Formation and movement of multiple fire whirls

Abstract

This paper presents an experimental study on the formation and movement characteristics of multiple fire whirls on the line fire under different slope angles (θ). Experiments were performed in a rotating screen facility with independently controlled heat release rate per unit length (Q˙l) and shear wind speed (us). The evolution of the line fire under different shear wind speeds and slope angles involved six regimes, including free line fire, interacting line fire, moving fire whirls, stationary fire whirls, stationary unstable fire whirls, and moving unstable fire whirls. The critical us for the formation and movement of fire whirls decreased roughly with increasing θ. The moving fire whirl would become stationary when us reached the critical value, which may be explained by the balance between the buoyancy-induced velocity and us. A correlation was proposed for predicting the critical shear wind speed of the stationary fire whirl on the sloped surface. The spacing between adjacent fire whirls increased steadily with us under various θ, and the dimensionless spacing depended on the dimensionless us by the 0.5 power law for all θ. The formation frequency of the moving fire whirl increased steadily with θ, and was inversely proportional to us. The variation of the average moving speed of the fire whirls with us differed among different θ. The average moving speed decreased steadily with us for the relatively low slope angles (θ ≤ 15°), and increased first and then decreased with us for the relatively large slope angles (θ ≥ 20°). The steady decrease of the flame height with the large us was associated with the apparent bulge structure at the flame tip, and the critical us for the occurrence of the bulge structure decreased at the relatively large slope angles (θ ≥ 20°).