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A tool chain for the numerical prediction of high cycle fatigue (HCF) in an industrial centrifugal compressor during resonant stimulation, caused by rotor-stator interactions, is set up and validated against experimental investigations. The compressor under investigation is built up of an inlet guide vane (IGV), an open impeller, a vaned diffuser and an outlet volute. This type of centrifugal compressor is typically used in integrally geared compressors. The experimental investigation was performed on an open loop test rig of MAN in Oberhausen. Frequency domain based CFD simulations were used to investigate the fluid-structure-interactions. The resulting forced response alternating stresses were examined at a cyclic symmetric model by the use of a FE analysis. Two nodal diameters of one particular eigenmode are investigated. Once the excitation is caused by the diffuser and the other time by the IGV. The aerodynamic excitation and damping forces are changing along a speed line from choke to surge. Therefore, three different operating conditions are experimentally and numerically investigated in detail for each resonance. The numerically predicted alternating stresses occurring in the tuned system are compared directly to measurements with strain gauges on the impeller blade and to non-intrusive tip timing probe data. Objective of the paper is to show that the numerical tool chain is able to predict the alternating stresses due to rotorstator-interaction correctly. Especially, the simultaneous calculation of excitation and damping in one frequency domain based CFD simulation is not commonly used. Although the magnitude of the predictions and the measurements show some offset, the qualitative validity of the numerical prediction could be proved. This enables a quick evaluation of critical operating conditions. Systematic evaluations of the aerodynamic excitation and damping mechanisms are conducted. These analyses improve the understanding of the damaging potential of HCF due to different excitation sources. INTRODUCTION Rotor-stator-interactions can cause high unsteady pressure fluctuations on the stator and rotor blade-rows of a centrifugal compressor stage. If the frequency of these pressure fluctuations is equal to an eigenfrequency of a bladerow, this blade-row is stimulated in resonance. The resulting vibration amplitude of the blades causes high alternating stresses within in the material, which can lead to a large high cycle fatigue (HCF) of the blades. The magnitude of the induced alternating stresses is depending on the material properties, the excitation force and the damping of the system. Equation (1) gives the general equation of motion for a single degree of freedom system [e.g. (Platzer & Carta, 1988)] s s s , . (1) [M] is the mass and [K] the stiffness matrix. The damping matrix [C] consists of the mechanical, the material and the aerodynamic damping. As the impeller is manufactured of one piece, no mechanical damping is existing. The consensus – which will be confirmed later – is that the aerodynamic damping is approximately an order of magnitude greater than the material damping and thus, this contribution is nearly negligible. The aerodynamic excitation force is standing on the right-hand side of equation (1). The compressor stage under investigation consists of an IGV, an open impeller, a vaned diffuser and an outlet volute. Typically, the compressor stage is utilized in integrally geared compressors and running on a fixed speed. Thus, with regard to the frequencies of pressure fluctuations induced by rotorstatorinteractions, in most cases a resonant free steady operation of the compressor is possible. But even in these alleged resonant free compressor designs excitation of eigenmodes can occur, if unsteady phenomena like rotating stall arise with the corresponding frequencies, c.f. (Jenny & Bidaut, 2016), (Chen, et al., 2017), (Wagner, et al., 2011). Within this paper these phenomena are not discussed, but

Computational and Experimental Investigation of Mechanical Stresses Induced by Rotor-Stator Interaction in an Industrial Centrifugal Compressor