Multicomponent Effects on the Supercritical CO₂ Systems: Mixture Critical Point and Phase Separation

Semi-closed supercritical CO₂ (sCO₂) gas turbine is a promising candidate for the next generation power cycles with high efficiency and almost 100% carbon capture. In this study, the multicomponent effects on the sCO₂ systems are investigated. A real-fluid modeling framework based on the vapor-liquid equilibrium (VLE) theory is implemented to predict the phase boundary and real mixture critical point, and to capture the phase separation in computational fluid dynamics (CFD) simulations. A novel VLE-based tabulation method is developed to make the CFD solver computationally more affordable. VLE-based thermodynamic analyses show that a small amount of combustion-relevant impurities (e.g., H ₂O , CH ₄, and O ₂) can significantly elevate the mixture critical point of the sCO ₂ systems. As a result, the so-called “supercritical” CO ₂ systems might be in the subcritical two-phase zone where phase separation occurs. At the relevant conditions in this study (100–300 bar), phase separation only has a small influence on the CO ₂/ H ₂O / CH ₄/ O ₂ mixture density, but has a considerable influence on the heat capacity of the mixture. VLE-based CFD simulation of a laminar premixed sCO ₂ shock tube shows that expansion waves can trigger significant condensation in the systems and the latent heat of the condensation can change the temperature and density fields in the systems. To understand the phase separation during mixing, VLE-based large-eddy simulations (LES) of turbulent jet-in-crossflows in the sCO ₂ systems are conducted, and the results show that when two subcritical gas or supercritical gas-like streams mix, the mixture can partially condense to subcritical liquid phase. Higher pressure, lower temperature, and higher H ₂O concentration can enhance the phase separation phenomenon in the systems.