Experimental Measurements of the Natural Frequencies and Mode Shapes of Rotating Disk-Blades-Disk Assemblies from the Stationary Frame

Abstract

Determining the natural frequencies and mode shapes of rotating turbomachinery components from both rotating and stationary reference frames is of paramount importance to avoid resonance problems that could affect the normal operation of the machine, or even cause critical damages in these components. Due to their similarity to real engineering cases, this topic has been experimentally analyzed in the past for disk-shaft assemblies and rotor disk-blades assemblies (bladed-disk or blisk). The same topic is less analyzed for disk-blades-disk assemblies, although such configurations are widely used in centrifugal closed impellers of compressors, hydraulic pumps, pump-turbines, and runners of high head Francis turbines. In this paper, experimental measurements, varying the rotating speed of a disk-blade-disk assembly and exciting the first natural frequencies of the rotating frame, have been performed. The rotating structure is excited and measured by means of PZT patches from the rotating frame and with a Laser Doppler Vibrometer (LDV). In order to interpret the experimental results obtained from the stationary frame, a method to decompose the diametrical mode shapes of the structure in simple diametrical components (which define the diametrical mode shapes of a simple disk) has been proposed. It is concluded that the resonant frequencies detected with a stationary sensor correspond to the ones predicted with the decomposition method. Finally, a means to obtain equivalent results with numerical simulation methods is shown.

Experimental and Numerical Vibration Analysis of Octet-Truss-Lattice-Based Gas Turbine Blades

“…An increase in first natural frequency is always desirable in turbines because the operating speed of rigid rotors must be lower than the first natural frequency. Therefore, increasing the natural frequency can extend the operating speed of turbines as discussed by Alexandre Presas et al [38]. The overall natural frequencies at The solid blade EMA frequency response plot in Figure 6 shows three clear peaks at 1887.50, 4762.50 and 5837.50 Hz, which correspond to the first, second and third natural frequencies of the solid blade, respectively.…”

“…An increase in first natural frequency is always desirable in turbines because the operating speed of rigid rotors must be lower than the first natural frequency. Therefore, increasing the natural frequency can extend the operating speed of turbines as discussed by Alexandre Presas et al [38]. The overall natural frequencies at the second and third modes were above the operational speed of most turbines and were therefore given not much importance.…”

Analytical Modeling of Transverse Vibrations and Acoustic Pressure Mitigation for Rotating Annular Disks

“…e natural frequencies and mode shapes of rotating turbo-machinery components from both rotating and stationary reference frames were experimentally analyzed by Presas et al [15]. Yang et al [16] analyzed thermos-elastic coupling vibration and stability of rotating circular plate in friction clutch.…”

Spatially Resolved Experimental Modal Analysis on High-Speed Composite Rotors Using a Non-Contact, Non-Rotating Sensor

“…If the rotor is a disk-like structure (The diametrical mode shapes of disk-like structures can be approximated to be purely sinusoidal and nodal circles can be neglected [ 26 ]. Vibration modes of other structures can contain multiple diametrical and circular components at once, which results in additional peaks in the stationary frame spectrum) in vacuum or air, and the peaks in at every excited natural frequency are split into two peaks each, according to [ 26 ] where M is the number of nodal diameters. For demonstration purposes, the deflection signals from a rotating flat disk that oscillates sinusoidally were simulated according to RFM and SFM.…”

“…However, in case of a continuous measurement from the stationary frame, the response spectrum results from the circumferential trajectory of the sensor. The spectrum has therefore inherently different characteristics than the single spot rotating frame spectra within the trajectory [ 24 , 25 , 26 ]. Furthermore, such stationary frame spectra are prone to ambiguities in mode determination, as both spatial information and vibration frequencies are encoded into the response spectra.…”