LES of reacting mixing layers: Species concentration boundedness and inflow conditions

The present work carries out large-eddy simulations of the high-Reynolds number, low Mach number, chemically reacting mixing layer experiments by Slessor et al.1 The objective is to gain an insight into the physics of the flow-field and also address the computational challenges associated with such a flow. In this work, we discuss issues related to species mass fraction (concentration) overshoots and undershoots, often observed near regions having sharp jump-discontinuity in species concentrations. We attempt to address the issue of spurious reactions and associated erroneous temperature rise, resulting from excursions of species concentration, by modifying the convective fluxes. Here, simulations are performed with incoming streams of fluids (that form the mixing layer) having turbulent boundary layer profiles. Synthetic turbulence is generated using digital filter approach by Klein et al.² to mimic the experimental flow conditions at the inlet of the test section. We present results from two-dimensional simulations of chemically reacting mixing layers, with modified fluxes, that show no signs of excursion in the species concentrations and nearly well-behaved temperature rise due to chemical reactions. Using the modified fluxes, three dimensional simulations were performed which showed good with the experimental data for velocity profiles, growth rate and product thickness. The spread of the temperature rise profile in the simulations compared well with the experiment, but over predicted the peak value of temperature.