CFD informed design of bench-scale experiments to characterize air entrainment into fuel beds induced by columnar vortices

Abstract

Recent experiments show that strong vortices, similar to fire whirls, can form far from a fire front in the region of smoldering fuel. These buoyancy-induced columnar vortices, visualized by entrained smolder smoke, were observed lofting hot embers into the air and in some cases lead to spot ignitions at the base of the vortex. Gaining insight on how the flow field of a buoyancy-induced columnar vortex could impact surrounding smoldering fuel is the focus of this study. Specifically, the potential air entrainment into a fuel substrate beneath the vortex. The flow field of such columnar vortices has been shown to drive air flow downward under certain conditions and, in the context of combustion, drive air deeper than typical entrainment, inducing spot ignitions and increasing burning and smolder rates. NIST's Fire Dynamics Simulator is utilized to successfully model buoyancy-induced columnar vortices. Then it is utilized to study the behavior of vortices as temperature and vorticity boundary conditions are changed. The flow field and fresh air entrainment potential are analyzed. The simulation results inform the experiment design and preliminary experimental results are presented. Understanding these high-risk phenomena will lead to better risk mitigation and more resilient Wildland-Urban interface communities.