Effects of N2 on Flame Structure and Emissions in Non-Premixed Swirl Combustion of Partially Cracked Ammonia

Abstract Partial cracking enhances ammonia combustion through the in situ production of H2, while the accompanying product N2 may alter the flow, reaction structure, and emission characteristics of the resulting cracked ammonia fuel. Nonpremixed swirl flames were investigated at swirl numbers S = 0.42, 0.73, and 1.27, cracking ratios from cr0 to cr60, and ϕ = 0.4–1.2 in an atmospheric, semienclosed combustor. At cr60, the NH3/H2/N2 and NH3/H2 flames were compared at identical absolute NH3 and H2 feed rates. At S = 0.73, a continuous reaction structure formed along the near-nozzle shear layer, and this condition was selected as the baseline. Increasing the cracking ratio contracted the OH and NH distributions toward the near-nozzle shear layers. The integrated NH signal peaked at cr20, whereas the NH signal area and axial extent decreased continuously, revealing local signal enhancement accompanied by spatial compression of the reaction layer. Retaining product N2 increased the fuel-side volumetric flow rate by 23.1% and reduced the combustible species mole fraction to 81.3%, while causing little change in the representative mean flow. Despite this limited change in the mean flow, the OH field became wider and more diffuse, and the axial extent of the NH reaction layer decreased. NO increased by 5.9% and 14.5% at ϕ = 0.6 and 0.8, respectively, while N2O increased from 0.57 to 13.71 ppm at ϕ = 0.9. The corresponding differences in combustion efficiency remained below 0.1 percentage points. Product N2 and the associated changes in dilution, thermophysical properties, and fuel flow had a greater effect on reaction structure and individual species emissions than on combustion efficiency. These results demonstrate that product N2 in cracked-ammonia fuel acts not merely as an inert diluent, but as an important factor influencing the reaction structure and pollutant formation.