A sub-grid, mixture - fraction-based thermodynamic equilibrium model for gas phase combustion in FIRETEC: development and results

Abstract

The chemical processes of gas phase combustion in wildland fires are complex and occur at length-scales that are not resolved in computational fluid dynamics (CFD) models of landscape-scale wildland fire. A new approach for modelling fire chemistry in HIGRAD/FIRETEC (a landscape-scale CFD wildfire model) applies a mixture–fraction model relying on thermodynamic chemical equilibrium to predict combustion flame temperatures and product species compositions. The mixture–fraction approach is common in combustor modelling applications. However, since individual flame sheets are not resolved in HIGRAD/FIRETEC, application of the mixture–fraction approach requires the development of a sub-grid model, which is based on the two assumptions (i) that combustible gases are concentrated into distinct pockets surrounded by air and combustion products and (ii) that reaction is limited by the mixing of the surrounding air with combustible gases from these pockets. The pocket radius and the thickness of the mixing zone are key parameters used in this model to characterise the sub-grid region where reaction occurs. The development of this sub-grid gas phase model is presented along with simulation results for various types of vegetation, including grass, California chaparral and ponderosa pine.