Research Papers

Blade Tip Clearance Flow and Compressor Nonsynchronous Vibrations: The Jet Core Feedback Theory as the Coupling Mechanism

[+] Author and Article Information
Jean Thomassin

 Pratt and Whitney Canada, 1000 Marie-Victorin, Longueuil, QC, J4G 1A1, Canada

Huu Duc Vo, Njuki W. Mureithi

 École Polytechnique de Montréal, 2500 Chemin de Polytechnique, Montréal, QC, H3T 1J4, Canada

J. Turbomach 131(1), 011013 (Oct 28, 2008) (9 pages) doi:10.1115/1.2812979 History: Received June 13, 2007; Revised August 24, 2007; Published October 28, 2008

This paper investigates the role of tip clearance flow in the occurrence of nonsynchronous vibrations (NSVs) observed in the first axial rotor of a high-speed high-pressure compressor in an aeroengine. NSV is an aeroelastic phenomenon where the rotor blades vibrate at nonintegral multiples of the shaft rotational frequencies in operating regimes where classical flutter is not known to occur. A physical mechanism to explain the NSV phenomenon is proposed based on the blade tip trailing edge impinging jetlike flow, and a novel theory based on the acoustic feedback in the jet potential core. The theory suggests that the critical jet velocity, which brings a jet impinging on a rigid structure to resonance, is reduced to the velocities observed in the blade tip secondary flow when the jet impinges on a flexible structure. The feedback mechanism is then an acoustic wave traveling backward in the jet potential core, and this is experimentally demonstrated. A model is proposed to predict the critical tip speed at which NSV can occur. The model also addresses several unexplained phenomena, or missing links, which are essential to connect tip clearance flow unsteadiness to NSV. These are the pressure level, the pitch-based reduced frequency, and the observed step changes in blade vibration and mode shape. The model is verified using two different rotors that exhibited NSV.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Rotor blade and NSV mode shape

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Figure 2

NSV blade vibrations and casing pressure

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Figure 3

Impinging trailing edge backflow (11)

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Figure 4

Tip velocity vector during RI (9)

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Figure 5

Pressure side velocity fluctuation (8)

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Figure 6

Resonant jet on a rigid plate, Ho (15)

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Figure 7

Jet core feedback theory

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Figure 8

Experimental setup

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Figure 9

Plate nodal circle mode shapes

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Figure 10

Mach sweep through resonance

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Figure 11

Forward wave transfer function

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Figure 12

Backward wave phase results

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Figure 13

Jet core feedback—NSV application

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Figure 14

NSV region on Campbell diagram

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Figure 15

NSV near EO6 on engine deceleration

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Figure 16

Feedback staging and vibration frequency shift



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