Experimental investigation on flame patterns of buoyant diffusion flame in a large range of imposed circulations

Abstract

This paper reports experimental efforts to reveal the flame characteristics of a buoyant diffusion flame under imposed circulations. Experiments were performed by a rotating screen facility with independently controlled heat release rate (Q˙) and imposed circulation (Γ). Nine flame patterns were distinguished and separated in the Q˙−Γ plot for the first time, including free buoyant flame, inclined flame, weak fire whirl, conical fire whirl, cylindrical fire whirl, transitional fire whirl (between conical and cylindrical fire whirls), curved cylindrical fire whirl, flame extinction and irregular flame. It was found that these flame patterns occurred within different specific ranges of Γ and Q˙, and the critical imposed circulations for the appearance of these patterns increased with Q˙. A conical fire whirl could be converted to a cylindrical fire whirl only when Q˙≥1.20kW. Otherwise, flame extinction of conical fire whirl occurred at a critical imposed circulation. The transitional fire whirl was characterized by the unstable floccule-like flame structure and noticeable flame expansion near the top of conical fire whirl. This flame instability, as considered to be induced by vortex breakdown, could propagate downwards to the burner surface to induce complete cylindrical fire whirl at critical Γ. The formation of cylindrical fire whirl highly extended the extinction limit of fire whirl. For Q˙≥6.25kW, the cylindrical fire whirl would be transformed into irregular flame with no flame extinction. At the lower limit of weak fire whirl, the flame was stabilized significantly, and the flame height increased noticeably with respect to free buoyant flame. The variations of flame height and flame diameter with imposed circulation differed greatly among different Q˙ in weak and conical fire whirls. In cylindrical fire whirl, the mean flame height decreased, while the flame diameter increased with Γ to maximum until the cylindrical fire whirl was curved.