Modeling of fire spread through a forest fuel bed using a multiphase formulation

We describe a multiphase formulation to study numerically the propagation o f a line fire in a forest fuel bed. One of the objectives of these studies is the improvement of knowledge on the fundamental physical mechanisms that control the propagation of forest fires. In complement of the experimental approach, this simulation tool can also be used for the development of sim plified operational models used for instance for the prediction of the rate of spread (ROS) of wildland fires. The decomposition of solid fuel constit uting a forest fuel bed as well as the multiple interactions with the gas p hase are represented by adopting a multiphase formulation. This approach co nsists in solving the conservation equations (mass, momentum, energy) avera ged in a control volume at a scale sufficient to contain several solid part icles in the surrounding gas mixture. After a presentation of the equations and closure sub-models used in this approach, some numerical results obtai ned for the propagation of a line fire in a pine needles litter are present ed and compared with experimental data obtained in laboratory. These result s show that the rate of spread of fire in the fuel bed is primarily control led by the radiative heat transfer. By increasing the fuel load (with a con stant packing ratio), the results show the existence of two modes of propag ation. A first area where the ROS varies linearly with the fuel load follow ed of a second where the ROS becomes independent of the load. By introducin g the optical thickness characterizing the fuel bed, this difference in mod e of propagation was interpreted like the demonstration of two modes of rad iative transfer (optically thin and thick, respectively). The analysis of t he distributions of the mass fractions of fuel and oxidant present in the g as mixture integrated through the depth of the fuel bed shows that the prop agation velocity could also be limited by the lack of oxygen or fuel availa ble in the ignited zone to maintain the pilot flame. (C) 2001 by The Combus tion Institute.