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°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 (°C), and the maximum heating efficiency of particle was found to be . 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.
Experimental and numerical study on moving hot particle ignition