Effect of imposed circulation on temperature and velocity in general fire whirl: An experimental investigation

Abstract

This paper presents experimental effort to reveal the independent effect of imposed circulation on the evolutions of thermal and velocity fields in general fire whirl. Experiments were performed by using a small-scale rotating screen facility with finely controlled imposed circulation (Γ) under fixed heat release rate of Q˙ = 7.5 kW. It was found that the centerline excess temperature and axial velocity varied consistently with the flame shape. The buoyant regime and circulation-controlled regime were distinguished in specific ranges of Γ. For the buoyant fire whirl, the centerline axial velocity increased by a 1/2 power law of height in the entire continuous flame. For the circulation-controlled fire whirl, the centerline axial velocity depended on height by a 1/3 power law at lower levels and differed slightly with height in the rest continuous flame. At a certain level, the variations of centerline excess temperature and axial velocity with Γ varied noticeably in three consecutive axial regions. In the bottom region, the behavior of centerline axial velocity was closely related to the occurrence of a special jump in flame size and the drop of its height with increasing Γ. In the upper region, the centerline axial velocity decreased first, then increased and decreased, and finally increased quickly again with Γ. It was found that the imposed circulations at the interval boundaries increased steadily with height. The centerline axial velocity increased a little faster than the tangential velocity with Γ in the circulation-controlled regime. The radial distributions of excess temperature and axial velocity varied significantly with Γ and height, and unimodal profile, plateau-type profile and hump-type profile were found. The radial profiles of tangential velocity had self-similarity within the mean flame height. The Richardson number increased with Γ and height in the buoyant fire whirl and exhibited some decrease in the circulation-controlled fire whirl.