We describe a numerical model to study the propagation of a surface fire in a fuel bed. The objective, in the long term, of these studies is to improve the mode of evaluation of the rate of spread (ROS) of a wildfire in simulations and thereby help to prevent and reduce the risks related to forest fires. The decomposition of solid fuel constituting a forest fuel bed as well as the multiple interactions with the gas phase are represented by adopting a multiphase formulation. This approach consists of solving the conservation equations (mass, momentum, energy, etc.) averaged in a control volume at a scale sufficient to contain several solid particles in the surrounding gas mixture. The calculations, which were carried out in pine needle litter on flat ground and without wind, are compared with experimental data obtained in the laboratory. These results show that the rate of spread of fire in the fuel bed is mainly controlled by radiative heat transfer. When the fuel-bed depth is increased near the extinction limit, the ROS increases quickly, reaches a linear mode, and tends to stay in a saturated mode for which the ROS becomes independent of the height of the litter. By introducing, the optical thickness characterizing the fuel bed, this difference in mode of propagation is interpreted as demonstrated: two modes of radiative heat transfer corresponding, respectively, to an optically thin and thick medium.
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