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Research Papers: Gas Turbines: Structures and Dynamics

Rotordynamic Force Coefficients of a Hybrid Brush Seal: Measurements and Predictions

[+] Author and Article Information
Luis San Andrés

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

José Baker

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843jose.baker@kbr.com

Adolfo Delgado

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

Pressure ratio Pr=Ps/Pd=absolute supply pressure/absolute discharge pressure.

Smaller load magnitudes lead to stick-slip (nonlinear) phenomenon with erratic seal behavior, while larger magnitude loads produce too large rotor displacements that endanger the seal life.

Refer to Ref. 16 or (19) for details on the physical model relying on equivalent stiffness and mass parameters representing the combined actions of the shaft+disk+seal, and derived from mechanical energy considerations for motions exciting the fundamental elastic mode.

J. Eng. Gas Turbines Power 132(4), 042503 (Jan 26, 2010) (7 pages) doi:10.1115/1.3159377 History: Received March 23, 2009; Revised March 29, 2009; Published January 26, 2010; Online January 26, 2010

Brush seals effectively control leakage in air breathing engines, albeit only applied for relatively low-pressure differentials. Hybrid brush seals (HBS) are an alternative to resolve poor reliability resulting from bristle tip wear while also allowing for reverse shaft rotation operations. A HBS incorporates pads contacting the shaft on assembly; and which under rotor spinning, lift off due to the generation of a hydrodynamic pressure. The ensuing gas film prevents intermittent contact, reducing wear, and thermal distortions. This paper presents rotordynamic measurements conducted on a test rig for evaluation of HBS technology. Single frequency shaker loads are exerted on a test rotor holding a hybrid brush seal, and measurements of rotor displacements follow for operating conditions with increasing gas supply pressures and two rotor speeds. A frequency domain identification method delivers the test system stiffness and damping coefficients. The HBS stiffness coefficients are not affected by rotor speed though the seal viscous damping shows a strong frequency dependency. The identified HBS direct stiffness decreases 15% as the supply/discharge pressure increases Pr=1.72.4. The HBS cross-coupled stiffnesses are insignificant, at least one order of magnitude smaller than the direct stiffnesses. A structural loss factor (γ) and dry-friction coefficient (μ) represent the energy dissipated in a HBS by the bristle-to-bristle and bristle-to-pad interactions. Predictions of HBS stiffness and damping coefficients correlate well with the test derived parameters. Both model predictions and test results show the dramatic reduction in the seal equivalent viscous damping coefficients as the excitation whirl frequency increases.

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Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Axial and cross-sectional views of a conventional brush seal

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Figure 2

Photograph of a hybrid brush seal (close up of pad and elastic supports)

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Figure 3

Isometric view of rotordynamic test rig for evaluation of hybrid brush seal

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Figure 4

Detail view of disk/shaft assembly

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Figure 5

Reference coordinate system for rotating tests with shaker loads

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Figure 6

Waterfalls of rotor displacements (X,Y) resulting from a periodic excitation load (22 N). Excitation frequency range of 20–90 Hz, Pr=1.7, and rotor speed: 600 rpm (10 Hz).

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Figure 7

Identified system rotordynamic stiffness versus frequency. Load magnitude=22 N for increasing supply pressure to discharge pressure ratios (Pr) and rotor speeds.

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Figure 8

Test data and identified HBS equivalent viscous damping for increasing rotor speeds (600 rpm and 1200 rpm) and two pressure ratios (Pr=1.7 and 2.4)

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Figure 9

Predicted nonsynchronous HBS stiffness coefficients versus excitation frequency at two supply to discharge pressure ratios, Pr=1.7 and 2.4. Rotor speeds: 600 rpm (10 Hz) and 1200 rpm (20 Hz).

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Figure 10

Predicted HBS viscous damping coefficients versus excitation frequency. Rotor speeds: 600 rpm (10 Hz) and 1200 rpm (20 Hz); supply to discharge pressure ratio, Pr=1.7 and 2.4.

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