On the effects of flow/mixture stratification on combustion/stability behaviors of dual-swirl oxy-methane flames: Experimental and numerical study

Oxy-methane (CH₄/CO₂/O₂) partially premixed flames were studied numerically and experimentally in a dual annular counter-rotating swirl (DACRS) burner for clean power production in gas turbines. The velocity of the primary (central) stream was fixed at 5 m/s with varying the secondary (annular) velocities to achieve velocity ratios (Vr) of 3.878, 3, and 2.27 over ranges of equivalence ratios for both the primary (φ_p: 0.4–1) and secondary (φ_s: 0.456–0.85) streams at fixed (OF) oxygen fraction of 34 %. Increasing φ_p (from 0.4 to 1.0) enhances the overall stability of combustion and supports flames of the ultra-lean secondary stream (from 0.595 to 0.456), thereby extending the combustor lean blowout limit to lower global equivalence ratios (φ_g: from 0.577 to 0.499). No flame flashback occurred within the operational φ_p domain up to the stoichiometric secondary stream, φs = 1.0. Variations in velocity ratios impact flame structure, stability, and lean blowout performance. Velocity ratios of 2.27, 3, and 3.87 correspond to expansion flame angles of 70°, 58°, and 42°, respectively, owing to the Coanda effect, wherein fluid flows tend to adhere to solid surfaces. The laminar flame speed is more responsive to variations in φ_s than φ_p. Better flow/flame interactions were achieved at lower Vr with bigger recirculation zones and better flame-holding stability. The intricate relationship between temperature distribution and velocity ratios emphasizes their significant role in influencing chemical reactions, particularly in elevating CO concentrations.