Modeling aspects of thermodiffusively unstable turbulent premixed flames

The modeling of hydrogen-air flames has become increasingly important due to ongoing efforts for the decarbonization of propulsion and power applications. Significant research has focused on developing accurate and efficient combustion models to simulate the propagation of lean hydrogen-air premixed flames. In this context, this study investigates key modeling aspects of thermodiffusively unstable turbulent premixed flames, leveraging an experimental dataset comprising Bunsen flames under various turbulence intensities and thermochemical conditions. The stability limits of the hydrogen-enriched-methane/air mixtures are assessed using highly resolved numerical simulations with detailed thermochemistry and transport. The thermodiffusive instability of the mixtures is quantified by comparing the numerical dispersion relation with the theoretical limit of the Darrieus–Landau linear growth rate. The ensuing results are consistent with the values of a thermodiffusive instability parameter ɛ based on theoretical models, which is therefore used throughout the analysis on modeling aspects. For thermodiffusively unstable mixtures, higher flame surface density and enhanced wrinkling are observed under sufficiently high turbulence intensities. The experimental measurements on the flame front position are spatially filtered to evaluated, similarly to flame resolved numerical simulation, the wrinkling factor. A wrinkling factor model, based on the volume averaged filtered flame surface density is proposed to represent the interactions between turbulence and intrinsic flame instabilities. This empirical model incorporates a measure of the overall turbulence intensity experienced by the flame, a thermodiffusive instability parameter based on theoretical considerations, and the LES grid size.