Identification of Eigenmodes and Determination of the Dynamical Behaviour of Open Impellers

Abstract

Whenever an open bladed impeller (without shroud) is implemented in a centrifugal compressor a key issue regarding the reliability of the compressor concerns the dynamical behaviour of this impeller. Due to the absence of the shroud this impeller can be operated at high tip speeds, hence allowing for higher volume flow. However the impeller reacts very sensitive to excitations leading to large blade vibrations with potential considerable dynamic stresses. In order to avoid resonance caused by fluctuating pressure distributions due to rotor-stator interactions special attention must be paid to the natural frequencies of the impeller which shall be determined with the highest accuracy possible. Therefore this paper focuses on the identification of the natural frequencies of the impeller first. To validate the finite-element (FE) model extensive measurements of the natural frequencies are performed at standstill. Especially the set-up and results obtained with the Shaker-Laser vibrometer-tests are described in detail and compared to other measurement methods such as the Laser-Scaning-Vibrometry (LSV) and strain gauges. Furthermore the prediction of the dynamic behaviour of the impeller is of considerable importance in the design of turbo compressors. For this purpose comprehensive measurements of the dynamic stress are carried out on the author’s company’s test rig. The paper describes the test arrangement including numerous strain gauges installed in different positions of the blades. The last part of this paper describes a simplified approach for the evaluation of the dynamic stresses in the impeller blade. A substantial list of comprehensive lectures about coupled CFD–structural analysis to determine the dynamic behaviour of impeller blades is available. However, despite of considerable improvement in software and calculation methods in the last years, such analyses are still expensive and require very large capacities. The presented alternative calculation method is based on a harmonic analysis which is calibrated to the measurements and is the base for a further estimation of the life time of the impeller.