Effect of Wind Velocity on Smoldering Ignition of Moist Pine Needles by a Glowing Firebrand

Firebrand ignition of wildland fuels is an important pathway of initiation and propagation of wildland and wildland-urban interface fires. The ambient wind plays an important role in smouldering ignition of wildland fuels by firebrands, but its influence is poorly stated. In this work, the effect of ambient wind on the ignition of a moist pine needle fuel bed by a glowing firebrand was investigated. Two ignition outcomes, smouldering or failed ignition, were observed. Wind was found to have a significant impact on the ignition process by affecting the glowing combustion state of the firebrand. As the wind velocity increased from zero to 4 m/s, the smouldering ignition probability firstly increased and then decreased, while the smouldering ignition delay time decreased. Under the same wind conditions, the ignition probability decreased while the smouldering ignition delay time increased as the fuel moisture content increased from 5.9% to 53.2%. A theoretical model based on the glowing combustion rate of firebrand and heat transfer between the firebrand and the fuel bed under the effect of wind was proposed. The model predicted the variation trend of the smoldering ignition delay time with the wind velocity well when FMC was less than 20%, but poor at higher FMC. It also predicted that the square root of ignition delay time had a good linear relationship with FMC. This work is a step forward in understanding of spot fires initiated by glowing firebrands.