Non-Ideal Fluid Effects of Supercritical Carbon Dioxide on Turbomachinery Loss Characteristics near the Critical Point

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Supercritical carbon dioxide (sCO2) is a promising working fluid for next-generation power cycles due to its high heat capacity and low viscosity, enabling efficient and compact designs. However, its complex thermophysical behaviour near the critical point poses challenges to accurate modelling. This thesis presents investigations on fundamental non-ideal fluid flow cases with sCO2 near the critical point, including isentropic flows, normal shocks, co-flow mixing, and turbulent boundary layers, providing deeper physical insights into the non-ideal fluid behaviours. A new isentrope model with path-specific exponents is proposed to describe non-ideal fluid isentropic processes. The new model provides an explicit analytical framework that can predict sCO2 stagnation states within 2% of EOS calculations but is 15-20 times faster than EOS-based methods, striking a balance between accuracy, simplicity, and computational efficiency. Normal shock behaviours of sCO2 near the critical point are investigated focusing on three parameters: post-shock Mach number, shock strength, and polytropic efficiency. The study highlights the significant deviation from perfect gas shock and shows stronger shocks and higher efficiencies for sCO2, especially near the critical point. Explicit regression models are provided for faster shock prediction, reducing percentage errors by 2 to 40%. A simplified co-flow mixing case was used to study mixing loss characteristics of sCO2. The results show that mixing loss sensitivity is greatest near the critical point, particularly due to temperature differences. A significant change in loss is reported if the static state is near the critical point, which is mainly caused by property gradient variation. A comparative DNS study between sCO2 and air turbulent boundary layer was carried out, including both the mean flow and turbulent fluctuation behaviours. Results indicate that sCO2 exhibits smaller temperature variation and liquid-like viscosity behaviours within the boundary layer. Additionally, the property ratio method was shown to provide reasonable corrections to Cf.

Recommended citation:
Jinhong Wang, "Non-Ideal Fluid Effects of Supercritical Carbon Dioxide on Turbomachinery Loss Characteristics near the Critical Point." Imperial College London, 2025.