Computational and Experimental Assessment of a MW-Scale Supercritical CO₂ Compressor Operating in Multiple Near-Critical Conditions

This work illustrates the results of a wide experimental campaign in the frame of the EU-funded project sCO₂-Flex, which focused on the investigation of a MW-scale sCO₂ compressor operating in plant-representative conditions. The experimental tests were carried out for four temperature levels between 304.15 K and 309.15 K at a fixed pressure of 79.79 bar, hence covering an extended thermodynamic region close to the critical point. The experimental results are thoroughly discussed with the support of steady computational fluid-dynamics simulations, assuming homogeneous flows and thermodynamic equilibrium for the two-phase flow description. Changing the upstream total state, two peculiar variabilities in the compressor pressure ratio and choking flow rate are experimentally and computationally observed. While the former is mainly related to the single-phase flow thermodynamics, the latter originates from the onset of two-phase flows. As the simulations predict the experimental choking with a maximum error of 3%, the corresponding two-phase speed of sound is analyzed to infer the underlying equilibria between phases. It is found that, for the tested conditions, two-phase flows quickly achieve thermodynamic equilibrium, and non-equilibrium or metastable effects arguably play a marginal role in the process.