Experimental Study on Flame Height and Radiant Heat of Fire Whirls

Abstract

Fire whirls occur occasionally in wild land fires and large scale urban fires. When a fire whirl occurs, the height of flame increases significantly and the fire damages could become more severe. In this study, small and middle scale experiments (up to 20 cm in diameter) were performed to understand the mechanisms of the flame height increase at fire whirls. In the experiments, fire whirls were established on a fuel pool using a fixed-frame-type fire whirl generator. At the small scale pools (\( d \le 5\,{\text{cm}} \)), the flame behaved as a laminar diffusion flame and the measured flame height almost agreed with our scale model based on Burke–Schumann theory. Meanwhile, at the middle scale pools (\( d \ge 10\,{\text{cm}} \)), the flame comes away from a laminar diffusion flame. The measured flame height was correlated to the dimensionless heat release rate at the conditions without swirling flow and also with swirling flow. The measured data at the middle scale pools almost laid on a line of slope 2/3 in the logarithmic diagram between the flame height and dimensionless heat release rate. Also the radiant emittance and shape of base part flame, which were important to understand the fire whirl phenomena, have been examined. It was found that the radiant emittance of flame increased considerably by the swirling flow at the middle scale pools. It was also confirmed experimentally that the viscous effect near the floor (Ekman layer) made the base part flame approach to the fuel pool.