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Research Papers

Prediction of Rotordynamic Performance of Smooth Stator-Grooved Rotor Liquid Annular Seals Utilizing Computational Fluid Dynamics

[+] Author and Article Information
Farzam Mortazavi

Mem. ASME
Department of Mechanical Engineering,
Texas A&M University,
College Station, TX 77843
e-mail: farzam.mortazavi@tamu.edu

Alan Palazzolo

Fellow ASME
Department of Mechanical Engineering,
Texas A&M University,
College Station, TX 77843
e-mail: a-palazzolo@tamu.edu

Contributed by the Technical Committee on Vibration and Sound of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received July 10, 2017; final manuscript received November 1, 2017; published online December 12, 2017. Assoc. Editor: Patrick S. Keogh.

J. Vib. Acoust 140(3), 031002 (Dec 12, 2017) (9 pages) Paper No: VIB-17-1313; doi: 10.1115/1.4038437 History: Received July 10, 2017; Revised November 01, 2017

Circumferentially grooved, annular liquid seals typically exhibit good whirl frequency ratios (WFRs) and leakage reduction, yet their low effective damping can lead to instability. The current study investigates the rotordynamic behavior of a 15-step groove-on-rotor annular liquid seal by means of computational fluid dynamics (CFD), in contrast to the previous studies which focused on a groove-on-stator geometry. The seal dimensions and working conditions have been selected based on experiments of Moreland and Childs (2016, “Influence of Pre-Swirl and Eccentricity in Smooth Stator/Grooved Rotor Liquid Annular Seals, Measured Static and Rotordynamic Characteristics,” M.Sc. thesis, Texas A&M University, College Station, TX). The frequency ratios as high as four have been studied. Implementation of pressure-pressure inlet and outlet conditions make the need for loss coefficients at the entrance and exit of the seal redundant. A computationally efficient quasi-steady approach is used to obtain impedance curves as functions of the excitation frequency. The effectiveness of steady-state CFD approach is validated by comparison with the experimental results of Moreland and Childs. Results show good agreement in terms of leakage, preswirl ratio (PSR), and rotordynamic coefficients. It was found that PSR will be about 0.3–0.4 at the entrance of the seal in the case of radial injection, and outlet swirl ratio (OSR) always converges to values near 0.5 for current seal and operational conditions. The negative value of direct stiffness coefficients, large cross-coupled stiffness coefficients, and small direct damping coefficients explains the destabilizing nature of these seals. Finally, the influence of surface roughness on leakage, PSR, OSR, and stiffness coefficients is discussed.

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References

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Figures

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Fig. 1

Negative stiffness

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Fig. 2

Schematic of the seal dimensions

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Fig. 3

Quasi-steady circular whirling motion in CFD modeling

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Fig. 4

Computational domains used for CFD simulation

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Fig. 5

Exploded view of the grid

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Fig. 7

Suppressed back flow and the streamlines at the inlet chamber

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Fig. 8

Flow reattachment in extension annulus

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Fig. 9

Pressure and swirl ratio variation at r=50.9mm in the inlet plenum, seal, and outlet chamber

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Fig. 10

Leakage, PSR, and OSR versus frequency ratio

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Fig. 11

Dynamic stiffness variation versus excitation frequency: (a) (Hii), (b) Re(Hij), (c) Im(Hii), and (d) Im(Hij)

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Fig. 12

Radial and tangential forces acting on the rotor

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Fig. 13

Effect of rotor speed on leakage

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Fig. 14

Effect of Δp on leakage

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Fig. 15

Effect of roughness on leakage, PSR, and OSR

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Fig. 16

Effect of roughness on radial and tangential impedance at Ω=0

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