Influence of load flexibility on combustion performance of combined micro-mixing flames within a wide equivalence ratio and air velocity ranges

Micro-mixing (MM) combustion technology offers significant advantages for fuel-flexible, low-emission gas turbines. To support load-flexible operation in systems fueled by challenging low heating value syngas, a systematic understanding of MM flame characteristics under wide operational ranges is required. This study investigates the combustion performance of combined MM flames across broad ranges of equivalence ratio (φ = 0.52, 0.56, 0.60, 0.64, 0.68, and 0.73) and air velocity (v_air = 30, 40, 50, and 60 m/s), analyzing flame structure, dynamics, and NO emission. Two distinct time-averaged flame types are observed: a flame lifted above the nozzle (Type A) and a flame anchored at the nozzle outlet (Type B). As φ increases or v_air decreases, the temporal distribution uniformity deteriorates, and the dominant oscillation frequency of the OH* chemiluminescence shifts lower. Dynamic Mode Decomposition (DMD) analysis reveals that the spatial distribution of the dominant mode closely aligns with the time-averaged OH* signal. NO emission increases rapidly with φ. However, across all test conditions, it remains at or below 5 μL/L (@15%O₂). Conversely, an increase in v_air reduces NO emission, which is attributed to a shorter gas residence time within the combustor. The results provide fundamental insights into MM flame behavior under conditions relevant to flexible plant operation.