Propagation characteristics of lean turbulent premixed ammonia–hydrogen flames

Direct numerical simulations (DNS) of fuel-lean turbulent premixed NH₃-H₂-N₂-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 H₂ 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%NH₃-25%H₂-15%N₂-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 H₂ mixed with the low-reactive NH₃ in moderate quantities, resulting in higher generation of flame elements extending into the product side and dynamic evolution of H₂. The flame structure analysis, in terms of conditional averages, revealed a distinctive variation in H₂ and H atom distributions. The flames with a higher H₂ fraction (40%NH₃-45%H₂-15%N₂-air) produced a second peak of H₂O₂ in the trailing edge region, indicating additional production in the intense reaction zone. Additionally, in the 60%NH₃-25%H₂-15%N₂-air flame, the reaction rate of H₂ exhibited a unique behavior, with H₂ being produced in the intermediate flame zone and rapidly consumed in the reaction zone, differing from other cases.