Abstract
Fire whirls can occur during large wild or urban fires. Their consequences are often devastating. Although there are some published investigations on such whirling fires, understanding of their structure and formation mechanisms is limited. The present study aims to help fill these knowledge gaps through numerical simulations using the large eddy simulation techniques (LES). The aim is to develop a predictive capability for the onset of wind -driven fire whirls over L-shaped fire sources. Large eddy simulations were conducted using an in-house modified FireFOAM solver, which was previously validated by the authors ' group for pool fire simulations. The predictions captured all three types of fire whirls, which were experimentally observed over L-shaped fuel sources in a crossflow. These include the stationary fire whirl over the fire source ( Type I ), the fire whirl periodically shedding off from the fire source ( Type II ), and the stationary fire whirl in the open area downstream of the fire source ( Type III ). The detailed predictions were analysed to examine the influence of the crosswind speed. The temporal evolution of the predicted flow structures revealed that the Type III fire whirl is likely responsible for the generation of Type I and Type II fire whirls. Heat transfer mechanisms were found to differ among the three types of fire whirls with Type II and Type III being primarily influenced by thermal diffusion and Type I being driven by advection. Further insights have also been gained through detailed analysis of vorticity generation and evolution. It was found that the vorticity in Type I fire whirls is mainly generated by the tilting and baroclinic torque; Type II fire whirls are likely to be generated by the vorticity enhanced by baroclinic torque and some tilting effects; while the tilting, traction and stretching effects are all responsible for the generation of vorticity in Type III fire whirls.