Unsteady Flow in a Turbocharger Centrifugal Compressor: 3D-CFD Simulation, Impeller Blade Vibration and Vaned Diffuser-Volute Interaction

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.