Effects of Impeller Gap on Rotor Vibration in a High Speed Centrifugal Compressor: A Numerical and Experimental Analysis (TECHNICAL NOTE)

Centrifugal compressors produce high pressure ratio and rotate at speeds. So, a tiny unbalance can produce severe vibration. In this paper, effects of impeller gap on rotor vibration in a high speed centrifugal compressor is investigated. For this purpose, a numerical and experimental analysis are carried out. The moving reference frame method in FLUENT software is used for modeling of geometries. Then, the motion of rotation components is introduced using UDF (User-Difined-Function) writing in C++ software and Define CG Motion macro. By using cloud points, three-dimensional geometry model of blades of this compressor is prepared. Finally, two-dimensional geometry of diffuser is added to blades and the final geometry is presented. Fluid flow inside the centrifugal compressor with and without considering blades vibration is studied. The numerical and experimental analysis of power spectrum density to determine the dominant vibration frequency cause of horizontal and vertical forces exerted on the compressor is studied. Results show that the dominant frequency of vibrations of forces exerted on the compressor is in the range of 9800 Hz, that is in good agreements with those reported by earlier researchers. Also, the main reason of centrifugal compressors shaft vibration is static and dynamic unbalance in shaft and other components of the compressor. In other words, the forces exerted on compressors blades do not affect the centrifugal compressor vibration. In the numerical studies, distribution of pressure, temperature, velocity and velocity vectors at different times are studied. Horizontal and vertical forces exerted on the compressor is represented. The mass flow rate of the compressor output for different cases of A/G ratio is presented and does not depend on amplitude of vibrations.