Numerical Investigation of Unsteady Flow Phenomena in a Centrifugal Compressor Operating Near Surge With a Geometrically Reduced Model

Abstract

Abstract Deep surge is a time and system dependent phenomenon that might be triggered by a flow instability. This instability could be subsequently used as an indicator for the surge margin. Based on former numerical investigations concerning steady flow phenomena in a centrifugal compressor, published at “Turbo Expo 2021” by Hochschule RheinMain and TU Darmstadt the investigations have been continued. A speed line is divided into four sections based on its slope, which are examined for instabilities by means of transient simulations with a numerical reduced model. For this purpose, a detailed frequency analysis is carried out to detect unstable flow behaviour. It is shown that the characteristic of the speed line is in a good agreement with the analysed frequency spectrum and the associated unsteady flow phenomena. These are for instance a highly changing incidence triggered by vortex tubes at the impeller inlet. In addition, the computations show an instability in the vaneless space behind the trailing edge, the occurrence of which is in a relationship to the experimental stability limit. Regardless of the system volume, the phenomenon could be used as an indicator for the stability of the compressor wheel. A second speed line is then used to validate this relationship.