On the flame width of turbulent fire whirls

Flame shape is a fundamental property of flames and is critical for scaling the flame size, heat release rate, and radiative heat flux. This paper presents a combined analytical and experimental effort to reveal the effect of imposed circulation on the flame width of turbulent fire whirls. Experiments of free buoyant flame without rotation were performed for comparison using propane burner diameters of 0.30 m and 0.40 m with heat release rates of 25–300 kW to ensure substantially turbulent flow. The turbulent fire whirl experiments used propane (0.30 m in burner diameter) and heptane (0.15–0.50 m in pool size) as fuels, and the heat release rates ranged from 25 kW to 669 kW. Results showed that the imposed circulation strongly restricts the flame pulsations in fire whirls compared to free buoyant flames with equal heat release rates. The turbulence suppression in fire whirls is quantified by the turbulent diffusivity that is expressed as a decreasing function of the Richardson number (Ri). Based on the concepts of equal axial convection and radial diffusion times and turbulence suppression, a semi-physical model of the mean flame width is derived, which couples the known heat release rate, Richardson number, turbulence viscosity parameter, and axial velocity parameters. The model predictions are in good agreement with experimental data of turbulent fire whirl and free buoyant flame. Turbulence suppression was found to play a dominant role in the radial flame contraction of a turbulent fire whirl compared to a free buoyant flame with the same fire size.