Modeling transport and combustion of firebrands from burning trees

Burning embers, commonly called firebrands, are lofted by a fire's buoyant plume and transported downwind to ignite new fires (spot fires) ahead of the main fire. In the present work, transport and combustion of firebrands from burning trees are numerically investigated. A three-dimensional physics-based model is used to precompute the steady-state gas flow and thermal fields induced by a crown fire into which firebrands will be injected. Another preliminary study of the thermal degradation and combustion of woody fuel particles is conducted to determine the burning characteristics of firebrands. Then the trajectories and burning rates of disk-shaped firebrands lofted by the crown fire plume and transported downwind are calculated for various values of the wind speed, UwindUwind, and of the fire intensity, I  . Firebrands of different sizes and densities are first launched from a specified location at the top of the canopy. Firebrands that fall on the ground are in a flaming or a glowing state depending on the product ρfw0×τ (ρfw0: initial firebrand density, τ  : firebrand thickness). Results show that for firebrands that remain longer in the thermal plume, the distance covered upon landing is independent of the initial particle diameter and correlates well with I0.1Uwind0.9(ρfw0×τ)−0.2. The normalized brand mass fraction at landing also correlates with the flight time normalized by the product ρfw0×τ for flaming firebrands or by the product ρfw0×D05/3 (D0D0: initial firebrand diameter) for glowing firebrands. For firebrands released from random locations within the canopy, the above correlations remain unchanged, thereby demonstrating the generality of the developed approach.