Spotting ignition of pine needle bed by multiple hot steel particles

Ignition of natural fuels by numerous small hot metal particles is an important pathway for wildfire initiation associated with power lines and mechanical operations. This work explores the ignition of a pine needle bed by hundreds of 2-mm diameter stainless steel hot particles deposited as a particle spray over an initial landing area, with fuel moisture content (MC) of up to 45% and wind speed up to 8 m/s. Increasing the particle number or mass facilitates direct flaming ignition, whereas the smoldering-to-flaming (StF) transition significantly lowers the ignition threshold in terms of particle area densities, defined as the ratio of total particle mass to the initial particle scattering area, with a minimum particle area density found at 0.4 g/cm² for 5% MC. A localized dispersion of particles ignites the fuel more effectively than distributed ones due to reduced environmental cooling and stronger inter-particle heating. Fires initiated by multiple particles also exhibit more extensive and faster fire spread than those from a single particle of equivalent energy. The ignition via StF transition is promoted by higher wind speed and lower fuel MC, where opposed smoldering spread exceeds concurrent spread under a small wind (∼ 0.5 m/s). The ignition delay of smoldering and StF transition depends on the location and extent of the reaction zone, while being insensitive to the total mass and distribution of particles. Finally, a simplified heat transfer analysis links the spotting ignition threshold and delay time under different moisture and wind conditions based on a minimum char-layer thickness.