Validation of a Numerical Model Based on an Optical Surface Scan and Experimental Modal Analyses Under Vacuum Conditions

A safe operation of compressors requires a profound understanding of the vibrational behaviour of their components. The focus of this work is on the identification of the modal quantities of an impeller of a radial compressor. The prediction of the mistuned mode shapes and eigenfrequencies is carried out by a finite element model which is adapted to surface scan data via mesh morphing to capture geometrical deviations from CAD. A validation of this model is given by an experimental modal analysis, which is conducted under vacuum conditions to eliminate the damping and coupling influence of the surrounding medium. In consequence, the better separation of peaks in the frequency response function (FRF) enables the identification of all modes. The modal parameters as eigenfrequencies, damping ratios and modal constants are extracted from the measurement data by an improved multi-degree-of-freedom-fit algorithm which is capable to process the entire frequency band of a measured FRF. This is necessary due to a high density of modes preventing the separate assessment of certain frequency ranges without errors at the edges of the frequency band. In addition, a laser scan is carried out to support the assignment of measurement data to results from the numerical model based on vibration modes. It can be shown that a majority of the roughly 600 modes in the range of interest can be assigned to measurement data in terms of eigenfrequencies, modified cyclic symmetry mode shapes and blade mode shapes.