Effect of water vapor addition on the combustion characteristics in partially premixed swirling ammonia/methane/air flames

Ammonia (NH₃) combustion has been extensively studied for its potential in achieving zero carbon emissions. However, ammonia combustion is prone to high NO_x emissions. This study investigates the turbulent combustion and emission characteristics of partially premixed swirling ammonia/methane/air flames with water vapor addition, aiming for clean and efficient combustion. Various water vapor addition ratios (0 %, 5 %, 10 %, and 15 % by molar fraction) in hot air were examined. Large eddy simulations were conducted using the OpenFOAM solver with the Partially Stirred Reactor combustion model and the Okafor reaction mechanisms. The simulation results indicate that as the water vapor addition ratio increases, the flame height marginally increases, the flame temperature decreases, and the concentrations of free radicals such as H, O, and OH decrease, leading to a reduction in NO concentration. However, flame instability and intermittent local quenching become evident at a water vapor addition ratio of 15 %. Additionally, chemical kinetics analysis shows that the laminar burning velocity and adiabatic flame temperature decrease with increasing water vapor addition ratios, and the reaction rates in the NO formation pathway also decrease. The effects of water vapor addition on ammonia/methane flames can be ranked as follows: physical dilution effect, thermal effect, and chemical effect.