On the segregation of NH3 and H2 within the flame front in premixed turbulent combustion of NH3/H2/N2 blends

• Differences in reactivity and diffusivity lead to segregation between NH 3 and H 2 . • Differential diffusion causes local equivalence ratio variations. • Local stoichiometric or rich regions form, leading to diffusion flame behaviour. • Misaligned isosurfaces affect strain rates and scalar gradients in the flame. The study analyses the segregation of NH 3 and H 2 in globally lean premixed turbulent flames of NH 3 /H 2 fuel blends using direct numerical simulation (DNS) data of statistically planar turbulent flames. Turbulent premixed flames for two fuel blends, 60%NH 3 /25%H 2 /15%N 2 and 40%NH 3 /45%H 2 /15%N 2 , with an equivalence ratio of 0.81, were examined in the thin reaction zones regime. Differences in chemical reactivity and differential diffusion between NH 3 and H 2 lead to local variations in equivalence ratio within the flame, significantly affecting species distribution compared to one-dimensional (1D) laminar premixed flames. The equivalence ratio variation within the flame causes locally either stoichiometric or fuel-rich pockets despite the globally lean condition in the cases considered here. This also enables localised diffusion mode burning, which is stronger for H 2 in the 60%NH 3 /25%H 2 /15%N 2 blend, whereas it is stronger for NH 3 in the case of 40%NH 3 /45%/15%N 2 H 2 blend. The transition from lean premixed to non-premixed combustion at the rear end of the flame leads to the misalignment of the normal vectors of NH 3 , H 2 , and temperature isosurfaces, impacting reaction-diffusion balance. The displacement speeds of H 2 isosurfaces exceed those of NH 3 , leading to differences in effective normal strain rates, which along with local equivalence ratio variation, influence the behaviour of the scalar gradient magnitude. These findings suggest that the modelling of premixed combustion of NH 3 /H 2 blends must account for variable equivalence ratio combustion and non-premixed burning mode, even for globally lean mixtures.