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

Abstract

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.