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Research Papers: Gas Turbines: Structures and Dynamics

Identification of Squeeze Film Damper Force Coefficients From Multiple-Frequency Noncircular Journal Motions

[+] Author and Article Information
Adolfo Delgado

Department of Mechanical Engineering. Texas A&M University, College Station, TX 77843delgadoa@ge.com

Luis San Andrés

Department of Mechanical Engineering. Texas A&M University, College Station, TX 77843lsanandres@tamu.edu

As verified with single frequency load excitations. For a single frequency excitation force, the ensuing damper displacement presents multiple harmonic components (1× and 3× predominantly).

In an intershaft system, this condition simulates the case where the motion response vector for each shaft is in phase and out of phase with each other.

J. Eng. Gas Turbines Power 132(4), 042501 (Jan 22, 2010) (9 pages) doi:10.1115/1.3159374 History: Received March 22, 2009; Revised March 29, 2009; Published January 22, 2010; Online January 22, 2010

In rotor-bearing systems, squeeze film dampers (SFDs) provide structural isolation, reduce amplitudes of rotor response to imbalance, and in some instances, increase the system threshold speed of instability. SFDs are typically installed at the bearing supports, either in series or in parallel. In multispool engines, SFDs are located in the interface between rotating shafts. These intershaft dampers must ameliorate complex rotor motions of various whirl frequencies arising from the low speed and the high speed rotors. The paper presents experiments to characterize the forced response of an open ends SFD subject to dynamic loads with multiple frequencies, as in a jet engine intershaft damper. The test rig comprises of a stationary journal and a flexibly supported housing that holds the test damper and instrumentation. The open ends SFD is 127 mm in diameter, 25.4 mm film land length, and has a radial clearance of 0.125 mm. The damper is lubricated with ISO VG 2 oil at room temperature (24°C, feed pressure 31 kPa). In the experiments, two orthogonally positioned shakers deliver forces to the test damper that produce controlled amplitude motions with two whirl frequencies, one fixed and the other one varying over a specified range that includes the test system natural frequency. The test data collected, forces and motions versus time, are converted into the frequency domain for parameter identification. The identified viscous damping coefficients are strong functions of the amplitude of journal motion, lying within predictions from classical formulas for circular centered orbits and small amplitude motions about an eccentric journal position. The damper inertia coefficients agree well with predictions derived from a fluid flow model that includes the effect of the feed groove.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic view of intershaft damper configurations: (a) squeeze film rotates with low speed (LS) rotor, (b) squeeze film rotates with pressure high speed (HS) rotor, and (c) double ball bearing-squirrel cage design. Reproduced from Ref. 6.

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Figure 2

Schematic view of test rig for SFD dynamic forced response and flow visualization (21)

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Figure 3

Cut view of open-end SFD and detail view of squeeze film land

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Figure 4

Equivalent representation of test SFD with mechanical parameters

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Figure 5

Measured bearing orbits (y versus x) due to two fixed amplitude load vectors (a) F(1) and (b) F(2). Multifrequency excitation (constant 25 Hz+sine-weep 30–120 Hz). Damper clearance circle noted.

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Figure 6

Time traces of excitation force vector F(1) and ensuing x(t) and y(t) bearing displacements. Fixed amplitude load. Multifrequency excitation (constant 25 Hz+sine sweep 30–120 Hz). Maximum motion amplitude ∼60 μm. Damper radial clearance c=0.125 mm.

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Figure 7

DFTs of input force vector F(1) and ensuing x and y displacements

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Figure 8

Real and imaginary parts of direct impedance function Hxx versus frequency. Fixed load amplitude. Multiple frequency excitation (constant 25 Hz+sine sweep 30–120 Hz).

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Figure 9

Squeeze film damping coefficients identified from fixed load amplitude—multifrequency sine-sweep forced excitations (constant 25 Hz+sine sweep 30–120 Hz). Predictions for circular centered orbits (CCO) and radial motions about an off-centered journal static position (ORM).

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Figure 10

Time traces of excitation force vector F(3)—varying amplitude—and ensuing x(t) and y(t) bearing displacements. Multifrequency excitation (constant 25 Hz+sine sweep 30–120 Hz). Maximum motion amplitude ∼50 μm, damper radial clearance c=0.125 mm.

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Figure 11

DFTs of input force and ensuing x-displacement. Time data shown in Fig. 1. Three load conditions giving approximately constant motion amplitudes. Excitation force vector F(3).

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Figure 12

Imaginary part of impedances Hxx and Hyy derived from two load conditions giving approximately constant displacement amplitudes: 15 μm and 50 μm

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Figure 13

Squeeze film damping coefficients identified from varying load amplitude—multifrequency sine-sweep forced excitations (constant 25 Hz+sine sweep 30–120 Hz). Predictions for circular centered orbits (CCO) and radial motions about an off-centered journal static position (ORM).

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