Characteristics of Supercritical Transitional Temporal Mixing Layers

Several Direct Numerical Simulation realizations of supercritical, three-dimensional, temporal mixing layers are used to investigate aspects of interest to turbulent combustion. The realizations are based on a model previously developed that accounts for the enlarged transport matrix at supercritical conditions (Soret and Dufour effects), for real gas equations of state, and for variable Schmidt and Prandtl numbers. This model is exercised for two very different sets of binary species, heptane/nitrogen and oxygen/hydrogen, to obtain transitional states for mixing layers excited at different perturbation wavelengths. Visualizations of the transitional states show that all layers develop convoluted regions of high density gradient magnitude (HDGM), which result both from the distortion of the initial density stratification boundary and from mixing. The species mass fraction in these HDGM regions is very weakly dependent of the perturbation wavelength. The existence of the HDGM regions, independent of the perturbation wavelength, indicates that they may be a feature of spatial mixing layers, and furthermore have a similar mass fraction composition. Evaluations of the applicability of the assumed PDF method for describing supercritical transitional flows shows that neither the ß density nor the Gaussian are appropriate representations of the conserved scalar, and that the PDFs of the partial densities and the temperature are correlated, invalidating the typical model of the reaction rate as the product of the marginal PDFs