Secondary Flows in the Return System of a Centrifugal Compressor Stage

This paper presents an analysis of the flow in the return system of a centrifugal compressor with a flow coefficient of 0.15. Based on the detailed experimental and numerical data, the areas of high losses and potentials for improving the return system geometry are revealed. Special emphasis is placed on the interaction of the flow in the return system components, including the U-bend, the vaned return channel, and the final L-bend. Strong flow redistribution occurs due to the sharp curvature of the U-bend, forming a blockage area at the hub near the vane leading edge. This causes a strong passage vortex, which is further intensified by the pressure gradient induced by the L-bend. Additionally, the flow near the shroud accelerates due to the large blockage area near the hub at the exit of the U-bend, resulting in high friction losses. To identify the main causes of loss, a method was evolved. It was validated and supplied by pneumatic measurement data. On this basis, analytical approaches were taken to quantify the total pressure losses due to friction, secondary flows, incidence, and trailing edge flow. As a result, approximately 60% of the return system losses arise due to friction. Another 30–40% are caused by secondary flows. It can be concluded that the results of the investigation contribute to the understanding of the secondary flow structures inside a centrifugal compressor return system of high mass flowrates. By combining the knowledge acquired in respect to the sources of highest losses with the experimental data, a well-founded basis for future optimization is achieved and the validation of numerical approaches is possible.