Investigation of Differential Diffusion and Strain Coupling in Large Eddy Simulations of Hydrogen-Air Flames

Large Eddy Simulations with flamelet-based thermochemistry are used to investigate the behavior of a premixed hydrogen-air flame stabilized by a bluff-body. Validation against experimental data is carried out first to demonstrate the model’s ability to predict both velocity field and flame structure. The capability of the model in predicting differential diffusion effects is then assessed, in particular regarding the coupling between differential diffusion, tangential strain and curvature, and their effect on mixture fraction redistribution and reaction rate variation. Results indicate that unstretched flamelet thermochemistry is capable of capturing the increase in mixture fraction caused by positive resolved strain, as well as negative variations of mixture fraction due to negative curvature. Furthermore, the model is observed to mimic the effects of negative Markstein length to a certain extent, so that positive tangential strain causes reaction rate increase. The interplay between resolved stretch and preferential diffusion is also shown to lead to a shorter flame length which is in better agreement with experimental observations as compared to simulations under unity Lewis number assumption. These findings highlight that the macroscopic effects of differential diffusion and stretch on the premixed hydrogen flame, characterized by significant strain levels, can be predicted using a flamelet-based approach and without recurring to strained flamelets database, which implies important simplifications in the combustion modeling of turbulent hydrogen-premixed flames and offers valuable insights for the design of novel combustors.