Effect of Flow-Induced Radial Load on Brush Seal/Rotor Contact Mechanics

Abstract

Brush seals comprised of special-alloy wire bristles are currently being used in lieu of traditional labyrinth seals for turbomachinery applications. This advancement in seal technology utilizes close-packed bristles that readily undergo lateral deformation arising from aerodynamic loads as well as loads imparted by the rotor surface. Thus, during operation, filament tips remain in contact with the rotor surface, which, in turn, inhibits leakage between successive stages of the turbine, and increases engine efficiency. However, contact forces generated at the interface of the rotor and fiber tips can lead to eventual bristle fatigue and wear of the seal/rotor system. Therefore, it is important that reliable modeling techniques be developed that can help identify complex relationships among brush seal design parameters, in-service loads, and contact forces that arise during the operation of turbomachinery. This paper is concerned with modeling and evaluating bristle deformation, bending stress, and bristle/rotor contact forces that are generated at the interface of the fiber and rotor surface due to radial fluid flow, and augments previous work reported by the author’s, which assessed filament tip forces that arise solely due to interference between the bristle/rotor. The current problem derives its importance from aerodynamic forces that are termed “blow-down,” that is, the inward radial flow of gas in close proximity to the face of the seal. Thus, bristle deformation, bristle tip reaction force, and bristle bending stress is computed on the basis of an in-plane, large-displacement mechanics analysis of a cantilever beam that is subjected to a uniform radial load. Solutions to the problem are obtained for which the filament tip is constrained to lie on the rotor surface, and includes the effect of Coulombic friction at the interface of the fiber tip and rotor. Contact forces are obtained for a range of brush seal design parameters including fiber lay angle, flexural rigidity, and length. In addition, the governing equation is cast in non-dimensional form, which extends the range of applicability of solutions to brush seals having a more general geometry and material composition.