How mixing process mitigates C-N interactions: A quantitative LES study of dual-annular NH3/CH4 flames

The mixing process plays a profound role in the local thermochemical environment, flame structure, and NO x formation of NH 3 /CH 4 flames. Previous experiments have shown that separating fuel nitrogen and hydrocarbon stream can affect C-N interactions and reduce NO x emissions, but the underlying mechanism remains inadequately understood. In this work, large eddy simulations with detailed chemical kinetics are performed for NH 3 /CH 4 flames in a dual-annular burner featuring flexible straight/swirl jets configurations. A novel dual-flow mixing strategy that combines a fuel-rich, NH 3 -blended primary stream with a fuel-lean, NH 3 -free secondary stream is invesitaged. First, the LES results together with chemiluminescence measurements show that increasing the primary equivalence ratio (Φ pri ) from 0.90 to 1.35 significantly reduces the OH concentration and the overall reaction reactivity. At the same time, it enhances the ammonia decomposition due to the internal recirculation, thereby effectively suppressing the NH 3 -to-NO x conversion pathway. Second, to quantitatively investigate the mixing process of multiple fuels with air, we define the mixture fraction of CH 4 and NH 3 contributed components, f NH3 and f CH4 . Under primary fuel-lean conditions, both f NH3 and f CH4 are strongly affected by the methane splitting ratio S CH4 . A higher S CH4 contributes to reduced formation of HCN, a key intermediate associated with hydrocarbon-ammonia coupling. Finally, a fuel C-N separation factor S C-N is further proposed to quantify the separation between fuel carbon and nitrogen components. The results show that the C-N separation and therefore the mixing process are more sensitive to methane staging under primary fuel-lean conditions than under primary fuel-rich conditions. The HCN and NO concentrations are closely associated with S C-N . These results suggest that methane staging can reduce emissions by enhancing fuel nitrogen/hydrocarbon separation and weakening C-N coupling, especially under primary fuel-lean conditions. Novelty and significance statement How the mixing process influences NOx formation in NH 3 /CH 4 separation flames remains poorly understood at a fundamental level. This study introduces, for the first time, the fuel carbon/nitrogen separation factor ( S C-N ) as a quantitative scalar metric to diagnose the intricate multi-component mixing process in staged NH 3 /CH 4 flames and its critical link to NOx formation pathways. Moving beyond conventional macroscopic descriptors such as equivalence ratio or fuel staging ratio, this metric reveals how deliberate spatiotemporal separation of fuel nitrogen and hydrocarbon streams decouples key chemical interactions particularly those involving HCN intermediates, thereby unveiling a mixing-controlled mechanism for NOx reduction. S C-N provides a novel theoretical framework for designing low-emission ammonia combustion systems.