Effects of Ammonia Substitution and Fuel Injection Strategy on Non-Premixed Methane–Air Swirl Flames

The use of ammonia (NH3) as a carbon-free fuel is a promising pathway for low-carbon combustion, but its low reactivity and high NOx formation present significant challenges for stable operation and emissions control. In swirl-stabilized non-premixed systems, the effects of fuel injection strategy, swirl intensity, and NH3/CH4 blending remain insufficiently understood. This study presents a systematic experimental comparison of three fuel injection configurations (radial, axial, and central-rod) on the stability and emission characteristics of NH3/CH4/air swirl flames. Experiments covered NH3 fractions from 0% to 100% at two swirl numbers (Sn = 0.8 and 1.4), with equivalence ratio (0.6–1.2) variations at 50% NH3 and Sn = 1.4. Emissions (NOx, CO, CO2, H2) are measured using gas analyzers, complemented by optical diagnostics for flame structure assessment. Results show that injector configuration is a key parameter governing both flame stability and pollutant formation. Radial injection provides the widest stability range and enables stable combustion up to 100% NH3, attributed to enhanced recirculation and improved flame anchoring. In contrast, axial and central-rod configurations exhibit reduced stability limits and earlier flame destabilization with increasing ammonia content. A trade-off between stability and emissions is observed, with radial injection enhancing flame stability. Increasing NH3 substitution reduces measured NOx emissions at high ammonia fractions, but the associated rise in H2 and changes in CO/CO2 suggest reduced combustion efficiency and possible incomplete NH3 oxidation. These findings highlight fuel injection strategy as a key parameter for balancing stability and emissions in NH3/CH4 combustion systems.