Stability and Combustion/Emission Characteristics of Stratified Dual-Swirl Oxy-Methane Flames: Effects of Oxygen Fraction

The stability, combustion, and emission features of stratified oxy-methane (CH₄/O₂/CO₂) flames stabilized over a dual annular counter-rotating swirl (DACRS) burner, developed for gas turbine combustion applications, were investigated experimentally. The experiments were performed at fixed velocity ratio (V_r = V_p/V_s = 3.0) in both the primary and secondary streams at a constant primary stream velocity, V_p of 5 m/s and at fixed primary stream equivalence ratio, φ_p = 0.9, and over ranges of oxygen fractions (OF_p for the primary stream, OF_s for the secondary stream) and secondary stream equivalence ratios. Measurements of flame macrostructure, temperature profiles, and exhaust emissions were recorded to characterize the flames and validate future numerical models. The testing findings revealed no flame flashback within the operational ranges of OF_p and OF_s and up to φ_s = 1.0. However, the near stoichiometric operation of the primary stream (φ_p = 0.9) at OF_p = 0.38 permitted the main secondary flame to tolerate exceptionally lean conditions (φ_s = 0.397 at OF_s = 0.34 and φ_s = 0.223 at OF_s = 0.39), raising the thresholds for the flame blowout. Increasing OF_p from 0.21 to 0.38 significantly reduced φS at blowout from 0.537 to 0.223, corresponding to a decrease in the combustor's global equivalence ratio (φ_g) at blowout from 0.554 to 0.254 at global oxygen fraction (OF_g) from 0.38 to 0.39. Lower OF_p values caused earlier flame lift-off, indicating the greater influence of OF_p on flame macrostructures.