| Effects of Soret and Dufour cross-diffusion, are investigated in the context of both binary and ternary species mixing and combustion in isotropic turbulence at supercritical pressure. The compressible flow formulation is based on a cubic real gas state equation, and includes generalized forms for heat and mass diffusion derived from non-equilibrium thermodynamics and fluctuation theory. A previously derived formulation of the generalized binary species heat and mass fluxes is first extended to the case of ternary species, and appropriate treatment of new thermal and mass diffusion factors is described. Direct numerical simulations (DNS) are then conducted for both binary and ternary species mixing and combustion in isotropic turbulence.; For the case of binary mixing of nitrogen and heptane, results from two simulations at uniform mesh resolution are compared. One case is based on the complete diffusion formulation, whereas in the second simulation only “standard” Fickian and Fourier mass and heat flux terms are considered as a basis for comparisons. The evolutions of the two mixing processes are shown to be nearly the same at early times; however, at long times the pressure gradient based Soret diffusion acts as a forcing function resulting in a statistically stationary scalar fluctuation distribution. An analysis of the potential impact of Soret diffusion on probability density function (PDF) turbulence models reveals that the form of conditional expectations requiring modeling are altered from typical “low pressure” behavior at long times.; DNS of ternary species systems undergoing both pure mixing and a simple chemical reaction are then conducted. It is shown that stationary scalar states persist for the ternary species problem as well; however, the production and magnitude of the scalar variance is found to be altered for the intermediate molecular weight species as compared to the binary species case. For combustion of non-stoichiometric mixtures, a binary species mixture, characterized by stationary scalar states, results at long times after one of the reactants is depleted. A discussion of potential impacts to real combustion at high pressures, and recommendations for future research are made. |
