Experimental investigation of the physical mechanisms governing the spread of wildfires

Frédéric Morandini (1) , Xavier Silvani (1)

1 SPE - Sciences pour l'environnement

One of the objectives of the present study is to gain a deeper understanding of the heat transfer mechanisms that control the spread of wildfires. Five experimental fires were conducted in the field across on plots of living vegetation. The lengths of the ignition lines were set in the range of 20-30 m to reproduce wildfire front conditions as closely as possible. The experiments were performed under various vegetation properties, wind conditions and plot topography to highlight different fire spread behaviours. This study focused on characterising heat transfer ahead of the flame front. The temperature and heat flux were measured at the top of the vegetation as the fire spread. The results showed the existence of two different fire spread regimes that were either dominated by radiation or governed by mixed radiant-convective heat transfer. For plume-dominated fires, the flow strongly responds to the great buoyancy forces generated by the fire; this guides the fire plume upward. For wind-driven fires, the flow is governed by inertial forces due to the wind, and the fire plume is greatly tilted towards unburned vegetation. The correlations of the temperature (ahead of the flame front) and wind velocity fluctuations change according to the fire regime. The longitudinal distributions of the radiant heat flux ahead of the fire front are also discussed. The data showed that neither the convective Froude number nor the Nelson convection number--used in the literature to predict fire spread regimes--reflect the observed behaviour of wind-driven fires.

Mots clés: radiation; wildfires; field experiments; temperature; heat transfer; radiation.

Domaines: Génie des procédés

Frédéric Morandini, Xavier Silvani. Experimental investigation of the physical mechanisms governing the spread of wildfires. International Journal of Wildland Fire, 2010, 19, pp.570-582. ⟨10.1071/WF08113⟩. ⟨hal-00591022⟩