Insights from swirl number and ambient pressure variations with a hydrogen/ammonia swirl stabilized diffusion flame

Contemporary research into decarbonized fuels such as H 2 /NH 3 has highlighted complex challenges with applied combustion, with marked changes in thermochemical properties leading to significant issues such as limited operational range, flashback, and instability, particularly when attempts are made to optimize emissions production in conventional lean-premixed systems. Non-premixed configurations may address some of these issues but often lead to elevated NO x production, particularly when ammonia is retained in the fuel mixture. Optimized fuel injection and blending strategies are essential to mitigate these challenges. This study investigates the application of a 75 %/25 % mol H 2 /NH 3 blend in a swirl-stabilized combustor, operated at elevated conditions of inlet temperature (500 K) and ambient pressure (0.11–0.6 MPa). A complex, nonmonotonic relationship between swirl number and increasing ambient combustor pressure is demonstrated, highlighting the intricate interplay between swirling flow structures and reaction kinetics, which remains poorly understood. At medium swirl (SN = 0.8) an increase in pressure initially reduces NO emissions, diminishing past ∼0.3 MPa, with an opposing trend evident for high swirl (SN = 2.0) as NO emissions fall rapidly when combustor pressure approaches 0.6 MPa. High-fidelity numerical modeling is presented to elucidate these interactions in detail. Numerical data, generated using Detached Eddy Simulations (DES), were validated against experimental results to demonstrate a change in flame anchoring on the axial shear layer and marked change in recirculated flow structure, successfully capturing the features of higher swirl number flows. Favorable comparisons are made with optical data and a reduction in NO emissions with increasing pressure is demonstrated to replicate changes to the swirling flame chemical kinetics. Findings provide valuable insights into the combustion behavior of hydrogen-rich ammonia flames, contributing to the development of cleaner combustion technologies. • Investigates H 2 /NH 3 (75/25 mol) combustion under varying swirl and pressure conditions. • Reveals a non-monotonic effect of swirl number and pressure on NO emission trends. • Combines experiments and DES to analyse flame anchoring and recirculation structures. • Validated DES simulations capture pressure-induced shifts in flow and combustion behaviour. • Provides insights for designing low-emission, hydrogen-rich combustors for future energy.