Large eddy simulations and experimental studies of the influence of equivalence ratio on the combustion characteristics of turbulent NH3-CH4 premixed flames

As one of the most promising zero-carbon fuels, ammonia has attracted widespread attention. However, ammonia combustion faces problems such as high nitrogen oxide emissions. This work aims to investigate the combustion characteristics of fuel gas mixture of 60% NH3 and 40% CH4 by volume, under five different equivalence ratios, i.e., 0.7, 0.85, 1.0, 1.15, and 1.3, respectively. Both large eddy simulation (LES) and experimental test are conducted. The results reveal that flame temperature and nitric oxide (NO) emissions exhibit an initial increase, followed by a decrease with rising equivalence ratios. Notably, the highest temperature is observed at phi = 1.0, while peak NO emission is found at phi = 0.85. As the equivalence ratio changes, the variation of turbulent flow fields and mass recirculation rates is not significant. On the contrary, NO and OH radicals exhibit distinct shifts in relation to the equivalence ratio. The NO emissions predicted by LES agree well with the experimental results. A chemical reaction network (CRN) analysis is also conducted, which effectively predicts NO variation trends and clarifies NO generation pathways and key mechanisms. The CRN analysis highlights variations in the sensitivities of crucial constituents, such as NH3, OH, and NO, to variations in the equivalence ratio.