Formation and flame characteristics of the blue whirl

Author：Chen, Z. X., Liu, Z. Y., Liu, L., Xiao, H. H.

Journal：Proceedings of the Combustion Institute

DOI:  10.1016/j.proci.2022.08.129

Keywords: Blue whirl, Fire whirl, Vortex breakdown, Circulation, Flame width, vortex breakdown, Thermodynamics, Energy & Fuels, Engineering

Abstract: 

The blue whirl is a newly-discovered fire phenomenon that has attracted widespread attention because of its unique flame structure and potential advantages in fuel-spill cleanup. This paper presents an experi-mental and theoretical study of blue whirl generated from a traditional fire whirl over a water surface in an apparatus consisting of two offset, quartz half-cylinders. The added ventilation overhead can provide an ad-ditional circulation that drives the rapid formation of the blue whirl, and the blue whirl can be even formed directly without going through the fire whirl state when ventilation intensity is sufficiently strong (external axial velocity VZ0 > 0.451 m/s). The effects of heat release rate (HRR, Q ), gap size ( S) between the cylinders, and external axial velocity on the circulation ( I') and flame width (Wf) of blue whirl were examined in the experiment. The results show that both the HRR and gap size have a strong influence on the characteristics of a stable blue whirl. Increasing HRR leads to an increase in both circulation and flame width, while in-creasing gap size causes a decrease in circulation. The influence of gap size on flame width depends on the dominant regime of the inflow through the gap itself, i.e., Poiseuille-like and disturbed flow for small ( S < 25 mm) and large ( S > 35 mm) gaps, respectively. The maximum receding distance and the criterion for the onset of vortex breakdown were obtained through a theoretical analysis of blue whirl formation. Further dimensional analysis shows that the dimensionless circulation ( r) is linearly proportional to the ratio of normalized HRR ( Q *) to normalized gap size ( S *) when Q */S * > 0.025, and the dimensionless flame width (Wf*) is substantially dominated by normalized HRR. This work can provide a basis for quantifying and modeling blue whirl.