CFD investigation of cross-flow effects on fire-wind enhancement

Investigation of fire-wind (fire cross-wind) interaction is highly instrumental in dissecting the potential effects of bushfire attacks on buildings. The increase of free-stream wind velocity downstream of the fire source due to the interaction of wind and fire is referred as fire-wind enhancement which has been recognized as one of the destructive consequences of bushfirewind interaction. Although occurrence of the fire-wind enhancement phenomenon has been reported in previous studies, the mechanisms and contributing factors affecting the phenomenon have not been reported in the literature. This study applies Computational Fluid Dynamics (CFD) technique to fundamentally investigate the effects of cross-wind on fire-wind enhancement. Fire-FOAM solver which is based on OpenFOAM platform was used to solve thermo-fluid governing equations. A module has been added to the solver to extract different components of flow acceleration and the corresponding fireinduced flow momentum. Experimental data of buoyant diffusion flame was used to validate the numerical model. A selected range of simulation scenarios with different free-stream wind velocities under constant fire intensity has been performed to identify the effects of free-stream wind velocity on fire-wind enhancement. The outcome of the research indicated that as a result of interaction of cross-wind and fire, a longitudinal (horizontal) favorable pressure gradient is generated which leads to enhancement of wind downstream of the fire. It was also shown that the normalized fire-induced pressure gradient decreases when free-stream wind velocity increases. Therefore, for constant fire intensity, the flow field with a higher free-stream wind velocity undergoes a lower enhancement. Introduction Interaction of wind and fire is a two-way problem. That is wind causes a change of fire plume geometrical features and also fire affects flow (wind) aerodynamic properties. Many studies have focused on the effects of wind on fire plume geometrical properties [3]. Fire plume tilt angle and flame length were investigated in [4,5]. As for the effects of fire on the wind, it was experimentally shown that interaction of wind and fire can result in the increase of wind velocity downstream of the fire [2]. Thermal expansion and low-density area within the plume region were found to be the reasons for distortion in the velocity profile [12]. Therefore, the knowledge of thermal and geometrical details of plume region will help better understanding of the phenomenon. Fire-wind enhancement is one of the mechanisms of bushfire attacks. Lambert [7] and McRae et al [9] reported that wind can be intensified due to the interaction with bushfire and cause damages to buildings and vegitations. He et al [1] and Kwok et al [6] numerically investigated bushfire-wind interaction and showed that it can potentially enhance the near ground wind velocity downstream of the bushfire source up to 50%. They showed that accompanied by the increase of wind velocity, bushfire-wind interaction results in an increase of turbulence and wind gust. Coanda effects were postulated to be the reason why the wind is enhanced downstream of the bushfire source. As a result of the increase in wind velocity due to fire-windinteraction, the pressure coefficient around the buildings downstream of the bush-fire source was shown to be increased. There are two controlling factors in the process of fire-wind interaction: one is the longitudinal momentum flux which is determined by the wind velocity for given ambient air density; and the other is the buoyancy flux which is determined by gravitational acceleration and density difference between the momentum flow and fire induced buoyancy flow. The focus of the current study is to employ Computational Fluid Dynamics technique to fundamentally investigate the effects of wind velocity on the fire-wind enhancement phenomenon, whereas the effects of buoyancy flux would be reported elsewhere.