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International audience

International audience; The bulk density as definition represents the ratio between the packing ratio and the density of the vegetation. Therefore, it is directly related to the fuel load, the height and to the porosity of the vegetation. In fact, the bulk density plays an important role in fire propagation and behavior. Due to its dependence on the fuel porosity, the bulk density influences heat transfers inside the fuel bed, so, it can affect directly the rate of spread. Or, the bulk density influences also the fire intensity and flame characteristics (residence time, height and depth) due to its dependence of the fuel load and fuel bed height. However, despite the important influence of the bulk density on fire propagation, the literature does not clarify its impact on fire behavior, different points of view can be exanimated. So, the aim of this study is to investigate the role played by the bulk density upon both propagation parameters and heat transfer of a surface fire through a homogeneous vegetation layer. Investigations were conducted numerically using “FireStar2Dâ€, a complete physical model based on multiphase formulation. Also, experimentally, tests were constructed at the university of Corsica at laboratory scale under no wind and no slope condition. In order to study the elementary effect of the bulk density on fire behavior, three different cases were evaluated: (a) variable fuel load with a constant bulk density, (b) variable fuel load and variable bulk density, (c) variable bulk density with a constant fuel load. Case (a) was only studied numerically, the obtained results are in agreement with the literature: the rate of spread increases with the fuel load until a specific value where the ROS becomes independent of it. Case (b) was evaluated numerically and experimentally using a fix fuel bed height. The numerical and the experimental results showed that the ROS is barely affected by both fuel load and bulk density. Finally, the results of the last case, with a constant fuel load, showed numerically the same tendency proposed by Rothermel: the rate of spread reaches a maximum value at an optimal packing ratio that depends of the surface-volume ratio of the vegetation. Or, experimentally the ROS decreases with the increase of the bulk density. Different variables such as the optical thickness, the fire intensity, the residence time, the radiation and convection heat fluxes have been analyzed.

International audience; Field-scale experiments have been conducted on steep sloped terrains in Speluncatu and Letia, north-western and southern regions of Corsica. This work lies within the GOLIAT project framework and it was provided by the Fire and Rescue Service of North Corsica and the Corsican DFCI (Défense de la Forêt Contre l’Incendie) Group. This work reported high intensity fires propagating through shrub vegetation areas (Genista Salzmannii) lying between 60 cm and 85 cm. These sites were selected because of the density of the vegetation, the high slope angle values with a wind direction aligned with the main slope, which can generate a fire close to wildfire behaviour. A detailed experimental protocol is used in order to determine the propagation conditions and the fire behaviour using UAV cameras and heat flux gauges. In order to investigate the different phenomena encountered in these types of fires, numerical simulations were conducted using a complete physical fire model, based on multiphase formulation, namely FireStar2D. Numerical predictions were used to examine the fire front dynamics related to the fire’s rate of spread and fireline intensity. Despite the unfavourable wind and humidity conditions, experimental results analysis showed that the fireline intensity was higher than 7 MW/m, which means that these fires fall into the category of the very high fire severity. Numerical results predicting the fire’s rate of spread, fireline intensity and fire impact were in good agreement with the experimental data.

International audience

International audience; la présente étude propose de mesurer en laboratoire le champ de vitesse 2D vertical dans le plan médian d'une litière de frisure de pin 12/100 d'un demi-mètre carré et d'obtenir simultanément une image des zones de réaction par imagerie de chimiluminescence OH* perpendiculairement à la propagation du feu. L'incendie se propage après un allumage en ligne dans deux configurations de pente et sans vent extérieur. Pour ces deux cas, le but est de caractériser la géométrie des zones de réaction et le champ aérodynamique autour des flammes, de suivre leurs évolutions au cours de la propagation du feu, et également, grâce au couplage des deux techniques, trouver des éventuelles corrélations entre structures de flammes et champs de vitesse instantanés.

National audience; Afin d'étudier les phénomènes intervenant lors de la propagation des feux de forêt et de modéliser leur comportement, il est nécessaire de posséder des données expérimentales. Ce papier présente l'utilisation de la stéréovision comme instrument de mesure pour l'estimation de caractéristiques géométriques de feux en propagation et le positionnement de capteurs.

International audience

International audience

International audience; Fireline intensity is one of the most relevant quantity used in forest fire science. It helps to evaluate the effects of fuel treatment on fire behaviour, to establish limits for prescribed burning. It is also used as a quantitative basis to support fire suppression activities. Conversely, it has been poorly used as a key quantity to test the new generation of physical and detailed models of fire spread that have been developed these last ten years. The reason of the little use of fireline intensity by modellers may to be found in the difficulty encountered to measure this information accurately. Its measurement is particularly tricky for different reasons: difficulty in measuring the weight of the fuel consumed in the active fire front, difficulty to differentiate the heat yield from char and gas. An inverse method to obtain fireline intensity is through the observation of the flame length. Many researchers have established relationships between flame length and fireline intensity. In the the present work, we directly measure the fireline intensity at laboratory scale by oxygen consumption calorimetry. The results are then used to establish a new relationship between fireline intensity and flame length. Different formulations for fireline intensity versus flame length are also tested. Among them the famous Byram's formulation is considered.