Unsteady Flow in a Turbocharger Centrifugal Compressor: Three-Dimensional Computational Fluid Dynamics Simulation and Numerical and Experimental Analysis of Impeller Blade Vibration

[+] Author and Article Information
Hans-Peter Dickmann

 ABB Turbo Systems Ltd, CH-5401 Baden, Switzerlandhans-peter.dickmann@ch.abb.com

Thomas Secall Wimmel

 ABB Turbo Systems Ltd, CH-5401 Baden, Switzerlandthomas.secallwimmel@ch.abb.com

Jaroslaw Szwedowicz

 ABB Turbo Systems Ltd, CH-5401 Baden, Switzerlandjaroslaw.szwedowicz@ch.abb.com

Dietmar Filsinger

 ABB Turbo Systems Ltd, CH-5401 Baden, Switzerlanddietmar.filsinger@ch.abb.com

Christian H. Roduner

 ABB Turbo Systems Ltd, CH-5401 Baden, Switzerlandchristian.roduner@ch.abb.com

J. Turbomach 128(3), 455-465 (Feb 01, 2005) (11 pages) doi:10.1115/1.2183317 History: Received October 01, 2004; Revised February 01, 2005

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.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Schematic of a Campbell diagram

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Figure 2

Excerpt of the investigated compressor performance map

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Figure 3

Working principle of the inducer casing bleed system

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Figure 4

Turbocharger test facility

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Figure 5

CFD domain and principle of the outlet boundary condition

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Figure 6

Typical steady state pressure distribution at a volute inlet applied as outlet boundary condition for a compressor stage

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Figure 7

Transient main blade power of blade 1 depending on different number of internal time steps

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Figure 8

Snapshot of absolute velocities in the entire system

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Figure 9

Snapshot of velocity vectors in the inducer casing bleed system

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Figure 10

Pressure distribution at rotor inlet and positions of planes in Fig. 1. Black circles in planes B mark the positions, where the FFT has been applied to the circumferential pressure distributions.

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Figure 11

Local pressure distributions at rotor inlet for 4 planes. Black circles mark borders to the impeller casing bleed system

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Figure 12

Comparison of the resulting excitation spectra for operation points OPcC and OpcS (Figs.  1011)

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Figure 13

Impeller main blade powers (3 revolutions)

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Figure 14

Impeller main blade powers (1 revolution)

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Figure 15

Blade power vs time (iteration) and circumferential position of the main blades at closest and farest distance to the volute tongue

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Figure 16

Resolution of the CFD and FE meshes

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Figure 17

Dispersion diagram for the first disc mode of the analyzed impeller at operating rotational speed

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Figure 18

Mode shape for the resonance i,n=1,4

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Figure 19

Complex excitation pressures (normalized) on external FE faces of the impeller sector model for the EO 4 of interest

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Figure 20

Normalized dynamic equivalent stress in the impeller when exciting the mode i,n=1,4

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Figure 21

Tip timing measurement of blade vibrations with optical emitters and sensors

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Figure 22

Calculated eigenfrequency and measured eigenfrequency

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Figure 23

Experimental results versus calculated blade amplitudes




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