Validation of an integrated ignition-combustion model for premixed hydrogen-air flames under lean conditions

Recent climate changes, driven by greenhouse gas emissions, along with global regulations aimed at mitigating these effects, have intensified research on carbon-free fuels. Among these, hydrogen stands out as one of the most promising options.In this study, use is made of a recent 1D kernel expansion model developed by the authors, which is based on the conservation equations of mass, energy and deficient reactant, and on the transient thermo-diffusive theory of flames to estimate the reactant and temperature gradients at the outer flame surface. The kernel expansion model accounts for the variability of thermodynamic properties both inside and outside the flame volume, including high-temperature ionization and dissociation effects.The kernel expansion model is coupled with a flame propagation combustion model, which accounts for the effects of both thermo-diffusive instability and turbulence. The former, induced by a less-than-unity Lewis number and hence key for lean hydrogen combustion, is modeled introducing an equivalent flame consumption speed to quantify the increase in kernel expansion velocity. The latter is modeled using the fractal geometry theory, which considers the additional flame wrinkling produced by the turbulent eddies at different scales. A proper transition between kernel expansion and unstable turbulent flame is carried out when non-linear flame stretch effects become negligible. The model is validated against experimental data from literature obtained for premixed hydrogen-air flames propagating in an optically accessible spherical bomb. The data refer to quiescent and turbulent mixtures at multiple equivalent ratios (from 0.45 to 0.97) and turbulence intensities (up to 2.81 m/s). The capabilities of the present model are assessed by comparing its predictions with the measured time histories of chamber pressure and flame radius. A good agreement is achieved without the need for case-by-case tuning, confirming the consistency of the integrated ignition-combustion model proposed in this work.