Local flame acceleration and wrinkling in lean hydrogen flames: Roles of differential diffusion and strain

A series of two-dimensional direct numerical simulations (DNS) of expanding lean premixed hydrogen–air flames is performed to investigate flame wrinkling dynamics, nonlinear cellular evolution, self-acceleration, species transport, and strain-rate redistribution under thermo-diffusively unstable conditions. The DNS is initialized using flame-front disturbances extracted from experimental PIV-Mie scattering images, providing an experiment-to-DNS framework for studying laboratory-informed broadband cellular flame fields rather than idealized sinusoidal perturbations. The simulations capture the progressive development of multi-scale cellular structures, including cell growth, cusp formation, cell splitting and merging, and flame-front restructuring, and show good agreement with experimental observations of spherically propagating hydrogen flames in terms of flame morphology and statistical features. The experimentally informed cellular flame field reveals a strain-mediated acceleration mechanism. Broadband flame-front wrinkling generates alternating regions of positive and negative normal strain rate, together with strong tangential strain localization near concave cell valleys. These strain-rate structures reorganize the local velocity-gradient field and enhance flame-induced gas-flow deformation. The analysis shows that flame acceleration, particularly in the strongly thermo-diffusively unstable case, exceeds that expected from wrinkled flame surface-area growth alone, indicating additional contributions from local transport, curvature, and strain-rate effects. Further examination of radical-species distributions shows that OH, H, and O are strongly localized near convex flame-cell tips, where enhanced curvature, hydrogen consumption, water production, and normal strain rate are coupled. These results identify flame-cell tips as locally intensified reaction zones, consistent with thermal-leading-point behaviour in a realistic nonlinear cellular flame field. The findings provide new physical insight into how experimentally observed cellular disturbances promote self-acceleration in lean hydrogen–air flames beyond simple geometric wrinkling effects.