Impact of the bluff-body material on the flame leading edge structure and flame–flow interaction of premixed CH4/air flames

In this paper we investigate the interaction between the flame structure, the flow field and the coupled heat transfer with the flame holder of a laminar lean premixed CH4/air flame stabilized on a heat conducting bluff body in a channel. The study is conducted with a 2-D direct numerical simulation with detailed chemistry and species transport and with no artificial flame anchoring boundary conditions. Capturing the multiple time scales, length scales and flame-wall thermal interaction was done using a low Mach number operator-split projection algorithm, coupled with a block-structured adaptive mesh refinement and an immersed boundary method for the solid body. The flame structure displays profiles of the main species and atomic ratios similar to previously published experimental measurements on an annular bluff body configuration for both laminar and turbulent flow, demonstrating generality of the resolved flame leading edge structure for flames that stabilize on a sudden expansion. The flame structure near the bluff body and further downstream shows dependence on the thermal properties of the bluff body. We analyze the influence of flow strain and heat losses on the flame, and show that the flame stretch increases sharply at the flame leading edge, and this high stretch rate, together with heat losses, dictate the flame anchoring location. By analyzing the impact of the flame on the flow field we reveal that the strong dependence of vorticity dilatation on the flame location leads to high impact of the flame anchoring location on the flow and flame stretch downstream. This study sheds light on the impact of heat losses to the flame holder on the flame–flow feedback mechanism in lean premixed flames.