Interaction between flaming and smouldering in hot-particle ignition of forest fuels and effects of moisture and wind

Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6-14 mm and 600-1100 degrees C) was dropped onto pine needles with a fuel moisture content (FMC) of 6-32% and wind speed of 0-4 m s(-1). Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as T-p,T-crt = 1800(1 + 4FMC)/d + 500(degrees C), and the maximum heating efficiency of particle was found to be eta(sp) = 10%. As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.