Effect of Air Coflow on Lean Premixed Flames on a Stratified Burner

Abstract

The characteristics of lean premixed flames with and without air coflow were investigated to reveal the effects of the position of injected air and of the velocity ratio on the lift-off flame configuration. For lean premixed flames without air coflow, four configurations under decreased equivalence ratios were classified as cone flame, lift-off flame, near-blowout flame, and extinction. With air coflow, the variation of the position of injection showed that the outer air injection had a definite impact on the lift-off flame, whereas for the inner one there was no impact. The present results reveal that there are effective velocity ratios at which a flame configuration is altered from a lift-off flame to a cone-like flame. The key mechanism is that the flow field in the co-recirculation zone altered from entrainment flow for a lift-off flame to an ‘impinging’ reverse flow for a cone-like flame. This wall-like air coflow accumulated the hot combustion products in the co-recirculation zone, resulting in a lift-off flame base propagating nearer the burner exit. The temperature distribution and the intensity of chemiluminescence in the co-recirculation zone demonstrated enhanced stabilization of the flame.

Keywords:

• Air coflow
• Flame stability
• Lean combustion
• Recirculation zone