Role of Distributed Interbristle Friction Force on Brush Seal Hysteresis

Abstract

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.