A model describing supercritical-pressure, multi-species turbulent mixing has been developed to simulate situations prevailing in diesel, gas turbine, and HCCI (homogeneous charge compression ignition) engines. It is also a situation occurring in atmospheric planetary science, such as the Venus atmosphere. Previously, there had been no model to describe this high-pressure mixing under turbulent conditions.
The model is based on the conservation equations coupled to a real-gas equation of state and using transport properties valid for high-pressure multi-species flows. Direct Numerical Simulations were performed of a mixing layer composed of two streams of different initial compositions for realistic Schmidt and Prandtl number values. There were five species involved that are typically encountered in oxidation reactions: fuel (here n-heptane), oxygen, nitrogen, carbon dioxide, and water. The results allowed understanding of the intricacies and the thermodynamics of high-pressure mixing. These intricacies include the identification of the phenomenon of reverse species diffusion, which plays a major role in establishing the characteristics of the flow. Reverse (sometimes called “uphill”) diffusion happens in a mixture of species when one of the species locally diffuses against its mass fraction gradient or its molar fraction gradient, resulting in the formation of regions of high concentration of this species so that effectively this species separates from the other species in the mixture. It was found that when the initial composition conditions of the two streams were such that reverse diffusion occurred, the flow was more turbulent, which was attributed to the larger gradients in the flow that acted akin to a solid mesh, creating turbulence. However, increasing turbulence hindered the formation of reverse diffusion, a fact which has also been observed experimentally. Therefore, reverse diffusion and turbulence are competing phenomena determining the characteristics of a flow. It was also found that unless one uses the complete diffusion matrix including the off-diagonal terms, one cannot predict the small scale structures created by diffusion, a fact which would render the simulations unable to correctly predict ignition times since ignition is a phenomenon directly depending on species diffusion.