An Investigation on the Three-Dimensional and Unsteady Flow in a Multistage Axial Supercritical Carbon Dioxide Turbine

Abstract This study investigates the aerodynamic design and analysis of a low aspect ratio, multistage, axial-flow sCO2 turbine for integration in power cycles for small modular reactors and concentrated solar power systems. The secondary flows developing across the flow path are analyzed using low- and high-fidelity modeling approaches, to evaluate their impact on aerodynamic performance. An in-house low-fidelity design tool (zturbo), developed at Politecnico di Milano and featuring multiple loss correlations, was coupled to a nonlinear optimization algorithm to create an optimized preliminary mean-line design (MLD) of a five-stage axial sCO2 turbine flow path, with an optimal total-to-total efficiency of 93.9%. Fully three-dimensional (3D) numerical simulations of the turbine first stage, featuring the lowest aspect ratio blade (approximately 0.5), were performed using both steady-state and time-resolved approaches. The impact of vortex–blade and vortex–vortex interactions on the stage efficiency was highlighted, with unsteady interactions causing 10% higher secondary losses compared to the steady-state model. Finally, fully 3D numerical simulations of the complete five-stage axial sCO2 turbine were performed to investigate the development of secondary flows in a multistage configuration. The secondary loss estimates obtained by the computational fluid dynamics (CFD) simulations were compared with those evaluated by applying multiple empirical loss correlations. Results indicate that literature-based empirical loss correlations provide acceptable performance estimates for the overall turbine performance, but a margin of improvement is evident in the estimate of secondary losses, which appear overly conservative for low aspect ratio blades. Conversely, industrial correlations developed in-house aligned more closely with high-fidelity results.