Propagation and stability of isolated reactive fronts in ultra-lean hydrogen-air flames under intense heat losses

This study employs three-dimensional (3D) numerical simulations to determine the minimum equivalence ratio for which a premixed hydrogen-air flame can propagate within a 4 mm gap bounded by cold isothermal walls. This limiting condition provides a quantitative measure of practical interest in hydrogen safety. Using a previously validated one-step reduced chemical-kinetic mechanism with a simplified transport model, the complex flame dynamics occurring near this propagation limit is investigated and compared with recent experimental measurements. The simulations confirm that the interplay between conductive heat losses to the walls and diffusive–thermal instabilities is responsible for fragmenting the continuous reactive front into isolated flame kernels capable of sustaining flame propagation under conditions that would otherwise be impossible. The simulations identify the critical equivalence ratio that triggers the transition between two different isolated flame morphologies: the circular flame and the double-cell flame. Circular flames, which propagate slowly as an oblate-spheroidal surface, are sustained primarily by diffusion, which supplies fuel from all directions. In contrast, for double-cell flames, comparable diffusive and convective fluxes increase sensitivity to far-field perturbations, which can lead to an unstable motion or a fractal-like propagation via successive flame splitting. Notably, both flame morphologies maintain symmetry relative to the gap centerplane. By mapping these regimes as a function of the equivalence ratio, regions of stable coexistence are identified and characterized. While the simulations accurately capture observed regime transitions, some quantitative discrepancies remain in the predicted velocity and size of these isolated flames when compared with reported experimental measurements. Novelty and significance statement The novelty of the present work lies in the detailed 3D numerical characterization of the two different isolated flame-cell regimes that emerge near the extinction limit of hydrogen–air mixtures, providing insights into the internal cell structure that remains inaccessible to current experimental analysis. Crucially, we show that these isolated cell structures survive well below the standard flammability limit predicted by planar flame theory, challenging established safety assumptions for hydrogen-based systems.