Determining a safety condition in the prevention of eruptive fires

Abstract

Eruptive fires are one of the main causes of human losses in forest fire fighting. This phenomenon is fairly rare, unpredictable and thus extremely dangerous for firefighters (or civilians). Many casualties result from several accidents identified in the last fifty years around the world. Indeed, people involved in fire fighting activities are not prepared for facing such an unpredictable phenomenon. Very few literature is available to support either modelling or occurrence prediction for this phenomenon. Eruptive fire behaviour usually describes an extreme case of dynamic fire behaviour in which a sudden increase of the fire front rate of spread (ROS) in a short lapse of time is observed. So the question of predicting an eruption’s occurrence is a crucial point for the fire fighters’ safety. In this study, it is assumed that the eruption is due to physical considerations. The mechanism responsible for this erratic behaviour is the pioneering interpretation proposed by Viegas (2005) which consists in a feedback between the ROS and a convective air flow created by the fire itself. A physical modelling for this “induced wind” is given and is coupled with a simplified physical propagation model for surface fires. If the solution of the coupled system usually converges, it may diverge under certain conditions, leading to a fire eruption. It is then possible to obtain a physical condition which gives the impossibility (or not) for a fire to turn into an eruptive fire and the model is able to predict the occurrence of a fire eruption according to the triangle of fire. The model is tested by comparing its numerical results on the one hand to a set of experiments carried out at laboratory scale and on the other hand to an outdoor wildfire (Kornati accident).