Thermally-driven vortex resulting from the linear heat wires of different shapes under cross wind

Abstract

Fire whirl is a typical thermally-driven vortex which often occurs in large forest and urban fires. This paper presents an experimental study of the thermally-driven vortex induced by the linear heat wire under cross wind. The straight, C- and S-shape wires were designed to simulate the fire front of different shapes. A wide range of heat release rate per length and wind speed were used to test the critical conditions under which vortex flows occur. It is found that the vortex can appear over and away from the heat wire under certain conditions. As the wire shape became complex from the straight to C-shape and to S-shape, the clockwise vortex was firstly observed and then was replaced by the vortex pair that rotated oppositely. The critical conditions of strong vortex over the heat wire well follow the previous dimensionless correlation that couples the heat release rate per length and wind speed, but the characteristic length should be different for the three types of wires. A physical mechanism of vortex formation is proposed, which indicates that the boundary vorticity near the ground surface is the possible eddy source for the vortex formation under critical conditions. The conditions of similar vortex phenomena induced by S-shape wires of two different length scales follow the previous scaling law that was built for straight line fires, but the interaction of upward thermal plume over S-shape wire results in a minus intercept of linear fitting. This work can provide a positive reference for the numerical simulation of the fire spread under cross wind.