Experimental investigations on a single stage centrifugal compressor showed that measured blade vibration amplitudes vary considerably along a constant speed line from choke to surge. The unsteady flow has been analyzed to obtain detailed insight into the excitation mechanism. Therefore, a turbocharger compressor stage impeller has been modeled and simulated by means of computational fluid dynamics (CFD). Two operating points at off-design conditions were analyzed. One was close to choke and the second one close to the surge line. Transient CFD was employed, since only then a meaningful prediction of the blade excitation, caused by the unsteady flow situation, can be expected. Actually, it was observed that close to surge a steady state solution could not be obtained; only transient CFD could deliver a converged solution. The CFD results show the effect of the interaction between the inducer casing bleed system and the main flow. Additionally, the effect of the nonaxisymmetric components, such as the suction elbow and the discharge volute, was analyzed. The volute geometry itself had not been modeled. It turned out to be sufficient to impose a circumferentially asymmetric pressure distribution at the exit of the vaned diffuser to simulate the volute. Volute and suction elbow impose a circumferentially asymmetric flow field, which induces blade excitation. To understand the excitation mechanism, which causes the measured vibration behavior of the impeller, the time dependent pressure distribution on the impeller blades was transformed into the frequency domain by Fourier decomposition. The complex modal pressure data were imposed on the structure that was modeled by finite element methods (FEM). Following state-of-the-art calculations to analyze the free vibration behavior of the impeller, forced response calculations were carried out. Comparisons with the experimental results demonstrate that this employed methodology is capable of predicting the impeller’s vibration behavior under real engine conditions. Integrating the procedure into the design of centrifugal compressors will enhance the quality of the design process.

Experimental investigations on a single stage centrifugal compressor with radial inlet duct showed that measured alternating strains of the rotating blades depend considerably on the circumferential position of the diffuser ring to the volute tongue. By modeling of the entire turbocharger compressor stage with volute and inducer casing bleed system included, 3D unsteady flow simulations provided comprehensive insight into the excitation mechanism. A part load operating point was investigated experimentally and numerically. For operating conditions due to resonance transient CFD was employed, since only then a meaningful prediction of the blade excitation, induced by the unsteady air flow, is expected. The CFD results show primarily the interaction between the volute tongue and the two different vaned diffuser ring positions. It is shown that pressure and flow angle vary significantly due to the circumferential position of the flow entering the volute and the turning impeller blades. The geometrical arrangement of the volute and suction elbow imposes a non-axisymmetric flow field, which excites rotating blades periodically. These vibrations depend on the circumferential assembly position of the vaned diffuser. Outflow and reverse flow at the tongue region also differ with respect to the vaned diffuser ring position. The time dependent pressure distribution on the impeller blades resulting from the CFD calculation was transformed into the frequency domain by Fourier decomposition. The complex modal pressure data were imposed as exciting load on the structure which was simulated by the FEM. By applying a fine FE mesh the measured resonant frequencies for the lower modes were reproduced very well by FEM. After determining the 3D mode shapes of the impeller by means of a free vibration calculation, forced response simulations without considering transient vibration effects were carried out for predicting the resonance strain amplitudes which were computed for both minimum and maximum experimental modal damping ratios. Comparisons with the experimental results at the strain gauges demonstrate that this employed methodology is capable of predicting the 3D impeller’s vibration behavior under real engine conditions up to 8 kHz. Considering strong influence of mistuning on real impeller vibrations, a new method for the comparison of experimental and numerical data has been successfully introduced. In general, this approach is based on the resonance sensitivity assessment, which takes into account the excitation, damping and mistuning parameters. Then, the measured resonance strain amplitudes of all experimental tests match very well the predicted scatter range of numerical results.

Experimental investigations on a single stage centrifugal compressor showed that measured blade vibration amplitudes vary considerably along a constant speed line from choke to surge. The unsteady flow has been analysed to obtain detailed insight into the excitation mechanism. Therefore, a turbocharger compressor stage impeller has been modeled and simulated by means of Computational Fluid Dynamics (CFD). Two operating points at off-design conditions were analysed. One was close to choke and the second one close to the surge line. Transient CFD was employed, since only then a meaningful prediction of the blade excitation, caused by the unsteady flow situation, can be expected. Actually, it was observed that close to surge a steady state solution could not be obtained; only transient CFD could deliver a converged solution. The CFD results show the effect of the interaction between the inducer casing bleed system and the main flow. Additionally, the effect of the non-axisymmetric components, such as the suction elbow and the discharge volute, was analysed. The volute geometry itself had not been modeled. It turned out to be sufficient to impose a circumferentially asymmetric pressure distribution at the exit of the vaned diffuser to simulate the volute. Volute and suction elbow impose a circumferentially asymmetric flow field, which induces blade excitation. To understand the excitation mechanism, which causes the measured vibration behavior of the impeller, the time dependent pressure distribution on the impeller blades was transformed into the frequency domain by Fourier decomposition. The complex modal pressure data were imposed on the structure that was modeled by Finite Element Methods (FEM). Following state-of-the-art calculations to analyze the free vibration behavior of the impeller, forced response calculations were carried out. Comparisons with the experimental results demonstrate that this employed methodology is capable of predicting the impeller’s vibration behavior under real engine conditions. Integrating the procedure into the design of centrifugal compressors will enhance the quality of the design process.