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Summary An internal fire whirl can be generated readily in a tall shaft model with appropriate gap width at one corner. Experimental study was carried out to investigate the relationship between the characteristics of an IFW and the corner gap width in a 9‐m‐tall vertical shaft model. The vertical shaft had a 2.1 m by 2.1 m square section with gasoline pool fire of different diameters burning inside. The gap width was varied to investigate its impact on fire whirl characteristics, such as flame development, swirling intensity, flame height, flame temperature, and heat release rate of the gasoline pool fire. Vigorous flame swirling motions were generated when the ratio of the gap width to the shaft section perimeter was within the range 0.16 to 0.21. From the flame streamline angle, it was observed that the swirling component was much stronger than buoyancy component near the bottom of burning region. The swirling component decreased and became roughly the same as buoyancy near the middle. Finally, it diminished to being much weaker than buoyancy near the top of the fire. These observations suggest that the Froude number Fr decreased from a large number to 1, and then continued to decrease to 0.

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Experiments on fire whirls generated by a gasoline pool fire in a vertical shaft were carried out. Vortex motions of swirling flame induced by buoyancy above the pool fires were observed to be more vigorous with increase in height. Upward flame motions were resulted due to increase in buoyancy. The phenomenon was described mathematically by solving the vorticity transport equation with reasonable assumptions and appropriate boundary conditions. Analysis gave swirling attenuation along the vertical direction. By measuring heat release rates and fire whirl diameters at different heights, vertical variation of circular speeds of fire whirls was derived.