Vortex strength and size of fire whirls without flames around a long narrow fire source

Abstract

Strong winds generated by fire-induced whirls without flames cause destructive damage. Laboratory experiments were conducted to clarify how the strength and size of this type of fire whirl are influenced by the wind speed, heat release rate (HRR), and fire source direction. This study uses a long narrow fire source placed parallel to the wind (parallel arrangement); the results were compared with those of previous experiments where the fire source was placed perpendicular to the wind (perpendicular arrangement). It was found that the direction of the fire source significantly impacts the circulation and radius of the fire whirl. The absolute value of the circulation (|Γ|) for the parallel arrangement is smaller than that for the perpendicular arrangement. The radius has the same tendency. |Γ| for the parallel arrangement tends to increase with increasing wind speed, which is the opposite of the tendency for the perpendicular arrangement. When the HRR increases, |Γ| for the parallel arrangement changes little, although |Γ| for the perpendicular arrangement increases. The results of both arrangements show that fire-induced whirls with large |Γ| and large radii, that is, large intense whirls, occur under low wind speeds and large HRRs in perpendicular arrangements downwind of the fire.