STUDY ON THE CASING RECESS DESIGN METHOD FOR A SUPERCRITICAL CARBON DIOXIDE AXIAL TURBINE

Supercritical carbon dioxide (S-CO2) closed Brayton cycles have recently received extensive attention for their potential applications in various fields such as power generation and mobile propulsion. Due to the high density of CO2, S-CO2 turbines have much smaller sizes compared to gas turbines at the same level, making tip leakage flow a critical factor affecting turbine performance. In this study, the flow field of a small-scale S-CO2 axial turbine was analyzed through CFD simulations. The effects of blade tip clearance and casing recess design on turbine performance were investigated. The results indicate that the tip leakage flow in the blade passage hinders the mainstream flow and serves as a significant source of losses. As the tip clearance ratio increases, the intensity of leakage flow increases, leading to a noticeable decrease in turbine efficiency. With an increase in the depth of the casing recess, the turbine efficiency initially increases and then decreases. When the clearance size is larger, the corresponding optimal recess depth for the highest turbine efficiency shows an increasing trend. The impact of recess position on efficiency is not significant in this study. The recessed casing increases the intensity of the tip passage vortex, which develops along the pressure surface and involves the nearby fluid, suppressing the intensity of the tip leakage flow. A suitable combination of casing recess parameters can help improve turbine efficiency.