Comprehensive study on the design and analysis of supercritical carbon dioxide (sCO2) radial outflow turbines (ROTs), focusing on tip clearance effects, loss correlations, and performance evaluation

• Thermodynamic study on sCO 2 turbine efficiency as a function of tip clearance. • Defining correlations between loss coefficients and tip clearance for sCO 2 turbine. • Details about energy transfer between tip leakage, passage and counter vortices. • Detecting connections between the vortices and the loss mechanisms in sCO 2 turbine. • Comparing the static pressure loss and entropy loss mechanisms in sCO 2 turbine. Supercritical carbon dioxide (sCO 2 ) energy cycles have significant potential to enhance the thermal efficiency of power systems compared to traditional steam and organic Rankine cycles (ORCs). The design and development of sCO 2 turbines with higher efficiency can substantially improve the thermal-economic feasibility of future sCO 2 power cycles. Recently, radial outflow turbines (ROTs) have gained attention for sCO 2 applications due to their high efficiency and compact size. This research presents a detailed and validated design for sCO 2 ROTs with a capacity of 2.7 MW and operating at 200 bar and 23,500 RPM. The initial design is based on a one-dimensional approach (Soderberg’s correlation), followed by a three-dimensional unsteady analysis (k-ω turbulence method) to examine pressure and entropy losses related to the physics of supercritical phase flow. This analysis is conducted for different relative tip clearances (R-TC St = 0 to 23.80 %-R-TC Ro = 0 to 18.18 %) to present an accurate correlation for tip leakage loss at different rotational speeds. The designed ROT model applies supercritical fluid formulations from the REFPROP database to provide precise estimations. The results indicate that as the relative tip clearance (R-TC) increases, particularly beyond R-TC Ro = 9.09 % and R-TC St = 11.90 %, the stagnation pressure and entropy loss coefficients, and consequently the turbine efficiency and power output stabilize at a certain level. Additionally, the relationship between the position and the spread of leakage and passage vortices, as well as the associated loss coefficients, has been investigated. The presented loss correlation can be applied to other working fluids or further refined to enhance the design of multistage ROTs.