Investigation of fire-driven cross-wind velocity enhancement

Abstract

Understanding the aerodynamics associated with the interaction of fire and cross-wind flow is of great importance because the consequence may have major implications in building design against bushfire (or wildland fire) attacks. However, a fundamental understanding of how the interaction of fire and wind can alter free stream flow aerodynamic properties has remained elusive. The scope of this study is to examine the pool fire and wind interaction under fixed wind velocity condition. This study dissects the fundamental mechanisms of how the interaction of horizontal momentum flow with a vertical buoyant plume leads to enhancement of wind velocity in the horizontal direction at a certain elevation. Changes in flow aerodynamics caused by the interaction of fire and wind were analysed using the computational fluid dynamics approach. The mechanisms causing the changes were explained. A module was developed and added to the FireFOAM solver to evaluate flow acceleration due to the pressure gradient, gravity, and viscous effects. The chosen computational model was validated against two sets of experimental data, namely, a buoyant diffusion fire plume in still air and the other in cross-wind condition. The numerical simulation revealed that due to the interaction of fire and wind, there is a negative longitudinal pressure gradient across the plume axis, causing the flow to accelerate and the velocity profile to alter. It was also shown that the distortion in velocity profile depends on the location downstream of the fire plume. The height of the distortion increases whilst the magnitude of the distortion diminishes as the longitudinal distance from the fire source increases. Investigation of the effects of heat release rate on wind enhancement further showed that fire with a higher heat release rate causes a greater pressure gradient and a lower density, culminating in higher flow acceleration and consequently increase of wind enhancement.