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TECHNICAL BRIEFS

Role of Distributed Interbristle Friction Force on Brush Seal Hysteresis

[+] Author and Article Information
H. Zhao

Department of Mechanical Engineering,  Marquette University, 1515 W. Wisconsin Ave., Milwaukee, WI 53233

R. J. Stango1

Department of Mechanical Engineering,  Marquette University, 1515 W. Wisconsin Ave., Milwaukee, WI 53233robert.stango@marquette.edu

1

Corresponding author.

J. Tribol 129(1), 199-204 (Jul 14, 2006) (6 pages) doi:10.1115/1.2401218 History: Received October 21, 2005; Revised July 14, 2006

Brush seals comprised of closely packed fine-diameter wires are an important innovation in seal technology for turbo-machinery. During service, brush seal bristles are subjected to a complex system of forces that are associated with various working loads including—but not limited to—aerodynamic forces, bristle tip∕rotor contact force, and interbristle interactions. The latter interactions are associated with contact forces that are exerted onto a bristle by adjacent fibers, as both forces and displacements are transmitted throughout the fibrous network. Such interbristle contact forces can be represented as uniformly distributed loads along the lateral surface of the fiber, or as applied discrete loads at various locations along the bristle length. In this paper, the role that uniformly distributed interbristle friction force plays in brush seal hysteresis is examined and reported. The origin of this frictional load is attributed to conjugate interbristle shear forces that arise due to compaction and aggregate displacement of the bristle pack during service. This, in turn, gives rise to a uniformly distributed internal micromoment that resists bending deformation. Numerical studies are reported for a brush seal whose bristle tips are subjected to rotor induced loading that is associated with bristle∕rotor interference or eccentric rotation of the shaft. In order to extend the range of applicability of numerical solutions, results are reported in terms of nondimensional brush seal design parameters. Results indicated that interbristle friction force can give rise to a delayed filament displacement as well as an incomplete bending recovery of bristles. The latter phenomenon can inevitably result in hysteretic “gapping,” i.e., the formation of an annular or crescent space between the rotor and bristle tips, thereby increasing vulnerability of the seal to leakage.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Downstream view of brush seal depicting interference parameter Δo* and inward radial flow-induced load q0, along with contact force Fres generated at interface of fiber tip and rotor; and (b) section view of annular front and back plate that constrain bristle pack. Arrows (inset) depict complex fluid flow through the bristle pack.

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Figure 2

(a) Depiction of partial brush seal with front and back plate that constrain bristle pack; and (b) section A-A view, depicting the compactive load gc around the bristle pack. Three fibers (1, 2, 3) are taken as an example from this bristle pack and examined for hysteresis phenomenon.

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Figure 3

(a) Three undeformed neighboring fibers subjected to the compactive load gc at an arbitrary cross section P-P; and (b) deformation of fibers under compactive load gc. The inset shows the relative movement of neighboring fibers and the corresponding friction force dfA‐A′ and dfB′‐B.

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Figure 4

(a) Segment of the deformed fiber subjected to the uniform compactive load gc and traction force fA‐A′ and fB′‐B on the top and bottom surface, respectively; inset depicts the system of forces acting on an arbitrary differential element; and (b) simplified model of interaction between neighboring bristles as uniformly distributed moment m* along the deformed fiber

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Figure 5

Inherent eccentricity of shaft during operation

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Figure 6

Relationship between dimensionless contact force at bristle tip and dimensionless penetration depth during loading and unloading Δ* for a transition seal with Rs∕H*=8.9, θ=45deg, and m*H*2∕EI=0.135. The span Δg∕H* indicates the (nondimensional) position where fibers remain, i.e., incomplete recovery of the bristle pack occurs during unloading Δ*.

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Figure 7

(a) Relationship between dimensionless contact force and dimensionless penetration depth for m¯=m*H*2∕EI=0, 0.045, 0.090, and 0.135 (results shown are for Rs∕H*=8.9, θ=45deg); and (b) relationship between Δg∕H* and nondimensional bending moment m*H*2∕EI

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Figure 8

(a) Relationship between dimensionless contact force and dimensionless penetration depth for θ=15deg, 30deg, and 45deg (results shown are for Rs∕H*=8.9, Δo*=0, m*H*2∕EI=0.135); and (b) relationship between Δg∕H* and bristle lay angle θ

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