The bottom boundary-layer structure of fire whirls

The bottom boundary layer of a fire whirl over a fuel pool is coupled to the outer rotating flow, the vaporization process, and the subsequent diffusional burning in the gas. As such, it plays an essential role in the dynamics of the fire whirl, particularly the resulting burning rate and flame height. In this paper, the bottom boundary-layer structure of a typical laminar fire whirl, generated by a rotating screen, is investigated both numerically and theoretically. The associated local similarity solutions of the momentum and coupling functions are derived, leading to the determination of the flame configuration as well as the surface gasification rate. It is demonstrated that the bottom boundary layer becomes thinner and the flame base is closer to the pool surface as the swirl intensity increases. As a result, the applied circulation increases the burning rate and thereby the flame height of the fire whirl by enhancing the convective heat transfer and the evaporation of the fuel, especially in the outer region of the fuel pool. The theoretical results agree well with the simulation results over a wide range of the Ekman number Ek and the Grashof number Gr, both qualitatively and quantitatively.