Propagation characteristics of lean turbulent premixed ammonia-hydrogen flames

Direct numerical simulations (DNS) of fuel-lean turbulent premixed NH3-H-2-N-2-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-2 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%H-2-15%N-2-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-2 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 H-2. The flame structure analysis, in terms of conditional averages, revealed a distinctive variation in H-2 and H atom distributions. The flames with a higher H-2 fraction (40%NH3-45%H2-15%N-2-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%H-2-15%N-2-air flame, the reaction rate of H-2 exhibited a unique behavior, with H-2 being produced in the intermediate flame zone and rapidly consumed in the reaction zone, differing from other cases.