Numerical simulation of grassland fires behavior using an implicit physical multiphase model

This study reports 3D numerical simulations of the ignition and the propagation of grassland fires. The mathematical model is based on a multiphase formulation and on a homogenization approach that consists in averaging the conservation equations (mass, momentum, energy ...) governing the evolution of variables representing the state of the vegetation/atmosphere system, inside a control volume containing both the solid-vegetation phase and the surrounding gaseous phase. This preliminary operation results in the introduction of source/sink additional terms representing the interaction between the gaseous phase and the solid-fuel particles. This study was conducted at large scale in grassland because it represents the scale at which the behavior of the fire front presents most similarities with full scale wildfires and also because of the existence of a large number of relatively well controlled experiments performed in Australia and in the United States. The simulations were performed for a tall grass, on a flat terrain, and for six values of the 10-m open wind speed ranged between 1 and 12 m/s. The results are in fairly good agreement with experimental data, with the predictions of operational empirical and semi-empirical models, such as the McArthur model (MK5) in Australia and the Rothermel model (BEHAVE) in USA, as well as with the predictions of other fully 3D physical fire models (FIRETEC and WFDS). The comparison with the literature was mainly based on the estimation of the rate of fire spread (ROS) and of the fire intensity, as well as on the analysis of the fire-front shape.