Supercritical Carbon Dioxide Mixing Loss Characteristics Near the Critical Point

Abstract

This paper aims to study the mixing loss characteristic of supercritical carbon dioxide (sCO2) near the critical point and shed light on the nonideal fluid effect on the mixing losses. As a simplified model of the mixing process in the trailing edge or tip leakage flow in turbomachines, a case of mixing in a constant area adiabatic duct involving two streams of parallel flows is studied by control volume analysis. Both perfect gas and nonideal fluid calculations are carried out for comparisons. The isolated and coupled effects of the two mixing streams' temperature, velocity, and pressure differences on the loss are investigated. The study shows that nonideal fluid mixing produces significantly higher losses when compared to the equivalent perfect gas estimation. It also reveals that the nonideal fluid mixing loss is sensitive to the average thermodynamic states of the mixing streams. A general trend is that the closer the temperature is to the critical point, the higher the loss is observed. This is particularly apparent for the temperature and pressure mixings, while the mixing loss due to the velocity difference is less affected. A significant variation of the mixing loss coefficient is reported if the static state of any of the two streams is near the critical point, which is mainly caused by the significant variation of the temperature gradient regarding the entropy along the isobar. In addition, a positive combined effect of the temperature, velocity, and pressure differences was found for mixing loss. The results also show that the change in the mixing loss due to the variation of one property difference between mixing streams is almost independent of the other property difference. This behavior matches the conclusion drawn in literature for perfect gases (Denton, J. D., 1993, “The 1993 IGTI Scholar Lecture: Loss Mechanisms in Turbomachines,” ASME J. Turbomach., 115(4), pp. 621–656). The results from this simple case contribute to understanding the mixing loss behavior differences between perfect gas and nonideal fluids in sCO2 turbomachinery and hold significance for the development of mean-line models.