Effect of circulation on flame heights over liquid fuel pools

Abstract

A striking feature of fire whirls is that they tend to have higher mean flame heights than free buoyant pool fires at the same heat release rates, which has been extensively studied in the past. Different from the mean flame height, the instantaneous flame height of a fire whirl reflects its dynamic nature in time domain, but has received less attention so far. This work experimentally investigated the effect of imposed circulation on flame heights over liquid fuel pools, including the mean and instantaneous flame heights, as a fundamental model of fire whirls. Both the rotating-screen and half-cylinder types of test facilities with different mechanisms for generating eddies were employed with circular ethanol pools of different diameters as fuel sources. Results showed that the scaling law proposed in previous studies can well estimate the mean flame heights of conical fire whirls, but does not work for the cases when a conical fire whirl-to-extinction transition or vortex breakdown occurs. Meanwhile, the characteristic flame pulsation frequency of a fire whirl is lower than that of a free buoyant pool fire with the same pool diameter, but increases with an increase in the imposed circulation due to enhanced air entrainment. A scaling law based on dimensional analysis is proposed to estimate the dimensionless flame pulsation frequency of a conical fire whirl by a combination of the fuel Froude number and the dimensionless circulation. Furthermore, with an increase in the imposed circulation, the instantaneous flame height transitions from a lower and less volatile position to a higher and more volatile position due to flame precession (an inherent instability of a conical fire whirl). Scaling laws for estimating the maximum instantaneous flame height difference in time domain and the flame precession frequency are deduced to summarize the data from small- and medium-scale tests.