Direct Numerical Simulation of a supercritical mixing layer

Direct Numerical Simulations (DNS) are conducted of a model hydrocarbon-nitrogen mixing layer under supercritical conditions. The temporally developing mixing layer configuration is studied using heptane and nitrogen supercritical fluid streams at a pressure of 60atm as a model system relevant to practical hydrocarbon-fuel/air systems. An entirely’ selfconsistent cubic ‘Peng-Robinson equation of state is used to describe all thermodynamic mixture variables as it was found to be accurate to better than 1% relative error through comparisons with highly accurate state equations over the range of variables used in this study (600.K 5 T 5 llOOK, .40atm I p < 80atm). Soret and Dufour contributions to the transport matrix are included ,in the formulation. The DNS results of two dimensional and three dimensional layers elucidate the unique thermodynamic and mixing features associated with supercritical conditions. Departures from the perfect gas and ideal mixture conditions are quantified by the compression ratio and by the mass diffusion factor, both of which.show reductions from the unity value. .Jt is found that the qualitative aspects of the mixing layer may be different according to the specification of the thermal difhrsion factors whose values are generally unknown: the constant Bearman-Kirkwood (BK) thermal diffusion factor excites fluctuations that the constant Irwing-Kirkwood (IK) one does not, and thus enhances mixing. Combined with the effect of the mass diffusion factor, constant positive large BK thermal difhrsion factors retard diffusional mixing; whereas constant moderate IK factors tend to promote mixing. Constant positive BK thermal diffusion factors also tend to maintain density gradients, with resulting greater shear snd vorticity. These conclusions about IK and BK thermal diffusion factors are species pair dependent, and therefore are not universal. Increasing the temperature of the lower stream to approach that of the higher stream results in increased layer growth as measured by the momentum thickness. The three dimensional mixing layer exhibits slow formation of, turbulent small scales, ’ Cbpyright ‘@ 2000 by the Ck **Senior Engineer; ***Senior Research’ Scientist, Associate AIAA Fellow (corre; sponding author, josette.b$lan@jpl.nasa.gov). and transition to turbulence does not occur even for a relatively long nondimensional time when compared to a previous, atmospheric conditions study: The primary reason for this delay is the initial density stratification of the flow, while the formation of strong density gradient regions both in the braid and between the braid planes may constitute a secondary reason for the hindering of transition through damping of formed turbulent eddies.