Experimental Investigation of Ultra-Lean Combustion Characteristics of N₂-Diluted Hydrogen and Cracked Ammonia

Hydrogen and ammonia are promising carbon-free fuels for power generation and propulsion systems, but challenges such as flashback, NO_x formation, and poor flame stability hinder their practical deployment. A practical solution is partial cracking of NH₃ into H₂/N₂ mixtures, which enhances reactivity while moderating flame temperature. However, the coupled influence of hydrogen fraction (HF) and bulk inlet velocity on flame stability, flashback, and emissions in swirl-stabilized premixed combustors remains poorly resolved. This study experimentally investigates the premixed combustion characteristics of N₂-diluted hydrogen and cracked ammonia in air using a swirl-stabilized burner, focusing on the concurrent effects of HF (40–100%) and bulk inlet velocity (15–25 m/s) under lean conditions (equivalence ratio = 0.3). The stability map was developed to identify flashback limits, while temperature distributions, NO_xemissions, oxygen concentration (OC), and flame macrostructures were analyzed. Increasing velocity enhanced flame temperature, for instance, at 40% HF, axial peak temperatures increased from 724 K (15 m/s) to 834 K (25 m/s). The flame length decreased with increasing HF, irrespective of flow velocity, thereby necessitating burner surface cooling and combustor downsizing for hydrogen combustion. NO_xemissions remained 6 ppm and below across all cases, and oxygen concentration trends indicated improved combustion efficiency at higher HF and velocities. Cracked ammonia flames (75% H₂, 25% N₂) exhibited strong dependence on equivalence ratio, with higher sensitivity under ultra-lean conditions. The results provide insights into controlling flame temperature and stability through velocity–composition coupling, highlighting the need for surface cooling and combustor downsizing in hydrogen combustion. These findings support key trade-offs and directly inform the design of low NO_xhydrogen–ammonia gas turbine combustors.