The role of preferential diffusion on the ignition dynamics of lean premixed hydrogen flames

High-fidelity Large-Eddy Simulations (LES) are used to study the ignition dynamics of two fuel/air mixtures with distinct Lewis numbers Le , unveiling the impact of preferential diffusion during flame expansion including its stabilization above the burner. The simulations cover a CH4/air 4 /air mixture with a unity Lewis number Le approximate to 1 and a lean H2/air 2 /air mixture with a sub-unity Lewis number Le approximate to 0.34. . 34. Both mixtures are injected at a fixed bulk flow velocity of Ub b = 5 m s-1, -1 , with the equivalence ratio adjusted to match the laminar burning velocity S degrees degrees = 0.25 . 25 m s-1. -1 . LES results, including non-reacting flow velocity fields and ignition dynamics, are validated against a large experimental dataset encompassing non-reacting PIy, pressure overshoot, and flame visualization via OH-PLIF. This validation process significantly bolsters confidence in the chosen numerical approach. To elucidate the influence of preferential diffusion on flame propagation during the ignition process, the absolute flame speed is analyzed from kernel initiation, through complete consumption of the fresh gases to flame stabilization. It is found that despite having a lower thermal expansion ratio (Pu/Pb), u / P b ) , the H2/air 2 /air flame still exhibits an enhanced absolute flame speed compared to the CH4/air 4 /air flame. This results in a similar pressure time-series over the full ignition process. An analysis isolating the effects of thermal expansion ratio and stretch effect reveals that this unexpected observation arises from the interplay between preferential diffusion, particularly evident in sub-unity Le mixtures, and the effects driven by the thermal expansion rate. Finally, the role of preferential diffusion and flame stretch on the local flame burning rate is investigated and it is demonstrated that LES can capture the effects of local enrichment observed in DNS studies.