Turbulent cylindrical fire whirl

Author：Lei, J., Liu, Z. H., Huang, P. C., Zhang, L. H., Liu, N. A.

Journal：Combustion and Flame

DOI:  10.1016/j.combustflame.2023.112879

Keywords: Cylindrical fire whirl, Formation, Flame height, Flame width, Vortex breakdown, combustion dynamics, flame height, diffusion flame, buoyant, circulation, laminar

Abstract: 

The cylindrical fire whirl, characterized by nearly constant flame width in the axial direction and open flame tip, is a unique flame pattern of the buoyant diffusion flame under strong imposed rotation. This paper presents a combined experimental and analytical study to investigate the formation and flame geometry of turbulent cylindrical fire whirls. A medium-scale rotating screen facility (diameter: 2.0 m, height: 10.0 m) was utilized for experiments with independently controlled and finely regulated imposed circulation ( F) and heat release rate ( Q = 15-250 kW), by using propane as the fuel. It was found that the formation of cylindrical fire whirl is accompanied by the downward movement of the unstable flame bulge, and the propagation velocity agreed well with the predictions of the classical vortex breakdown theory. The physical analysis showed that the onsets of cylindrical fire whirls would occur at specific critical values of Re / B m , where Re and B are the dimensionless imposed circulation and the dimensionless fire buoyancy flux, respectively. The analytical results agree well with data from small to medium-scale experiments and field observation, with m = 1/4 and 1/3 for laminar and turbulent fire whirls, respectively. Contrary to the general fire whirl, the mean flame height decreases, and the mean flame width increases steadily with F in the cylindrical fire whirl. A flame height correlation is derived from the radially-integrated momentum and mixture fraction equations, and a flame width correlation is developed based on the concept of equal axial convection and radial diffusion times for the turbulent cylindrical fire whirl. The two models both couple the heat release rate and the imposed circulation, and agree well with the experimental data of this work. The enhanced turbulence level induced by the flow instability of vortex breakdown is suggested to be the dominant mechanism for flame height reduction and flame width increase in the turbulent cylindrical fire whirl. & COPY; 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.