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Research Papers: Gas Turbines: Structures and Dynamics

Experimental Study on Impeller Blade Vibration During Resonance—Part I: Blade Vibration Due to Inlet Flow Distortion

[+] Author and Article Information
Albert Kammerer, Reza S. Abhari

LEC, Laboratory for Energy Conversion, Department of Mechanical and Process Engineering,  ETH Zurich, 8092 Zürich, Switzerland

J. Eng. Gas Turbines Power 131(2), 022508 (Dec 29, 2008) (11 pages) doi:10.1115/1.2968869 History: Received April 08, 2008; Revised April 08, 2008; Published December 29, 2008

Forming the first part of a two-part paper, the experimental approach to acquire resonant vibration data is presented here. Part II deals with the estimation of damping. During the design process of turbomachinery components, mechanical integrity has to be guaranteed with respect to high cycle fatigue of blades subject to forced response or flutter. This requires the determination of stress levels within the blade, which in turn depend on the forcing function and damping. The vast majority of experimental research in this field has been performed on axial configurations for both compressors and turbines. This experimental study aims to gain insight into forced response vibration at resonance for a radial compressor. For this purpose, a research impeller was instrumented with dynamic strain gauges and operated under resonant conditions. Modal properties were analyzed using finite element method and verified using an optical method termed electronic-speckle-pattern-correlation-interferometry. During the experiment, unsteady forces acting on the blades were generated by grid installations upstream of the impeller, which created a distorted inlet flow pattern. The associated flow properties were measured using an aerodynamic probe. The resultant pressure fluctuations on the blade surface and the corresponding frequency content were assessed using unsteady computational fluid dynamics. The response of the blades was measured for three resonant crossings, which could be distinguished by the excitation order and the natural frequency of the blades. Measurements were undertaken for a number of inlet pressure settings starting at near vacuum and then increasing. The overall results showed that the installed distortion screens generated harmonics in addition to the fundamental frequency. The resonant response of the first and the second blade mode showed that the underlying dynamics support a single-degree-of-freedom model.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Radial compressor research facility

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Figure 2

Impeller main blade modal shapes for Modes 1 and 2. (a) Mode 1; (b) Mode 2.

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Figure 3

Arrangement and dimensions within the inlet section

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Figure 4

Distortion screen geometry

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Figure 5

Velocity distortion by Koo and James (13) and velocity ratio dependency on mass flow. (a) Velocity ratios for λ=0.5: (b) predicted velocity ratio as function of the mass flow rate.

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Figure 6

Measured normalized axial velocity distribution (Vax/Vax,mean); a distortion screen is not installed. Support strut arrangement is shown.

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Figure 7

FRAP measured normalized velocity distribution upstream of the impeller

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Figure 8

Computed amplitude and phase angle of the unsteady pressure on blade surface using unsteady CFD. Measured inlet boundary conditions were applied for the five-lobe case.

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Figure 9

Impeller performance map

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Figure 10

Typical response of the main blade without a distortion screen installed upstream of the impeller

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Figure 11

Strain response spectrum for EO5 excitation using a five-lobe screen

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Figure 12

Dynamic response envelope during Mode1/EO5 resonance

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Figure 13

Maximum response amplitude dependency on inlet pressure for Mode1/EO5

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Figure 14

Maximum strain variation from mean

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Figure 15

Results for Mode1/EO6 case

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Figure 16

Results for Mode2/EO12 case

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Figure 17

Comparison of maximum strain amplitude

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