Effects of velocity ratio and hydrogen/oxygen enrichments on lean blowout and combustion behavior of stratified CH₄/H₂/O₂/CO₂ flames in a dual swirl combustor

This work experimentally evaluates how the velocity ratio (Vr = pilot-stream velocity/main-stream velocity), hydrogen enrichment, and oxygen concentration affect lean blowout limits, flame structure, temperature, and emissions in a dual annular counter-rotating swirl (DACRS) combustor. Three velocity ratios (2.27, 3.00, 3.87, with pilot velocity fixed at 6 m/s) were tested alongside hydrogen fuel fractions (HF) of 0–15 % and O₂/CO₂ main-stream oxidizer oxygen fractions (OF_m) of 20–34 %. The results show that a lower Vr (stronger main swirl) and higher hydrogen/oxygen enrichments dramatically extend the lean blowout limit, enabling stable combustion at ultra-lean overall equivalence ratios (ϕ_m≈0.35), while also altering the flame shape. With Vr = 2.27, 15 % HF, and 34 % OF_m, the flame remained compact and attached even at very lean mixtures, and carbon monoxide emissions fell below 5 ppm, indicating near-complete combustion. In contrast, a high Vr and lower enrichments produced an elongated, lift-prone flame and incomplete combustion (CO up to 50–65 ppm). These findings underscore the significance of optimizing swirl intensity and fuel/oxidizer composition in oxy-fuel combustors to achieve enhanced flame stability, ultra-lean operability, and minimal emissions.