Enhanced Lean Blowout and Macrostructure Characteristics of Stratified CH4/H2/O2/CO2 Flames in a Dual Annular Counter-Rotating Swirl Combustor

Abstract This experimental study investigates the effects of hydrogen (H2) and oxygen (O2) fractions on stability and combustion characteristics of oxy-methane flames in a dual annular counter-rotating swirl combustor for wider operability of emission-free gas turbines. The pilot stream, comprising a mixture of H2 and carbon dioxide (CO2), serves as the pilot flame, while the mainstream, consisting of a mixture of methane (CH4), CO2, and O2, is the dominant flame. The experiments were conducted at a fixed velocity ratio of 2.27 across a range of mainstream equivalence ratios. The results demonstrated that increasing hydrogen and oxygen fractions significantly enhanced the combustor's lean blowout stability. At a hydrogen fraction of 15% and an oxygen fraction of 34%, the blowout equivalence ratio was reduced to 0.35, compared to 0.50 at zero hydrogen enrichment, representing a ∼43% decrease. The highest temperature of 1763 °C was recorded at an equivalence ratio of 0.73, showcasing enhanced combustion intensity at richer equivalence ratios. Comparatively, at oxygen fractions of 34% and 30%, with a hydrogen fraction of 15%, the carbon monoxide emissions dropped to a range of 2–4 ppm at an equivalence ratio of 0.63, significantly lower than 33 ppm recorded at an oxygen fraction of 26% and zero hydrogen enrichment. Flame imaging revealed that the flames became brighter, more compact, and stable with increasing hydrogen and oxygen fractions. These findings underscore the synergistic role of hydrogen and oxygen enrichment in extending lean blowout limits, improving thermal performance, and reducing emissions in dual annular counter-rotating swirl combustors.