Rotating Instabilities in Axial Compressors

Investigation of the Formation Mechanism in Stationary and Rotating Blade Rows

Rotating Instabilities (RI) can appear in the stable operating range of axial compressors as well as in other turbomachines. They are responsible for blade vibration and noise generation and have therefore to be avoided. RI mainly emerge for large tip clearance ratios or high incidence of the blading. The rotating flow structures of RI are fluctuating in time and space. There are several explanatory approaches concerning the underlying mechanism of RI. One hypothesis states that shear layer instabilities (SSI) near the endwall are the reason for RI. These SSI can appear if fluid layers move relative to each other and become unstable because of small disturbances.   

Aim of this project is to improve the physical understanding of the underlying mechanism and the properties of RI in axial compressors. The hypothesis of SSI as trigger of RI will be fundamentally investigated by means of experiments and CFD simulations in the vaneless channel, in the non-rotating linear compressor cascade and finally in the rotor blade row of a large scale axial compressor. The flow instabilities will be triggered in a targeted manner with different parameter variations, e.g. tip gap width and incidence. Further on, using Coandă-nozzles, an unstable shear layer will be generated and its influence on rotating flow structures will be investigated.

Focus of the detailed experimental and numerical investigations are on the development of model conceptions of the underlying mechanism and the properties of RI as well as the transferability of the insights from simplified experimental setups towards the rotating machine. It will be determined for which parameter combinations SSI appear and if they are responsible for RI. This will be a first step for the development of general rules with the aim to predict and avoid RI and thus contribute to an improved reliability of operation of turbomachines.