Effect of imposed circulation on the transition between blue whirls and fire whirls

Abstract

This paper presents an experimental study of the influence of imposed circulation on the transition between blue whirls and fire whirls. The imposed circulation (Γ0) is achieved through overhead ventilation and quantified under unignited conditions. The results show that the imposed circulation has a significant effect on the flame characteristics of both blue and fire whirls. The flame undergoes eight types of pattern as the imposed circulation increases, i.e., transitional blue whirl, stable blue whirl, turbulent transitional blue whirl, conical fire whirl, transitional fire whirl, cylindrical fire whirl, curved cylindrical fire whirl, and irregular flame. In particular, the stable blue whirl fails to form when the fuel surface diameter (d) exceeds a critical diameter (∼54 mm). Inlet circulation shows a linear increase with increasing Γ0, while both mass burning rate and flame height show a non-linear variation with Γ0. The flame width remains largely unvaried in the blue whirl regimes, but shows a rapid growth in the fire whirl regimes, as Γ0 increases. When d is less than the critical diameter of extinction, the blue whirl may wander beyond the fuel surface due to flame precession, and this can lead to flame extinction. An analysis of the transition from the blue whirl to fire whirl, combined with vortex breakdown theory, shows that the evolution in flame patterns is closely related to the change in the type of vortex breakdown. The vortex breakdown in the stable blue whirl is thought to be a conical mode rather than a bubble mode. The transition from the blue whirl to fire whirl involves a shift of vortex breakdown from bubble mode to conical mode, and ultimately to wide-open conical mode. This work provides insights into the mechanisms underlying the transition between blue and fire whirls.