Prediction and Validation of High-Performance Centrifugal Compressor Impeller Forced Response

High-performance centrifugal compressors call for a reduction in weight to relax rotor-dynamic constraints, and a reduction in blade thickness as a way to improve efficiency. In this tight design space, synchronous excitations on impellers are often unavoidable and may represent a potential aeromechanic risk, especially for high speed stages operated at high pressures. Therefore, forced response analysis has become a fundamental design step. The aero/structure unsteady interaction in impellers, mainly driven by both stationary row wakes and potential fields, and the possible overlap with structural frequencies, must be carefully analyzed and quantified since they may lead to high cycle fatigue failures under critical operating conditions. This paper focuses on the forced response analysis of an unshrouded high-speed centrifugal impeller incorporating aircraft engine technology designed for hydrocarbon processing. The methodology focuses on the effects of generic disturbances (inlet distortions, potential fields, aero-acoustic excitations) superimposed in amplitude and frequency on the time averaged flow field, and follows a rigorous numerical approach supported by validation against dedicated aeromechanic test data. Aerodynamic forcing frequencies and amplitudes, and the corresponding aero-mechanic damping jointly contribute to the vibratory response. An unsteady CFD analysis determines the impeller aerodynamic load which, in conjunction with FE (Finite Elements) modal analysis, allows for computing the modal force. A time-linearized Navier-Stokes computation predicts the aerodynamic damping, and allows mapping the mode shapes onto the CFD grid in order to supply the displacement boundary conditions. A purely numerical Goodman diagram has been drawn and the response level for a specific structural mode shape analyzed. The predictions compare favorably with test data, providing confidence in this method for practical high-speed radial compressor design.