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Multi-stage radial compressors are key components in many industrial applications. Today, downsizing the compressor to reduce capital expenditure (CAPEX) is of utmost importance. Accordingly, the request is to increase the pressure ratio in a most compact design, accepting nearly no penalties in the performance level. Here, the complex flow in stator parts of multistage centrifugal compressors was investigated and predictions from numerical simulations were compared with experimental data. A test rig was developed enabling steady-state data using five-hole-probes and time-resolved velocity data via hot-film-anemometry to be acquired. Reynolds-averaged Navier-Stokes (RANS) calculations showed significant deviations from the measured data in predicting the flow profiles in the return bend for high flow coefficients (φ≥0.12) were observed. Not only eddy–viscosity models but also Reynolds stress models show similar discrepancies. Large Eddy Simulation (LES) was applied to acquire more detailed information about the flow. The flow profiles predicted by a hybrid RANS–LES calculation show better agreement with the experimental data but performance data still vary. Concluding, the RANS approach was found to under-predict turbulent transport in highly curved and complex geometries at high flow coefficients. This fact must be understood and respected when using CFD-techniques for flow calculation in wide return channels of centrifugal compressors. INTRODUCTION Multistage centrifugal compressors, used in a wide range of industrial applications, are often designed in a single shaft configuration. Each single compressor stage comprises a rotating impeller and a non-rotating diffuser and return channel. The need to lead the radial outward impeller flow back and then axially on to the next impeller inlet results in a challenging geometry design. This typically consists of a vaned or vaneless radial diffuser, a highly curved return bend, a vaned return channel and finally a 90°-bend. Simultaneously, the swirl induced by the preceding impeller must be eliminated completely before the next stage is entered. Key aims in optimizing the stator are to achieve a homogeneous and swirl-free outflow while minimizing total pressure losses and maximizing the increase of static pressure. Pursuing these goals is more challenging at off-design and especially under deep part load conditions. As the flow path is complex, the accessibility within return channels for measurement applications such as flow probes or optical measurement techniques is limited. Nevertheless, several experimental studies have led to a better understanding of flow behavior and thereby to improved design guidelines. To investigate stator flow, and in particular, return channels, Rothstein [1] tested stator geometries with different flow coefficients focusing on comparatively low flow rates and consequent narrow channel widths. He concluded that using a return vane with high curvature in the first part and low curvature in the last part to straighten the flow will result in a better exit flow profile and better performance. Lenke [2] subsequently applied an algebraic Reynolds Stress model and simulated the flow in Rothstein`s geometries. His findings support Rothstein`s conclusions. Recent publications by Aalburg et al. [3] and Rossbach ([4], [5]) focus on detailed flow phenomena with the aim of reducing the diffuser radius ratio, in turn motivated by the manufacturer need to reduce CAPEX by reducing machine size. However, with a lower diffuser ratio, the flow velocity inside the return bend and the return guide vanes (RGV) increases, promoting secondary flows and thus increased turbulence intensity due to higher shear rates. Numerical modelling in these contexts presents a particular challenge: modern turbulence models, such as

Deficits in Rans Modelling in the U-Turn of a Multistage Centrifugal Compressor