Flame Lift-off and Stabilization Mechanisms of Nonpremixed Jet Flames on a Bluff-body Burner

Abstract

A detailed regime diagram for bluff-body stabilized flames is proposed for the flame lift-off and stabilization limits. At low fuel velocities, the flame structure is classified into three stable modes: recirculation zone flames, jet-dominated flames, and jet-like flames according to the velocity ratio of annular to central jets. Two different flame stability limits can be identified between cold and combusting recirculation zones. For the former case, local flame extinction dominates lifting of the jet-like flames due to a strong interaction between the recirculating air flows and the jet flame front. A critical annulus Reynolds number is found at which the jet-like flame is least probable to lift off, whereas, for the latter case, partial quenching of the blue neck flame in jet-dominated flames is retarded due to the presence of a reignition source, the combusting recirculation zone. Thus, flame stability can be improved. It is further shown that stabilization of lifted flames is more sensitive to the co-flow air than the fuel jet velocity at the inception of flame lift-off, indicating the importance of diffusion flamelet quenching. At high fuel velocities, the annular air flows have little effect on the lift-off heights and premixed flame propagation becomes dominating. In the hysteresis region, the base of lifted flames is elevated with decreasing fuel velocities and the circular ring-shaped premixed flame in the leading front becomes more fragmented. When approaching the maximum lift-off height, the flame base consists mainly of separated, broken flamelets, suggesting an inhomogeneous fuel/air premixing, due to interaction with large-scale vortical structures. Some isolated flamelets with an arrow-headed structure, typical for a triple flame, can be observed at the flame stabilization position.