Resident mechanism of a holder-stabilized ultra-lean hydrogen enriched residual flame

Abstract

The lean premixed combustion near the flammability limit is a promising technology to achieve cleaner and higher efficiency combustion of gaseous fuels. The residual flame usually occurs in the vicinity of the flammability limit. The deep insight into this flame behavior is crucial to further improve the lean premixed combustion performance. In the present study, the ultra-lean 40%H₂–60%CH₄-air premixed residual flame stabilized on the heat-conducting holder in a preheated micro burner is observed experimentally and numerically, and its resident mechanism is analyzed quantitatively in terms of the effects of the stretch, preferential transport, and conjugate heat transfer. The stretch and heat-loss effects do harm to the anchoring performance of the residual flame root. By contrast, the preferential transport effect contributes to maintaining it via generating the local fuel-richer region. This is why the flame root can still maintain although it suffers a higher stretch rate compared to the corresponding extinction strain rate of a planar flame. The small stretch and heat-loss effects as well as the noticeable preferential transport effect contribute to maintaining the residual flame tip. More critically, the preferential transport effect increases and heat-loss effect decreases when the equivalence ratio reduces, which ensures that the residual flame tip still can maintain at the ultra-low equivalence ratio. To the best of our knowledge, such a detailed main factors visualization of the stable residual flame has not been reported yet. The present study helps us to further understand the ultra-lean residual flame dynamics.