Evolution from conical to cylindrical fire whirl: An experimental study

Abstract

This paper presents the first experimental effort on the evolution of the flow field from conical to cylindrical fire whirl, by using a small-scale rotating screen facility with finely controlled imposed circulation (Γ) under a fixed heat release rate of 4.0 kW. Stereo particle image velocimetry (SPIV) technique was employed to measure the instantaneous and mean velocity fields. Two-celled vortex structure and central recirculation zone are found to form and move downwards with increasing Γ in the plume region of cylindrical fire whirl. Within the mean flame height, the mean vortex core radius decreases slightly with Γ for conical fire whirl, while expands linearly after the onset of cylindrical fire whirl. The radial profiles of tangential velocity have self-similarity within the mean flame height for both conical and cylindrical fire whirls. In the plume region of the cylindrical fire whirl, the radial distribution of tangential velocity is similar to that of the Rankine vortex, and the decay of maximum tangential velocity and the expansion of the vortex core are faster than those of the conical fire whirl. With increasing Γ, the unimodal profile of axial velocity is found to change into the plateau profile and the wakelike profile in the plume region of the conical fire whirl and the transition location advances upstream steadily. For both conical and cylindrical fire whirls, the centerline axial velocities of certain levels and the mean flame height have consistent variation trends with Γ. Within the mean flame height for cylindrical fire whirl, the centerline axial velocity falls sharply at the axial position where the radial profile of axial velocity changes from the plateau to the wakelike profile. The height of this axial position is found to decrease steadily with the increase of Γ.