Research Papers

Improved Understanding of Stiffness in Leaf-Type Filament Seals

[+] Author and Article Information
Ingo H. J. Jahn

School of Mechanical and Mining Engineering,
University of Queensland,
St. Lucia, Brisbane 4072, Australia
e-mail: i.jahn@uq.edu.au

Gervas Franceschini

Structures and Transmissions,
Rolls-Royce plc,
Derby DE24 8BJ, UK
e-mail: gervas.franceschini@rolls-royce.com

Andrew K. Owen, Terry V. Jones

Department of Engineering Science,
University of Oxford,
Oxford OX1 3PJ, UK

David R. H. Gillespie

Department of Engineering Science,
University of Oxford,
Oxford OX1 3PJ, UK
e-mail: david.gillespie@eng.ox.ac.uk

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 29, 2014; final manuscript received August 25, 2015; published online October 13, 2015. Assoc. Editor: Jim Downs.

J. Turbomach 138(1), 011004 (Oct 13, 2015) (13 pages) Paper No: TURBO-14-1309; doi: 10.1115/1.4031579 History: Received November 29, 2014; Revised August 25, 2015

Filament seals, such as brush seals and leaf seals, are investigated as a potential improved seal for gas turbine applications. As these seals operate in contact with the rotor, a good understanding of their stiffness is required in order to minimize seal wear and degradation. This paper demonstrates that the filament and complete seal stiffness is affected in comparable magnitudes by mechanical and aerodynamic forces. In certain cases, the aerodynamic forces can also lead to an overall negative seal stiffness which may affect stable seal operation. In negative stiffness, the displacement of the seal or rotor into an eccentric position causes a resultant force, which, rather than restoring the rotor to a central position, acts to amplify its displacement. Insight into the forces acting on the seal filaments is gained by investigating a leaf seal, which consists of a pack of thin planar leaves arranged around the rotor, with coverplates on either side of the leaf pack, offset from the pack surfaces. The leaf seal is chosen due to its geometry being more suitable for analysis compared to alternative filament seals such as the brush seal. Data from two experimental campaigns are presented which show a seal exhibiting negative stiffness and a seal exhibiting a stiffness reduction due to aerodynamic effects. An empirical model for the forces acting on leaf filaments is developed based on the experimental data, which allows separation of mechanical and aerodynamic forces. In addition a numerical model is developed to analyze the flow approaching the leaf pack and the interleaf flow, which gives an insight into the causes of the aerodynamic forces. Using the empirical and numerical models together, a full picture of the forces affecting leaf stiffness is created. Validation of the models is achieved by successfully predicting seal stiffness for a further data set across the full range of operating conditions. The understanding of aerodynamic forces and their impact on filament and seal stiffness allows for their consideration in leaf seal design. A qualitative assessment of how they may be used to improve seal operation in filament seals is given.

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Fig. 1

Contacting filament seals: “(a) brush seal; (b) leaf seal; (c) finger seal, Proctor et al. [3]; and (d) compliant plate seal, Deo et al. [11]

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Fig. 2

Leaf rotor contact force. Negative Stiffness is a phenomenon, whereby FT decreases as the local incursion increase.

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Fig. 3

Definition of seal compression

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Fig. 7

Flow direction changes at the leaf edges (adapted from Ref. [16])

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Fig. 6

Pressure field mismatch between the sides of a leaf

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Fig. 5

Flow solution and interleaf pressure field (400–100 kPa)

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Fig. 8

Effect of different rear coverplates on the seal blow-down, segregated into the interleaf pressure force and inlet direction changed force

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Fig. 9

Effect of different rear coverplates on normalized seal leakage and leaf-to-rotor contact force

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Fig. 10

Slow speed stiffness test facility cross-sectional schematic

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Fig. 11

Slow speed stiffness test facility schematic showing the mode of eccentric operation

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Fig. 14

Force measured during a casing incursion cycles (prototype A, PU = 200 kPa)

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Fig. 15

Typical force versus displacement data (prototype A, PU = 200 kPa)

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Fig. 17

Torque characterization test data (prototype A)

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Fig. 18

Stiffness characterization test data (prototype A)

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Fig. 19

Photo of leaf rotor interface at various rotor positions to illustrate blow-down and rotor following: (a) incursion; (b) nominal; (c) excursion

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Fig. 12

Displacement cycles used to characterize the seal

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Fig. 13

Eccentric casing incursion cycle, measured by displacement sensor (prototype A, PU = 200 kPa)

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Fig. 20

Mass flow rate versus eccentricity (prototype A, PU = 200 kPa)

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Fig. 16

Geometric parameters used to define coverplates

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Fig. 21

Torque characterization test data (prototype B)

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Fig. 22

Force versus displacement graphs at a range of differential pressures used to obtain stiffness characterization test data (prototype B): (a) small rotor; (b) nominal rotor; (c) large rotor

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Fig. 23

Mid-range stiffness versus pressure for prototype A and B (bars show 95% confidence intervals)

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Fig. 24

Prototype B, normalized rotor torque after removal of mechanical torque contribution

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Fig. 25

Experimentally measured and predicted stiffness for prototype B

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Fig. 26

Relationship between leaf-to-rotor contact force gradient and change in seal stiffness




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