Effects of wind speed and heat release rate on the vortex strength and size of fire whirls without an inner core of flame

Dust-devil-like fire whirls without an inner core of flame cause considerable damage due to the strong winds during wildfires and urban fires. The purpose of this study is to clarify the effect of the crosswind speed (U) and heat release rate (HRR) on the circulation, size, and velocity of this type of fire whirl. A U of 0.06-0.54 m/s was investigated at HHRs of 0.84 kW and 2.23 kW. The particle image velocimetry measurements clarified the velocity fields of the vortices and complicated flow structures downwind of a long narrow fire source. The absolute value of the circulation of the vortex increases when the wind speed decreases in the range of 0.28 m/s. U. 0.54 m/s. The radius and velocity of the vortex have the identical tendency. This change in absolute value of the circulation with the crosswind speed is determined by the vortex radius and not by the average vorticity in the vortex. The absolute value of the circulation of the vortex increases when the HRR increases. This relationship is determined by the increase in average vorticity in the vortex and not by the radius of the vortex. This relationship does not depend on the crosswind speed.

Automated summary

This research clarifies the impact of crosswind speed and heat release rate on the circulation, size, and velocity of fire whirls without an inner core of flame. The study shows that the absolute value of the circulation of the vortex increases with decreasing wind speed and increasing heat release rate, and it is primarily influenced by the average vorticity in the vortex rather than the radius of the vortex.