Stability and combustion characteristics of dual annular counter-rotating swirl oxy-methane flames: Effects of equivalence and velocity ratios

An experimental study was carried out to investigate flow/flame interactions, stability, combustion, and emissions properties of CH₄/O₂/CO₂ stratified flames stabilized on a dual annular counter-rotating swirl (DACRS) burner for gas turbine combustion applications. The effects of two targeted parameters, namely equivalence ratios (for primary - ϕ_p, secondary - ϕ_s, and global - ϕ_g streams) and velocity ratio (V_r = V_p/V_s: primary to secondary stream inlet velocity ratio), are studied at fixed volumetric oxygen fraction (OF) in the oxidizer mixture (O₂ + CO₂) of both primary and secondary streams of 34 % at fixed V_p of 5 m/s. The experimental results indicated that no flame flashback was recorded in the domain of any operational ϕ_p up to stoichiometric operation of the secondary stream (ϕ_s = 1.0). At near stoichiometric operation of the primary stream, the main secondary flame can persist in extremely lean conditions (ϕ_s = 0.45 @ V_r = 3), thereby extending the thresholds of flame blowout. Moreover, raising ϕ_p from 0.4 to 1.0 results in significant reduction of ϕ_s at blowout from 0.595 to 0.456, corresponding to reducing the combustor ϕ_g at blowout from 0.577 to 0.499. At lower ϕ_p, the oxy-methane flame tends to lift and extinguish earlier.