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An internal fire whirl can be generated by burning a pool fire in a vertical shaft with appropriate sidewall ventilation provision. Earlier experimental results show that the flame swirling motion depends on the corner gap width providing ventilation. In this article, experiments on generating an internal fire whirl in a 9 m tall vertical shaft model will be reported. Flame shapes in burning a gasoline pool fire inside the shaft model with different corner wall gap widths and tray diameters were observed. Fuel mass of the pool fire, flame height of the internal fire whirl, and transient air temperatures were measured. From the results, it is further confirmed that an internal fire whirl cannot be generated when the gap width is too wide or too narrow. An internal fire whirl is developed in five stages. The pool fire is burning in a way similar to burning in open air at stage I. The burning rate of pool fire increases at stage II. Swirling flame motion starts to develop at stage III. An internal fire whirl develops fully at stage IV. Stage V is the decay stage. Furthermore, the created internal fire whirl can be divided into three zones. Zone I is at the lower part with the flame rotating violently. Zone II is in the middle part with a slower swirling rate. The upper part zone III has no flame rotation.

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