Species transport in a variable-density turbulent mixing layer considering stratified instability

Abstract

Mixing of miscible fluids with differing densities in turbulent mixing layers is of great importance in chemical engineering. In addition to the effect of density ratio R between two fluids, stratified instability will introduce additional complexity to turbulent mixing of species. We investigate this issue by performing direct numerical simulations of a stratified mixing layer involving two miscible fluids with R = 1, 3, and 6. We find the stronger decay of turbulence in the high-density stream in comparison with the low-density counterpart, which is attributed to the decrease in the shear rate and the increase in stratified instability. Moreover, it shows the dynamical connections between stably stratified mixing layers and wall-bounded turbulent flows, which can be supported by the hairpin vortical structures, the vertical profile of integral shear parameter S* in the low-density side (S* ≈ 10–30), and an inverse wave-number scaling for streamwise energy spectrum of velocity fluctuation. The statistical analysis of the budget of the mixture fraction variance shows that the turbulent mixing of species is significantly suppressed in the high-density side while enhanced in the low-density one, as a result of the larger reduction of turbulent production, turbulent flux, and a smaller reduction of dissipation in the high-density side. Finally, we find that the dissipation rate of the structure function of the mixture fraction variance is about four times larger than that of mixture fraction variance. These results can provide support for large eddy simulation of stratified turbulent mixing of miscible fluids with differing densities.