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Research Papers: Gas Turbines: Structures and Dynamics

On-Site Identification of Dynamic Annular Seal Forces in Turbo Machinery Using Active Magnetic Bearings: An Experimental Investigation

[+] Author and Article Information
Jonas S. Lauridsen

Department of Mechanical Engineering,
Technical University of Denmark,
Kgs. Lyngby 2800, Denmark
e-mail: jonlau@mek.dtu.dk

Ilmar F. Santos

Professor
Department of Mechanical Engineering,
Technical University of Denmark,
Kgs. Lyngby 2800, Denmark
e-mail: ifs@mek.dtu.dk

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received March 20, 2017; final manuscript received November 1, 2017; published online April 12, 2018. Assoc. Editor: Alexandrina Untaroiu.

J. Eng. Gas Turbines Power 140(8), 082501 (Apr 12, 2018) (9 pages) Paper No: GTP-17-1106; doi: 10.1115/1.4038755 History: Received March 20, 2017; Revised November 01, 2017

Significant dynamic forces can be generated by annular seals in rotordynamics and can under certain conditions destabilize the system leading to a machine failure. Mathematical modeling of dynamic seal forces are still challenging, especially for multiphase fluids and for seals with complex geometries. This results in much uncertainty in the estimation of the dynamic seal forces, which often leads to unexpected system behavior. This paper presents the results of a method suitable for on-site identification of uncertain dynamic annular seal forces in rotordynamic systems supported by active magnetic bearings (AMB). An excitation current is applied through the AMBs to obtain perturbation forces and a system response, from which the seal coefficients are extracted by utilizing optimization and a priori information about the mathematical model structure and its known system dynamics. As a study case, the method is applied to a full-scale test facility supported by two radial AMBs interacting with one annular center-mounted test seal. Specifically, the dynamic behavior of a smooth annular seal with high preswirl and large clearance (worn seal) is investigated in this study for different excitation frequencies and differential pressures across the seal. The seal coefficients are extracted and a global model on reduced state-space modal form is obtained using the identification process. The global model can be used to update the model-based controller to improve the performance of the overall system. This could potentially be implemented in all rotordynamic systems supported by AMBs and subjected to seal forces or other fluid film forces.

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Figures

Grahic Jump Location
Fig. 1

Test facility overview: ① AMB A, ② AMB B, and ③ seal house. Figure adapted from Ref. [23].

Grahic Jump Location
Fig. 2

Full section view of testrig with ① backup bearing, ② displacement sensor, ③ rotor and stator of the AMB, and ④ seal house. Figure adapted from Ref. [23].

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Fig. 3

Connecting the motor to the shaft: ① motor, ② encoder, ③ belt drive, ④ intermediate shaft pedestal, and ⑤ flexible coupling. Figure is adapted from Ref. [23].

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Fig. 4

Test bench AMB showing the AMB actuator and global reference frames. Figure is adapted from Ref. [24].

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Fig. 5

Cross section of the seal house: ① secondary discharge cavity, ② discharge cavity,③ inlet injection cavity, ④ inlet nozzle, ⑤ pressure sensor, and ⑥ seal surface. Figure is adapted from Ref. [24].

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Fig. 6

Cross section of the seal house at the inlet cavity section: ① inlet injection nozzle, ② inlet cavity, and ③ shaft. Figure is adapted from Ref. [24].

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Fig. 7

Updated/changed plant representation using upper LFT, Gupdated=Fu(Gfi,θ)

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Fig. 8

Active magnetic bearings-rotor model versus experimental time response. Displacements are shown in upper plot and currents are shown in lower plot. The PRBS current perturbation signal is indicated with dashed lines and the amplitude is scaled to fit the plot. The results are shown for bearing A and are similar for bearing B.

Grahic Jump Location
Fig. 9

Frequency response functions (FRFs) of AMB-rotor model versus experimental for parallel excitation of rotor in ζ-direction using a 0–150 Hz chirp signal. The model fits experimental data well up to approximately 100 Hz indicated with the dashed line. “Simulation” and “Experimental” shows FRFs from simultaneously excitation on bearing A and bearing B to the center movement of the seal in the excitation direction. “Experimental cross, bearing A” and “Experimental cross, bearing B” show FRFs from excitation on each bearing to the movement in the cross-coupled direction. Top: amplitude, middle: phase, and bottom: coherence.

Grahic Jump Location
Fig. 10

FRFs of AMB-rotor model versus experimental for parallel excitation of rotor in η-direction using a 0–150 Hz chirp signal. The model fits experimental data well up to approximately 100 Hz indicated with the dashed line. “Simulation” and “Experimental” show FRFs from simultaneously excitation on bearing A and bearing B to the center movement of the seal in the excitation direction. “Experimental cross, bearing A” and “Experimental cross, bearing B” show FRFs from excitation on each bearing to the movement in the cross-coupled direction. Top: amplitude, middle: phase, and bottom: coherence.

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Fig. 11

Example of the fit of seal coefficients identified at 40 Hz excitation. The simulated and experimental time responses are shown for displacement (upper) and for current (lower). A sinus excitation current is applied in the ζ-direction.

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Fig. 12

Seal coefficients versus excitation frequencies for different pressures across the seal. Coefficients are valid up to approximately 100 Hz as indicated with the dashed line. The solid lines indicate the constant seal coefficients chosen for model verification in Sec. 3.5.2.

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Fig. 13

FRFs of global model with constant seal coefficients versus experimental using 0.7 bar pressure across the seal. Top: amplitude, middle: phase, and bottom: coherence.

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Fig. 14

FRFs of global model with constant seal coefficients versus experimental using 1.0 bar pressure across the seal. Top: amplitude, middle: phase, and bottom: coherence.

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Fig. 15

FRFs of global model with constant seal coefficients versus experimental using 1.2 bar pressure across the seal. Top: amplitude, middle: phase, and bottom: coherence.

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