Reciprocal transitions between buoyant diffusion flame and fire whirl

This paper presents a theoretical and experimental investigation on the reciprocal transitions between a general buoyant propane diffusion flame and fire whirl. A small-scale rotating screen facility was used to produce laminar fire whirls. It was observed that under a very low imposed circulation, the initial vertical propane flame would start to incline and revolve around the facility central axis. For a given burning rate, the inclined flame was observed to be converted into fire whirl at a certain critical imposed circulation, while at another lower critical circulation the fire whirl would decay back to inclined flame. It was found that the formation of fire whirl involved the vortex growth from the flame tip to the burner exit. The linear hydrodynamic instability analyses demonstrated that laminar and turbulent fire whirls form at certain critical values of the governing parameters Rel/Bl1/4 and Ret/Bt1/3 respectively, where Re and B are respectively the dimensionless imposed circulation and the dimensionless buoyancy flux. The theoretical results agreed well with the data from small scale to very large scale experiments and field observations in this work and literature. For laminar fire whirl, the critical limit of inclined flame (denoting the maximum Rel/Bl1/4 under which an inclined flame can be sustained) is found to be 1.50 times the critical limit of fire whirl (denoting the minimum Rel/Bl1/4 under which a fire whirl can be sustained).