The flow surrounding fire whirls*

Despite significant research efforts, our current understanding of the flow structure and dynamics of fire whirls, including the reasons for their dramatic flame-lengthening effect and increased burning rate, is far from complete. The present study contributes to the needed understanding by investigating the steady axisymmetric structure of the cold outer flow surrounding fire whirls developing over localized fuel sources lying on a horizontal surface. Consideration is given to the three distinct flow regions arising from the disparity of length scales present in the problem, namely, an outer rotational inviscid region driven by the buoyant turbulent plume of hot combustion products that develops above the fire, a near-wall boundary layer, and a near-origin non-slender region where the radial inflow of the boundary layer collides. The resulting description, and in particular the terminal boundary-layer structure as the axis is approached, may be useful in future numerical computations of fire whirls.