Experimental investigation of the physical mechanisms governing the spread of wildfires

11 One of the objectives of the present study is to gain a deeper understanding of the heat 12 transfer mechanisms that control the spread of wildfires. Five experimental fires were 13 conducted in the field across on plots of living vegetation. The lengths of the ignition lines 14 were set in the range of 20–30 m to reproduce wildfire front conditions as closely as possible. 15 The experiments were performed under various vegetation properties, wind conditions and 16 plot topography to highlight different fire spread behaviours. This study focused on 17 characterising heat transfer ahead of the flame front. The temperature and heat flux were 18 measured at the top of the vegetation as the fire spread. The results showed the existence of 19 two different fire spread regimes that were either dominated by radiation or governed by 20 mixed radiant-convective heat transfer. For plume-dominated fires, the flow strongly responds 21 to the great buoyancy forces generated by the fire; this guides the fire plume upward. For 22 wind-driven fires, the flow is governed by inertial forces due to the wind, and the fire plume 23 is greatly tilted towards unburned vegetation. The correlations of the temperature (ahead of 24 the flame front) and wind velocity fluctuations change according to the fire regime. The 25 longitudinal distributions of the radiant heat flux ahead of the fire front are also discussed. 26 The data showed that neither the convective Froude number nor the Nelson convection 27