On the Mechanism of Dangerous Blade Vibration Due to Blade Flow Interactions on Centrifugal Compressors

Severe blade flow interactions at part-load operation conditions were investigated on a centrifugal compressor with a vaned diffuser leading to material stresses beyond the allowable values. By means of a number of measurement and analysis techniques it could be found, that a stationary periodic pressure field is produced on the circumference by the vibrating blade itself, which is induced at resonance conditions by the peripheral pressure nonuniformity due to the outlet tube. This peripheral pressure field of an integer wave number intensifies the blade resonance excitation from downstream leading to an additivity effect between wave amplitude and blade displacement. The significant role in this mechanism plays the reverse flow near the corner shroud/suction side in the impeller, occurring at part-load operation, which is controlled by the interaction of the tip angle of the vibrating blade and the flow angle at that location. It could be demonstrated that this dangerous blade vibration, in addition, is the source of a shift of the surge line toward higher mass flow, reducing the compressor operating range considerably in this operating zone.