Influence of equivalence ratio on combustion instability of a low-swirl flame under lean-hydrogen conditions.

With targeting combustion instability in a low-swirl combustor using hydrogen as fuel, Large-eddy simulations are performed employing a dynamically thickened flame model and a detailed chemical reaction model considering 9 chemical species and 20 reactions. Especially in this study, the main discussion is focused on the impact of equivalence ratios. The equivalence ratio of the lean hydrogen premixture is changed from 0.33 to 0.45 in 0.03 increments referring to the experimental study (Shoji et al., Proc. Combust. Inst., 2020) to investigate the difference in combustion instability intensity and oscillation phenomena for different equivalent ratios. The experimental study reported that the combustion instability intensity has a maximum value at an equivalence ratio of 0.33, and the intensity did not increase at higher equivalence ratios.

The obtained results in this numerical study reveal that the strongest combustion instability is observed at an equivalent ratio of 0.39, confirming that the correlation between pressure and heat release rate becomes strongest at this equivalent ratio. Although the difference in the equivalence ratio with maximum intensity exists, the trend of having maximum intensity not at the highest equivalence ratio but at a specific equivalence ratio is reproduced. This difference is attributed to the limited accuracy of the models employed in this study.