Modelling firebrand transport in wildfires using HIGRAD/FIRETEC

Abstract

Firebrand transport is studied for disc and cylindrical firebrands by modelling their trajectories with a coupled-physics fire model, HIGRAD/FIRETEC. Through HIGRAD/FIRETEC simulations, the size of possible firebrands and travelled distances are analysed to assess spot ignition hazard. Trajectories modelled with and without the assumption that the firebrands’ relative velocities always equal their terminal velocities are. Various models for the flight and combustion of disc- and cylindrical-shaped firebrands are evaluated. Eight simulations are performed with surface fuel fires and four simulations are performed with combined surface and canopy fuels. Firebrand trajectories without terminal velocity are larger than those from models with terminal velocity. Discs travel further than cylinders, as discs are aerodynamically more favourable. Thin discs burning on their faces and tall cylinders burning around their circumference have shorter lifetimes than thin discs burning from their circumference or longer cylinders burning from their ends. Firebrands from canopy fires, with larger size and potential to ignite recipient fuel, travel further than firebrands from surface fires. In the simulations, which included a line fire ignition in homogeneous fuels on flat terrain, the firebrand launching patterns are very heterogeneous, and the trajectories and landing patterns are dominated by the coupled fire–atmosphere behaviour.