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Research Papers: Gas Turbines: Structures and Dynamics

A Novel Bulk-Flow Model for Improved Predictions of Force Coefficients in Grooved Oil Seals Operating Eccentrically

[+] Author and Article Information
Luis San Andrés

 Texas A&M University, College Station, TX 77843-3123 lsanandres@tamu.edu

Adolfo Delgado1

 GE Research Center, Niskayuna, NY 12309 delgado@ge.com

Ref. [13] contains a comprehensive review of the past literature on force coefficients for grooved oil seals and squeeze film dampers.

The omission of a physically sound model for lubricant cavitation (vapor or gas) is not grave. Oil seals with their large pressure differentials rarely develop pressures below ambient. Open ends SFDs; on the other hand, are subject more to air entrainment than lubricant cavitation.

The experimental force coefficients reported equal to 50% of the measured values for the whole test arangement configuration, i.e., two seals in parallel.

1

Work conducted as a Research Assistant while at Texas A&M University.

J. Eng. Gas Turbines Power 134(5), 052509 (Mar 01, 2012) (10 pages) doi:10.1115/1.4004736 History: Received June 21, 2011; Revised June 22, 2011; Published March 01, 2012; Online March 01, 2012

Oil seals in centrifugal compressors reduce leakage of the process gas into the support bearings and ambient. Under certain operating conditions of speed and pressure, oil seals lock, becoming a source of hydrodynamic instability due to excessively large cross coupled stiffness coefficients. It is a common practice to machine circumferential grooves, breaking the seal land, to isolate shear flow induced film pressures in contiguous lands, and hence reducing the seal cross coupled stiffnesses. Published tests results for oil seal rings shows that an inner land groove, shallow or deep, does not actually reduce the cross-stiffnesses as much as conventional models predict. In addition, the tested grooved oil seals evidenced large added mass coefficients while predictive models, based on classical lubrication theory, neglect fluid inertia effects. This paper introduces a bulk-flow model for groove oil seals operating eccentrically and its solution via the finite element (FE) method. The analysis relies on an effective groove depth, different from the physical depth, which delimits the upper boundary for the squeeze film flow. Predictions of rotordynamic force coefficients are compared to published experimental force coefficients for a smooth land seal and a seal with a single inner groove with depth equaling 15 times the land clearance. The test data represent operation at 10 krpm and 70 bar supply pressure, and four journal eccentricity ratios (e/c= 0, 0.3, 0.5, 0.7). Predictions from the current model agree with the test data for operation at the lowest eccentricities (e/c= 0.3) with discrepancies increasing at larger journal eccentricities. The new flow model is a significant improvement towards the accurate estimation of grooved seal cross-coupled stiffnesses and added mass coefficients; the latter was previously ignored or largely under predicted.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Typical oil seal multiring assembly

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Figure 2

Schematic view of grooved annular cavity divided into flow regions

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Figure 3

View of rotating and whirling journal and coordinate system for bulk-flow analysis

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Figure 4

Coordinate system and sample FE mesh for oil seal model

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Figure 5

(a) Schematic view of streamlines in axially symmetric grooved annular cavity (ΔP = PsPd ). (b) CFD simulation of pressure driven streamlines across a 10c and 15c circumferential mid-land groove in an oil seal tested in Ref. [7]. (c = 86 mm, Ω=10,000 rpm, D = 117 mm.)

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Figure 6

Schematic view and dimensions of test (parallel) oil seal in Refs. [6,7]

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Figure 7

Predicted reaction forces for smooth seal and seal with inner land groove (cη  = 7c) versus journal eccentricity ratio. Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 8

Predicted seal direct stiffness coefficient (KXX , KYY ) versus journal eccentricity ratio. Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 9

Predicted cross-coupled stiffness coefficients (KXY , KYX ) versus journal eccentricity ratio. Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 10

Predicted cross-coupled stiffnesses (KXY , KYX ) versus shaft speed at two journal eccentricities (0, 0.3). Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 11

Predicted direct damping coefficients (CXX , CYY ) versus journal eccentricity ratio. Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 12

Predicted cross-coupled damping coefficients (CXY , CYX ) versus journal eccentricity ratio. Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 13

Predicted added mass coefficient (MXX , MYY ) versus journal eccentricity ratio. Experimental data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7].

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Figure 14

Seal leakage versus journal eccentricity ratio: predictions and test data for smooth seal and seal with inner land groove (cg  = 15c), 10,000 rpm, 70 bar [7]

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Figure 15

Predicted dynamic pressure fields in seal due to journal whirl motions (5 μm, ω = 200 Hz). (a) Classical theory [12] assumes null dynamic pressure in deep plenum; (b) Current model with effective central plenum clearance (cηI=12c). Film thickness noted.

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Figure 16

Predicted dynamic pressure fields in seal with inner land groove due to journal motions (5 μm, ω = 200 Hz). (a) Classical theory [12] assumes null dynamic pressure in deep plenum and inner groove; (b) Current model with effective plenum and inner groove clearances (cηI=12c, cηIII=7c).

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