Blade Excitation by Circumferentially Asymmetric Rotating Stall in Centrifugal Compressors

The excitation of blade vibration by circumferentially asymmetric rotating stall in a high pressure ratio, high mass flow centrifugal compressor with radial impeller and vaned diffusers has been investigated. This was accomplished experimentally by means of blade vibration measurements with strain gages mounted on the blades and simultaneous unsteady pressure measurements with fast response dynamic transducers. The earlier results showed rotating stall effects as the monofrequent excitation of blade vibration. However a more detailed analyses of blade vibration by very strong rotating stall in the speed range of 13,000–14,500 rpm displayed multiple frequency components of blade vibration besides the main excitation frequency caused by rotating stall. The frequency values were found to be related to frequencies of shaft rotation and rotating stall. The detailed analyses of the pressure signals of rotating stall obtained from dynamic transducers located at different circumferential positions on the shroud wall showed a different pressure pattern amplitude due to rotating stall. The circumferentially asymmetric amplitude profile of the rotating stall was an additional excitation of blade vibration. By aid of two-dimensioal Fourier analysis for the pressure pattern of rotating stall, a calculation method for blade vibration frequencies by circumferentially asymmetric rotating stall was developed. The predicted blade vibration frequencies corresponded to the frequency components on the blade vibration spectrum obtained from the strain gages.