Smouldering-to-flaming transition on wood induced by glowing char cracks and cross wind

Abstract

The shrinkage, deformation and cracking of the wood affect their smouldering and flaming dynamics, but the scientific understanding is still limited. We study the burning behaviours of disc wood samples with a diameter of 60 mm and thicknesses of 5–40 mm under external airflows up to 6 m/s. Results show that the smouldering-to-flaming (StF) transition can be observed at about 830 °C under external airflow, which is caused by the interactions between smouldering-induced crack and environmental airflow. The fully penetrated vertical char crack or pre-perforated hole promotes the StF transition because of (1) enhanced radiation between the two smouldering surfaces and (2) greater air supply under the chimney effect. As the wind velocity increases, both the smouldering surface temperature and crack size increase, so the transition to flaming becomes faster. For a larger wood thickness, a larger airflow is required to generate the crack and cause a StF transition. A numerical model is proposed to investigate the volatile convection and flaming ignition. Numerical analysis reproduces the StF transition, as an autoignition of a pyrolysate-oxygen mixture promoted by hot smouldering surfaces. The numerical model further reveals the effects of smouldering temperature and cross wind on the StF transition. This work deepens the understanding of the StF transition dynamics and provides insights into the wildfire ignition dynamics and fire hazards of timber structures.