Combustion of forest litters under slope conditions: Burning rate, heat release rate, convective and radiant fractions for different loads

A set of experiments at laboratory scale was conducted to study the combustion of forest fuel beds in order to quantify the contribution of radiant and convective heat transfer under slope condition. To proceed, a Large Scale Heat Release apparatus was used to measure the heat release rate and fire properties such as the mass loss rate, the geometry of the fire front and the heat transfer were assessed. Because of the slope and the size of the fuel bed, the mass loss rate and the heat release rate do not reach a quasi-steady state when the propagation takes place under slope condition. This is due to a V-shape distortion of the fire front, which leads to an increase of the burnt surface rate by the fire over time. The study of this quantity has shown that the heat release over time can be estimated with the fuel load and the time derivative of the burnt surface. The fractions of radiation and convection released by the fire in its environment were calculated. Under a slope of 20°, the convective fraction decreases from 74.9% to 61.1% whereas the overall radiant fraction ranges between 25.1% and 38.9% and increases with increasing fuel loads.