Enhancement of hydrogen combustion efficiency by helium dilution in a micro-combustor with wall cavities

For very lean H-2/air flames in a micro-combustor with cavity flame holders, the combustion efficiency decreases rapidly at sufficiently high inlet velocity due to the occurrence of "flame tip opening". To suppress this undesirable phenomenon, we used helium (He) to replace nitrogen (N-2) as dilution in the oxidant. Numerical simulation was conducted under an equivalence ratio of 0.4 for both N-2 and He dilutions. The results show that the combustion efficiency is greatly improved, remaining above 98% at 32 m/s in the case of He dilution. The analysis reveals that several reasons are responsible for these results. Firstly, the heat capacity of He is smaller than N-2, which leads to a higher temperature level of the gaseous mixture in reaction zone. Second, the effective Lewis number of the H-2 /O-2/N-2 mixture is larger than that of the H-2/O-2/N-2 mixture. Thirdly, the magnitude of stretch rate at the flame tip for He dilution is less than the N-2 counterpart. The combined effects of three aspects result in intensified reaction rate and heat release rate at the flame tip. As a consequence, the flame tip opening phenomenon can be significantly suppressed and combustion efficiency be notably increased.