Development of Flame Instability Induced by Perturbations of Finite Amplitude in Lean Hydrogen–Air Mixtures

Lean hydrogen-air flames exhibit pronounced susceptibility to intrinsic instabilities. Discrepancies in thermal and mass diffusivities subject these flames to coupled thermodiffusive and hydrodynamic instabilities, complicating practical implementation of hydrogen-based fuels. Small-scale perturbations on an initially planar flame front grow exponentially, inducing local acceleration and wrinkling. While early-stage evolution under infinitesimal perturbations is well-studied theoretically and numerically, real-world scenarios involve finite-amplitude perturbations (e.g., from obstacles, droplets, particles, or non-uniform flows)—a regime requiring further investigation. This study numerically examines instability development triggered by both small-scale perturbations and perturbations with amplitudes comparable to the flame thickness. Two initiation mechanisms are implemented: (1) localized temperature elevation and (2) finite-amplitude spatial distortion of the flame front. Results demonstrate that initial perturbation amplitude critically influences instability patterns, alters growth rates, and can destabilize modes otherwise stable under infinitesimal disturbances. These findings support development of subgrid combustion models and can be used for the analysis and interpretation of flame propagation phenomena in multiphase systems.