Insight into the ultra-lean residual flame stabilized on a high-temperature bluff-body

The lean premixed combustion LPC can achieve clean combustion of natural gas and reduce emissions of harmful gases. However, this combustion mode is usually difficult to sustain. To improve the anchoring performance of lean premixed flame LPF, the high-temperature bluff-body HTB with 900 K is employed in this work. Unexpectedly, a stable residual flame of the methane-air ultra-lean premixed mixture is found experimentally and computationally near the blow-off limit. A deeper insight into its anchoring mechanism is necessary to further promote the LPF stability. At first, the residual flame structure is revealed quantitatively, and it is found that the diffusion dominates the reactant flux which arrives at the residual flame rather than the convection. Then, the anchoring mechanism of the ultra-lean residual flame is revealed in terms of the effects of the preferential transport, stretch, and conjugate heat exchange. The recirculation zone right behind the HTB provides a good anchoring location for the residual flame base. The small value of the stretch rate contributes to the residence of the residual flame tip. The enhanced preferential transport effect by the HTB contributes to maintaining the residual flame by generating a relatively fuel-richer region compared with the incoming fresh mixture around it. In addition, the enhanced pre-heated fresh reactants by the HTB provide good ignition and combustion conditions around the residual flame, which contributes to its residence. To the best of our knowledge, such a detailed visualization of the main factors responsible for anchoring the residual flame stabilized by the HTB has not been reported yet. This study provides a new scheme to improve LPC performance. This study expands our understanding of the LPF dynamics stabilized by the bluff-body.