Wildland Urban Interface
Firebrand spotting is a major cause for structure losses in wildland-urban interface (WUI) fires. When firebrands land nearby and accumulate into groups or piles, they can act as a more competent ignition source compared to single firebrands. While experimental studies have demonstrated this, and measured the enhanced ability to ignite fuels, it is not understood under what critical conditions the firebrands act as an accumulation rather than individual isolated firebrands. This work is an experimental and numerical study of how the fuel bed thermal interactions influence the subsequent flaming ignition propensity of wood in contact with two electric heaters acting as idealized firebrands. The experiments use electrical cartridge heaters of two sizes (7.5 and in width) which produce an incident heat flux to the fuel (up to 60 kW/m2) to create controllable heating conditions comparable to those from firebrands. It is observed that decreasing the heater size requires higher heat fluxes to the fuel for ignition to occur. When a second heater is introduced within a certain critical distance it is found to hasten ignition or make ignition possible with a lower incident heat flux to the fuel. Numerical modelling is used to elucidate the importance of thermal interactions in the fuel on the propensity of flaming ignition. The numerical modelling is also used to examine a broader range of idealized firebrand sizes () and the separation distance between idealized firebrands in finer resolution. The model 2D captures the qualitative ignition behavior well and even shows quantitative agreement in most cases.