The use of carbon dioxide in supercritical thermodynamic conditions (sCO2) for closed gas cycles is attractive since it provides potential advantages in terms of overall cycle efficiency, size of the components, and dynamics of the system. The combination of the aforementioned features allows adapting the cycle to several alternative energy sources such as nuclear, solar, and waste-heat recovery.

Depending on the specific heat source exploited, several layouts were historically proposed for sCO2 power systems. All of them, however, share the use of at least one compressor operating in close proximity to the critical point, since the thermodynamic optimization of the cycle drives the compression towards conditions featuring the lowest work exchange. For electric power within 500 MW, the recommended machine is a centrifugal compressor.  As a result, the intake conditions of the compressor are close to saturation, making the fluid prone to cavitation or condensation (depending on the actual entropy level of the fluid at the intake, i.e. if lower or larger than the critical value). The intake of a turbo-compressor is indeed characterized by nearly-isentropic suction phenomena, induced by the flow-turning around the leading edge of the blades and by the acceleration due to the blade blockage.

In the present study, we consider a prototype sCO2 centrifugal compressor operating in proximity to the critical point, with intake entropy level of the fluid lower than the critical value. In this condition, the phase change occurs as evaporation/flashing, thus resembling cavitation phenomena observed in liquid pumps, even though with specific issues associated to compressibility effects occurring in both the phases. The flow configuration is therefore highly non-conventional and demands the development of proper tools for fluid and flow modeling, which are instrumental for the compressor design.

The papers discusses the modeling issues from, at first, a thermodynamic perspective and, then, highlighting the implications on the compressor aerodynamics. Then, we propose tailored models to account for the effect of the phase change in 0-D mean-line design tools as well as in fully 3D Computational Fluid Dynamic (CFD) simulations. In this way, a design strategy is build-up as a combination of mean-line tools, industrial design experience, and CFD for detailed flow analysis. The application of the design strategy allowed studying the actual flow configuration within a realistic sCO2 compressor. The study reveals that the onset of the phase change might alter significantly the performance and operation of the compressor, both in design and in off-design conditions.


Implications of Phase Change on the Aerodynamics of Centrifugal Compressors for Supercritical Carbon Dioxide Applications

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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