Combined Flame Shape Transitions and Their Effects on NO Emission for Micro-mixing Nozzles at Different Hydrogen Blending Ratios

Micro-mixing (MM) combustion technology has advantages in flame stability and low pollutant emissions. When addressing the changes in hydrogen blending ratio (χH2) in syngas, the influence of the zones among the MM nozzles on flame stability is crucial. Furthermore, the effects of the flame shape transitions on the NO emission are significant for nozzle design. In this study, we focused on the annular MM nozzles, investigating the combined flame shape transitions and their effects on NO emission at different χH2 and the air velocity at the nozzle inlet using OH-PLIF with OH* chemiluminescence techniques. The results reveal that the influence of the flame interaction zones (FIZ) on the outer recirculation zones among the adjacent nozzles (ORZ) is critical for anchoring the flames in the ORZ. When χH2 is below 60%, the flame interaction and outer recirculation dominate the flame stability. The flame interaction becomes the most pronounced at χH2 = 60%. As χH2 increases from 60 to 100%, the flames are progressively anchored in the center recirculation zones (CRZ), leading to the formation of M-shaped flames. The reaction zones gradually concentrate toward the nozzle outlets. Under all operating conditions, NO emission remains below 10 μL/L (@15%O2). The relative axial coordinate of the flame centroid (zc/D2) has the highest Pearson correlation with NO emission and the most pronounced influence (the highest importance calculated by the neural network) on NO emission. NO emission exhibits an exponential decay as zc/D2 increases. When zc/D2 exceeds 5, NO emission is lower than 5 μL/L (@15%O2).