LES of High-Re Reacting flows: Active Scalar Conservation and Boundedness

Abstract

We aim to bring predictive capabilities to reacting flow simulations at high Reynolds numbers. In particular, we are interested in non-premixed reacting flows with realistic inflow conditions and heat release. These flows find their application in Scramjet engines, used for hypersonic propulsion. Higher-order, low-dissipation simulations of non-premixed combustion flows are often subject to numerical errors due to the presence of sharp gradients in species mass-fractions. These dispersive errors lead to overshoots and undershoots in species mass-fractions, resulting in violation of conservation of mass. In reacting flows, these errors result in overshoots of temperature above that allowed by the adiabatic flame temperature rise, rendering the simulations unreliable. To overcome this issue, we develop a new switched, low-dissipation flux methodology that mitigates these errors. The new method is validated on a range of one, two and three-dimensional problems, showing its effectiveness and promise to provide reliable solutions. We use this newly developed method to simulate chemically reacting, spatially evolving subsonic and supersonic mixing layers at high Reynolds numbers. We investigate the effect of inflow conditions on subsonic reacting mixing layers, following the experiments of Slessor et al. [1], performed at the California Institute of Technology. Results from the simulations show close agreement to the experimentally measured velocity and temperature profiles, indicating that the entrainment and heat release is predicted with good accuracy. We also observe that varying the inflow conditions changes the nature of entrainment into the mixing layers, consistent with the past experimental observations. We also investigate the effect of heat release in supersonic reacting flows in an inclined ramp geometry, following the work of Bonanos et al. [2]. Probability density function plots and mass-fraction isosurfaces of `tracer' species reveal that heat release significantly alters the flow field.