Quantifying effects of using thermally thin fuel approximations on modelling fire propagation in woody fuels

In this paper, we quantify the effects of the thermally thin fuel approximations commonly made in numerical models that eliminate temperature gradients within a heated object. This assumption is known to affect the modeled ignition and burn behavior, but there is little research on its impact, particularly in larger fuels or in numerical models including moisture and chemical decomposition of fuels. We begin by comparing modeled to observed ignition times and burn rates. To constrain variability in the material properties of wood and focus on variability caused by fuels assumed to be thermally thin, we conduct experiments using thermogravimetric analysis (TGA) for samples of lodgepole pine. From these data, we derive material properties via optimization with genetic algorithms. We consider burnout experiments on large, woody fuels to confirm ignition time and mass loss rates for a range of fuel specimens and then recreate them with a numerical modeling platform to validate the model. Once validated, we use the model to explore the significance of thermally thin fuel assumptions by performing the same analyses on fuels assumed to be thermally thick and thermally thin. We quantify the ignition times and mass loss rates but also examine differences in thermal inertia of ignited fuels and how the compositions of fuels vary spatially and temporally. We find that fuels of around 1mm in thickness of both approximations show very similar ignition times, mass loss rates, and surface temperature histories. Fuels any larger will quickly show differences.