Blade Excitation by Broad-Band Pressure Fluctuations in a Centrifugal Compressor

The mechanism of blade excitation during the operation of a high-mass-flow, high-pressure-ratio centrifugal compressor has been investigated. This was carried out in the compressor operating range below 60 percent of design speed and in the zone of unsteady flow occurrence, where considerable blade vibration has been measured but no periodic unsteady pressure pattern such as rotating stall could be identified. Experiments conducted to study the mechanism of interactions between flow and blades were accomplished using several measuring methods simultaneously, such as measurements of blade vibration, flow angle at impeller inlet, unsteady pressure at different meridional and peripheral locations, as well as flow visualization by means of oil pattern. Analysis of the measurements showed typical broad-band characteristics of the unsteady pressure field and also for the blade vibration behavior. Results of flow angle investigations at the impeller inlet together with the analysis of oil pattern show that the broad-band pressure fluctuations and blade excitation can be attributed to a strong reverse flow near the suction side of the radial blade in the shroud zone. This reverse flow has its source downstream of the impeller and is extending back up to a location ahead of the impeller inlet. Similar results were obtained when the compressor was operated with vaneless and vaned diffuser configurations.