Aerodynamic Stability Enhancement of a SCO2 Centrifugal Compressor by Riblets of the Shroud

Aerodynamic stability enhancement is crucial for the stable operation of supercritical carbon dioxide (SCO2) centrifugal compressors. This paper investigates the mechanism of aerodynamic instability of shrouded SCO2 compressors and accordingly proposes a new method for stability enhancement via the casing treatment in terms of shroud riblets. First, the experimentally validated computational fluid dynamic (CFD) method is employed to investigate the flow mechanism of the compressor under near-surge condition. The significant backflow phenomena within the impeller were revealed. Further analysis indicated that the imbalance of the Coriolis force and pressure gradient in blade-to-blade direction pushed the low-momentum fluid toward the shroud suction side. Additionally, higher Reynolds number resulted in thinner SCO2 boundary layer at the inlet near end-wall, increasing passage vorticity and further intensifying the aggregation of low-energy fluid on the shroud suction side. Based on the flow mechanisms, the streamwise riblets on shroud were designed to impede the migration of low-energy fluid. The CFD results revealed that under low-flow condition, riblets inhibit the formation of inducer vortices and backflow, thereby enhancing impeller aerodynamic stability and reducing the surge mass-flowrate. Further research indicated that riblets obstruct the migration of low-energy fluid toward shroud suction side, reducing the accumulation of low-energy fluid and blockage, thereby increasing the flow area and aerodynamic stability. Moreover, additional riblets wake and friction losses contributed to the deterioration of compressor performance at middle/large mass-flowrate conditions. Specifically, riblets reduced the flow area between blades at near choke mass-flowrate, leading to more pronounced shock structures and compressor earlier choke.