The Wind Effect on the Transport and Burning of Firebrands

Firebrands, controlling spot fires, are often responsible for fast damages in wildland and urban fires. However, the behaviours of firebrands are difficult to predict. In this study, we conduct experiments in a wind tunnel to investigate the effect of wind on the smouldering burning and transport of firebrands. Three different sizes of disc wood particles (weighing about 1 g) are heated to generate smouldering firebrands, and then blown out by a horizontal wind of 5 or 7 m/s. In each experiment the transport distance (in the order of 1 m) and mass loss of firebrands are measured to examine their burning behaviours. For the first time, a bimodal distribution (burning and extinction modals) is observed for small firebrands under certain wind speeds (firebrands of 12-mm diameter and 5-mm thickness under a wind speed of 7 m/s in this work). Both the firebrand transport distance and mass loss in the extinction modal are smaller than those in the burning modal. The heat transfer analysis shows that there is a critical wind speed to quench the firebrand and produce a bimodal distribution, and its value increases with both the particle size and the heating duration. The predicted critical wind speed agrees well with experimental measurements.