Flow physics analysis and dynamic efficiency assessment of supercritical CO2 radial outflow turbines

Radial outflow turbines (ROTs ) provide a significant alternative as expanders in supercritical CO 2 (sCO 2 ) power cycles, offering high specific work, enabling mechanically feasible rotational speeds, and reducing the number of stages. This study presents a comprehensive design of an sCO 2 ROT using Soderberg’s loss model, coupled with a validated unsteady 3D-CFD simulation employing the k–ω SST turbulence model. The investigation explores the combined effects of pressure ratio (PR), rotational speed, and relative tip clearance (R-TC) on the turbine’s efficiency, as well as the interactions of unsteady internal flow structures at multiple stator-rotor angular positions. The results indicate that the ROT efficiency increases with the PR and levels off after reaching a certain threshold (PR s ). This threshold shifts to higher PR values as the R-TC increases, ranging from approximately PR s = 3 at zero clearance to PR s = 6 at the maximum R-TC of 23.80 % and 18.18 % for the stator and the rotor, respectively. Based on the results, increasing the R-TC, beyond 11.90 % (stator) and 9.09 % (rotor) has minimal impacts on the total-to-static efficiency. Also, beyond the PR s , the incremental effects of further increasing the PR on both torque magnitude and fluctuation amplitude begin to decrease. There is an optimal rotational speed (23500 RPM), as it delivers the highest total-to-static efficiency and the lowest torque fluctuations, even though the torque value decreases at higher rotational speeds. These findings provide valuable insights for optimizing the design and operation of sCO 2 ROTs to enhance their efficiency and rotor stability in advanced power cycle applications.