A Multiphase Approach in Modeling Heat Transfer from the Assemblage of Firebrands*

Wildfire propagation is driven by two main mechanisms that are (1) the spread of local fire front through convection and radiation heat transfer during direct exposure of fuels to flames and (2) firebrand showers, also known as ember attacks. Firebrand showers are the ignition of spot fires as a result of the generation, transport, and deposition of firebrands away from the fire line. It is known that the combustion state of the firebrands, their assemblage patterns, and the reciprocal interactions within the boundary layer influence heat transfer to the recipient surface significantly. However, systematic experimentation on the confluence of these parameters is very challenging. Thus, this work introduces a coupled CFD-DEM approach to modeling the transport and assemblage of firebrands through the boundary layer. A series of numerical simulations are conducted, and the results show the aptitude of the methodology in quantifying the influential factors on heat transfer from the firebrands to the exposed surfaces. In addition, simulations provide insights into forming a preheating zone ahead of the assemblage that may contribute to drying the fuel surface before the arrival of firebrands. This finding may improve our understanding of the time scales involved in spot fire ignition.