Compressors operating with carbon dioxide near the critical point experience complex aerothermodynamic phenomena, where deviations from perfect-gas similarity and two-phase flow effects dominate. Existing models inadequately capture the impact of intake thermodynamic conditions on the choked flow rate, leaving a gap in predictive capabilities for these machines. This work addresses this gap by deriving a correlation to predict the choked flow rate as a function of two generalized parameters: the cavitation/condensation parameter and the isentropic pressure-volume coefficient, which describe two-phase and non-ideal effects.

A database of 100 speedlines, generated through CFD simulations with varying thermodynamic conditions and fixed peripheral Mach number, was used to train a symbolic regression algorithm based on gene expression programming. This method was chosen to derive an explicit, easy-to-use analytical expression without assuming a priori functional forms.

Results showed that the choked flow rate could vary from 90% to 155% of the nominal value depending on thermodynamic conditions, highlighting the dominant role of the two parameters. The derived correlation demonstrated trends consistent with CFD predictions, with an accuracy of ±3 percentage points for most cases. However, an a-posteriori validation against varying peripheral Mach numbers and an alternative impeller geometry revealed significant discrepancies, underscoring the interplay between thermodynamic conditions, geometry, and aerodynamics. This analysis showed that the peripheral Mach number and the geometric features influence choking behavior unpredictably, limiting the correlation's general applicability.

While the proposed correlation is not adequate for quantitative scaling across designs, it provides preliminary insights into qualitative trends. For accurate predictions, high-fidelity CFD remains necessary, highlighting the inherent challenges of universal scaling for near-critical operations in sCO₂ compressors.

sCO2 energy conversion cycles technology development is reaching the MW-scale cycle demonstration phase. At the moment of this abstract writing, the STEP plant is going through the different phases of commissioning and has already produced electrical power. Other demonstration projects are reaching the end of the design phase and forecast a commissioning start in 2026. Nuovo Pignone Baker Hughes is part of the Desolination project consortium. Desolination project aims to decarbonise the desalination process in arid regions by demonstrating in a real environment the efficient coupling of a concentrating solar power plant to a direct osmosis desalination system. The sCO2 cycle involved in this process is a Rankine evolving a blended CO2 mixture optimized to enable condensation in hot ambient temperature areas.

Nuovo Pignone contributes to the Desolination project developing the pump and the expander of the sCO2 cycle. Expander development has reached the completion of the detailed design phase and the procurement of long lead material has started. This paper will provide a comprehensive overview of the expander development process, from conceptual design through to detailed design, displaying the key technology challenges peculiar of sCO2 expanders and illustrating the engineering methodologies applied to optimize the expander design.

This paper is dedicated to the performance map computation of centrifugal compressors operated with carbon dioxide at supercritical states (sCO$_2$) and inlet conditions in the vicinity of the critical point. Three-dimensional computational fluid dynamics (3D-CFD) simulations are utilised. First, different approaches to model two-phase flows are reviewed. Second, the impact of two-phase flow on the speed of sound and the choking limit is further investigated. The flow through Laval nozzles is analysed to simplify the problem to its fundamental aspects. The 3D-CFD calculations match well with the ones conducted with simple one-dimensional CFD programs and the theory of equilibrium phase change. Third, the throttle curve of an industrial-scale centrifugal compressor is computed and compared to the one for air. Total inlet conditions are supercritical in the so-called liquid-like region. A considerable shift of the choking line towards lower flow coefficients is found. The reason for this shift is a drop in the speed of sound when bubbles are formed in the liquid, and a two-phase flow is established while the flow is accelerated around the compressor’s leading edge. Finally, a log(p)-h diagram is provided, enabling a quick assessment of the risk of two-phase flow in centrifugal compressors.