An Investigation of the Three-Dimensional and Unsteady Flow in a Multistage Axial sCO2 Turbine

Abstract In this study, a low aspect ratio, multistage, axial sCO2 turbine for integration in power cycles for Small Modular Reactors and Concentrated Solar Power systems has been designed. The secondary flows developing across the flow path are analyzed using low- and high-fidelity modelling 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 of a five-stage axial sCO2 turbine flow path, with a resulting optimal total to total efficiency of 93.9 %. Fully 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 an 10% higher secondary losses compared to the steady-state model. Finally, fully 3D numerical simulations of the complete five-stage axial sCO2 turbine was performed to study the development of secondary flows in a multistage configuration. The secondary loss estimates obtained by the CFD simulations were compared with those evaluated by applying multiple empirical loss correlations. Results indicate that literature-based empirical loss correlation provide acceptable performance estimates for the overall turbine performance, but a margins of improvement is evident on the estimate of secondary losses, which appear overly conservative for low aspect ratio blades. Conversely, industrial correlations developed in-house more closely align with high-fidelity results.