Comparison of Firebrand Propagation Prediction by a Plume Model and a Coupled-Fire/Atmosphere Large-Eddy Simulator

Firebrand spotting is one of the most vexing problems associated with wildland fires, challenging the lives and efforts of fire-fighting planners. This work is an effort to model numerically the event of firebrand spotting for the purposes of reviewing past modelling approaches and of demonstrating a more current coupled fire/atmosphere approach. A simple, two-dimensional treatment of the process of firebrand lofting is examined under the restrictive conditions typical of a classical plume modelling approach. Using this approach, the differences in trajectories of combusting and non-combusting particles are investigated. Next, firebrand spotting is examined using a coupled fire/atmosphere LES (Large Eddy Simulator) in which the processes of firebrand lofting, propagation, and deposition are connected. The behaviour of combusting and non-combusting firebrands released from a moving grassfire into three-dimensional time-varying coupled atmosphere-wildfire induced circulations is examined. When these results are compared to the results of a classical plume model for firebrand spotting, it is found that firebrand propagation in the coupled LES simulated flow is significantly different from that obtained by the two-dimensional empirically-derived plume model approach. The coupled atmosphere-wildfire LES results are explorative and need to be subjected to direct testing.