Direct numerical simulation of turbulent premixed methane-ammonia-air jet flames

Methane-ammonia blended combustion has emerged as a promising strategy to reduce carbon emissions while maximizing the utilization of existing fossil-fuel-based burners, serving as a viable intermediate step towards carbon neutrality. In order to effectively implement methane-ammonia combustion strategies in practical combustors, it is crucial to understand the combustion characteristics of premixed methane-ammonia-air flames, particularly focusing on how turbulence and varying ammonia blending ratios influence flame structure and NO formation. Therefore, in the present study, turbulent premixed methane-ammonia-air jet flames at different ammonia blending ratios were investigated using two-dimensional direct numerical simulation (DNS). Local heat release rates, flame front curvature, tangential strain rates, NO production rates, and their correlations were systematically evaluated. Results indicate that, under turbulent flame interactions, flame front curvature has a dominant influence; specifically, flame elements convex toward the burned side exhibit increased heat release rates. These effects are consistently predicted across different ammonia blending ratios when appropriately normalized with their corresponding laminar flame quantities. However, a comparison between stretched laminar flames and local turbulent flame elements reveals that convex flame elements significantly enhance NO formation, which cannot be fully captured by laminar planar flame analyses alone. Therefore, it is essential to incorporate curvature effects for accurately evaluating NOx emissions in turbulent combustion.