Heat Transfer Characteristics of Lean Methane Flame in the Region near the Wall Boundary Layer

Since proportion of wall heat loss takes as high as 20-30% of the total engine heat loss, the reduction of wall heat loss is considered as an effective way to improve the engine thermal efficiency. The heat transfer near the wall boundary layer plays a significant role on the exploration about the mechanism of wall heat transfer which contributes to figuring out the approach to the reduction of wall heat loss. However, the near wall characteristics of heat transfer are still unclear. In this study, the premixed lean methane flame propagation was captured by the high-speed schlieren and the flame behavior in the near-wall region was investigated by the micro CH^* chemiluminescence. The temporal histories of the wall temperature and the heat flux are measured by the co-axial thermocouple. The factors including the convective heat transfer coefficient and non-dimensionless numbers, Nusselt number and Reynolds number, were used to characterize the near wall characteristics. Also, the characteristics of near-wall heat transfer were confirmed under various ambient pressures and lean mixtures. In addition, the flame temperature and thermal gradient near wall boundary layer were revealed by CFD simulation using Chemkin Code. The experimental and simulation results show that the moment of wall temperature rising becomes earlier with the increase of equivalent ratio. The corresponding near-wall thermal gradient derived from the heat flux shows a significant positive correlation with the mixture concentration. The increased ambient pressure leads to a faster flame propagation speed due to the strong buoyancy effect. However, the laminar burning velocity turns decreasing because of the low reactivity of the lean combustion at elevated pressures, leading to a small variation of wall heat flux. The flow field within the near-wall region becomes intense at elevated ambient pressures, resulting in increasing convective heat transfer with the flame. The wall boundary layer characteristics show a strong correlation with the wall heat transfer process, then greatly affecting the heat loss and flame quenching at near-wall region.