FGM modeling of thermo-diffusive unstable lean premixed hydrogen–air flames

Ultra-lean premixed hydrogen combustion is a possible solution to decarbonize industry, while limiting flame temperatures and thus nitrous oxide emissions. These lean hydrogen/air flames experience strong preferential diffusion effects, which result in thermo-diffusive (TD) instabilities. To efficiently and accurately model lean premixed hydrogen flames, it is crucial to incorporate these preferential diffusion effects into flamelet tabulated chemistry frameworks, such as the Flamelet-Generated Manifold (FGM) method. This is challenging because the preferential diffusion terms in the control variable transport equations contain diffusion fluxes of all species in the mechanism. In this work, a new implementation is presented; the full term is reduced by only considering the most contributing species. When carefully selecting this set of major species, preferential diffusion fluxes along the flame front, i.e., cross-diffusion, can be captured. This is particularly important for manifolds that include heat loss effects, where enthalpy is one of the control variables. The diffusion of the H-radical has a significant contribution to the enthalpy transport equation, and cross-diffusion of the H-radical is non-negligible. Two manifolds, without and with heat loss effects, and the set of major species are analyzed in an a-priori and a-posteriori manner. Simulations of TD unstable hydrogen-air flames with detailed chemistry and several FGM models show that accurately capturing cross-diffusion of enthalpy is important for correctly predicting the flame shape and dynamics.