Ground wind generated near the base by the massive convective column of very large-scale mass fires

Abstract

In large-scale mass fires generated in forests or by a nuclear event, the area of the fire is large (diameter 1 or more kilometers) whereas the flame height is relatively small (less than 10 m) creating a large turbulent buoyant plume. This paper determines a correlation for the magnitude of velocity such a flow generates near above the ground at the edges of the mass fire. This induced wind velocity is due primarily to the total cumulative buoyant plume above the mass fire. Therefore, this situation can be simulated by using a pure buoyant plume (for example of helium) representing the buoyant flow above the flame height and having a diameter representing that of the mass fire. A similarity and numerical study of turbulent buoyant helium plumes is presented to determine and correlate the induced velocity near the ground in terms of the total buoyancy or equivalent heat release rate and the diameter of the source fire. The similarity analysis is based on relations for large buoyant plumes which have also been recently supported by experiments. Simulations for validation using the fire dynamics simulator (FDS) were performed for literature data for a 1 m helium plume source. Subsequently, simulations were carried out for various source sizes and for a range of helium flow rates to deduce a correlation between the maximum horizontal velocity with the buoyancy rate and the fire plume diameter. Finally, additional simulations were performed for a 2D case where the fire source length was set the same as the computational domain in that direction with varying fire source widths and flow rates and it is found that similar relation as that deduced for a 3D plume also applies to the 2D case with the characteristic length being the length of the fire source. In this work, we do not address asymmetric fire source load conditions, which may or may not generate fire whirls.