m/s. Key results and conclusions Three distinct phenomena were observed, namely direct flaming ignition, smoldering-to-flaming (StF) transition and no ignition. The results reveal that the controlling parameter for ignition depends on the ignition pathway. The overall critical ignition temperature is governed by particle effective energy, which determines the ability of the particle to sustain smoldering within the fuel bed. In contrast, direct flaming ignition is primarily controlled by particle temperature and size. Although hollow particles exhibit critical temperatures for direct flaming ignition comparable with solid particles, their reduced effective energy suppresses self-sustained smoldering, thereby increasing the overall critical ignition temperature. Implications This study improves the understanding of the interaction between hot porous particles and natural fuels during wildfire initiation.
Wang et al. (Thu,) studied this question.