Impact of differential diffusion and reactivity of NH3 and H2 on flame front characteristics of lean premixed turbulent NH3/H2/N2 flames

Partial cracking of carbon-free ammonia (NH3) into hydrogen (H2) and nitrogen (N2) or its blending with H2 present a viable solution to challenges associated with its low reactivity and flame speed. Through direct numerical simulations of statistically planar lean turbulent premixed flames for NH3/H2 fuel blends, this study analyses effects of differential diffusion and reactivity in the characteristics of the flame front. Two fuel blends, 60%NH3/25%H2/15%N₂ and 40%NH3/45%H2/15%N2, with an equivalence ratio of 0.81, are considered. The analysis reveals significant variations in local equivalence ratio within the flame leading to either stoichiometric or fuel-rich pockets despite the globally lean condition. In addition, a localised diffusion mode of burning is observed, which is stronger for H2 in the 60%NH3/25%H2/15%N2 blend, whereas it is stronger for NH3 in the 40%NH3/45% H2/15%N2 blend. The transition from lean premixed to non-premixed combustion at the rear end of the flame causes the misalignment of the normal vectors of NH3, H2, and temperature isosurfaces, impacting reaction-diffusion balance. The findings suggest that the modeling of premixed combustion of NH3/H2 blends must account for variable equivalence ratio and non-premixed burning modes, even for globally lean mixtures.