Theoretical and Numerical Study on Flame Height of Axisymmetric Laboratory-Scale Fire Whirls

Abstract

A fire whirl may occur when a pool fire interacts with a swirling flow. This paper discusses the mechanism of flame-height increase of laboratory-scale axisymmetric fire whirls. The classical Burke-Schumann theory is first extended to include swirling flows. By applying a coordinate transformation, it is shown that the dimensionless flame height (flame height divided by pool diameter) is proportional to the Reynolds number (based on the fuel evaporation rate) regardless of the presence of swirling flow. Thus, swirling flows have no direct impact on the flame height of a pool fire. Instead, the presence of a swirling flow changes the shape of flame base, increasing the heat flux to the liquid surface and hence the fuel evaporation rate; the flame height is then increased because it is proportional to the fuel evaporation rate. These theoretical findings are validated by a series of numerical simulations conducted in this study as well as previous studies by other researchers.