High frequency dynamics in centrifugal compressors

Problems with centrifugal compressors relating to high frequency, i.e. Blade passing frequency (BPF) are increasing. Pulsations and vibrations generated in centrifugal compressors can lead to nuisance, due to strong tonal noise, and even breakdown. In several cases the root cause of a failure or a noise problem was the generation of strong pulsations at BPF. However, it appeared that no reliable methods exist with which such phenomena can be analysed in the design stage of a compressor. Therefore an R&D project was initiated with the objective to develop techniques with which the production and propagation of pulsations in the range of BPF can be predicted. In the first phase of the project, that has been nearly completed, the potential capabilities of CFD and Computational Aero Acoustics (CAA) techniques are evaluated. There are several steps in the approach. First of all an unsteady CFD simulation is required, which will generate the transient flow field and a time dependent pressure distribution on the impeller and stator blades. From these pressure distributions the acoustic sources will be computed. These acoustic sources , together with the time dependent flow field from CFD, will be used in an acoustic response calculation. This response calculation includes the effects of the mean flow and moving (mainly rotation) of the sources. For this a Linearised Euler Equation solver is applied that is part of the CAA (Computational Aero Acoustics) code that has originally been developed at TNO for evaluating aero acoustic noise generation in external flows. One of the sources are the so called Tyler Sofrin modes In this paper two intermediate results are shown. First a 2D CFD model of a rotor stator configuration is analysed. CFD can be used to get insight in the spinning mode patterns due to rotor-stator interaction with respect to number of lobes, frequency, and (relative) amplitude. Secondly, the first results of the combination of CFD and CAA are detailed. The results look promising but more analysis will be needed to make this method applicable for a real centrifugal compressor.