LES of Reacting Mixing Layers: Species Concentration Boundedness and Inflow Conditions

The present work carries out large-eddy simulations of the low-speed, high-Reynolds number, chemically-reacting mixing layer experiments by Slessor et. al. In particular, we study the low-heat release case with prescribed turbulent inflow conditions. The objective of the present work is to gain insight into the physics of the reacting shear layer and to address some associated computational challenges. This set of experiments are at subsonic conditions and use hydrogen and fluorine as the fuel and oxidizer, respectively. The hypergolic reaction between H_2 and F_2, as it was run in the Slessor et al. experiments, is characterized by a large Damköhler number, making the chemistry fast compared to the flow time scales: the product formation and temperature-rise in the flow is mixing-limited. In this work, we attempt to address the issue of overshoots and undershoots of species mass-frictions, often observed in LES of high-Reynolds number flows, by modifying the convective fluxes. We observe that the modified fluxes eliminate the global excursions of species mass-fraction concentration. A three dimensional simulation is performed by imposing synthetic turbulence at the inflow, generated using the digital filter approach of Klein et al., to mimic the experimental flow conditions. The velocity profiles, growth rate, and product thickness obtained from the simulations show a good match with the experimental data, but the peak value of temperature-rise is slightly over predicted.