Abstract
For very lean H2/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 (N2) as dilution in the oxidant. Numerical simulation was conducted under an equivalence ratio of 0.4 for both N2 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 N2, which leads to a higher temperature level of the gaseous mixture in reaction zone. Second, the effective Lewis number of the H2/O2/He mixture is larger than that of the H2/O2/N2 mixture. Thirdly, the magnitude of stretch rate at the flame tip for He dilution is less than the N2 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.
Published Version
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