Differential diffusion effects on chemical mode structure in lean premixed hydrogen flames via computational singular perturbation analysis

A species-dependent transport formulation incorporating non-unity Lewis numbers and thermal diffusion is employed within a reacting-flow large eddy simulation (LES) framework for hydrogen–air combustion, extending the standard unity Lewis-number closure while maintaining consistency with detailed kinetics and energy transport. The formulation is applied to a lean premixed hydrogen–air Bunsen flame, and the structural response to differential diffusion is characterized through comparison with the unity Lewis baseline. Non-unity transport produces a systematic displacement of the flame front, local mixture enrichment near the reaction zone, and a broader thermal layer. Computational singular perturbation (CSP) diagnostics are then applied to examine how transport modeling affects the intrinsic chemical dynamical structure of the flame. While the dominant reaction pathways remain largely unchanged, differential diffusion reorganizes the spatial distribution of chemical modes. In the inner core, the explosive mode region shifts downstream and widens while its growth rate decreases. HO2-mediated chemistry assumes a dominant role that persists beyond the flame front, substantially delaying the transition toward the NO-controlled slow manifold. These results demonstrate that simplified transport assumptions affect not only macroscopic flame observables but also the hierarchy of chemical time scales governing lean hydrogen flame evolution.