Propagation characteristics of lean turbulent premixed ammonia–hydrogen flames

Abstract

Direct numerical simulations (DNS) of fuel-lean turbulent premixed NH-H-N-air flames are analyzed to investigate propagation and flame structural characteristics under fixed velocity and length ratios. To comprehensively assess the impact of diffusive-thermal imbalances on hydrogen–enriched ammonia flames, additional solutions with unity-Lewis-number transport were analyzed and compared with those obtained using the mixture-averaged transport model. The increase of H fraction in the fuel leads to elevated mean turbulent flame speed and stretch factor, indicating the impact of thermal-diffusive instability. The turbulent flame speed of the 60%NH-25%H-15%N-air flame displays pronounced oscillations, a phenomenon absent in other mixtures considered in the current study. This behavior is attributed to the preferential diffusion of H mixed with the low-reactive NH in moderate quantities, resulting in higher generation of flame elements extending into the product side and dynamic evolution of H. The flame structure analysis, in terms of conditional averages, revealed a distinctive variation in H and H atom distributions. The flames with a higher H fraction (40%NH-45%H-15%N-air) produced a second peak of HO in the trailing edge region, indicating additional production in the intense reaction zone. Additionally, in the 60%NH-25%H-15%N-air flame, the reaction rate of H exhibited a unique behavior, with H being produced in the intermediate flame zone and rapidly consumed in the reaction zone, differing from other cases.