Experimental investigation of hydrogen enrichment effects on flame stability, thermoacoustic, and emission characteristics in premixed methane air flames

In this study, the influence of hydrogen enrichment on premixed CH₄/air flames in a dual annular counter rotating swirl (DACRS) combustor has been investigated. Experiments were carried out for jet and swirl flame configurations with hydrogen enrichment from 0 to 40% by volume. The primary objectives were to investigate the effects of hydrogen addition on static stability, thermoacoustic behavior, flame macrostructure, and emissions characteristics. Hydrogen enrichment extends the static stability significantly, improving the lean blowout (LBO) limit by 39% in jet flames and 43% in swirl flames at 40% hydrogen enrichment compared to methane air combustion. Flame macrostructures become more compact, shorter and anchored with increasing hydrogen fractions indicating enhanced flame stabilization. Thermoacoustic analysis exhibited that jet flames remain thermoacoustically stable across all hydrogen fractions as there was no coupling between acoustic pressure and heat release fluctuations. On the other hand, swirl flames do not exhibit any thermoacoustic instability in lean conditions below φ_g = 0.7 for all hydrogen fractions, offering advantages for low-emission combustion. However, swirl flames become increasingly susceptible to thermoacoustic instability at higher hydrogen fractions afterwards. In addition, emission analysis indicated substantial reductions in CO (67% in jet flames, 80% in swirl flames) and a moderate rise in NO_x at 40% hydrogen, which is attributable to higher flame temperatures and thermal NO_x pathways.