A numerical study of brush seal leakage flow

Abstract

With their increasing use in gas turbine engines, brush seals have drawn the recent attention of researchers. Among the challenging problems, perhaps leakage analysis is of primary importance in investigating sealing performance. This work presents a 2-D laminar flow analysis. Using a commercially available finite element analysis software FIDAPTM, the model predicts flow rate versus pressure drop for a given flow region. The brush seal flow domain is divided into representative smaller cells in which simulation takes place. The cells are chosen such that together they can represent whole seal. At the beginning, the model consists of a single cell of five bristles, one in the middle and four at each corner of a square. Then, other cells are added in order to extrapolate the results for a full seal. In parallel to the numerical work a simple analytical model has also been developed. This simplified solution of the 2D laminar Navier-Stokes equation predicts pressure drop across two bristles. Results from the numerical work are also compared to this analytical solution. They also show good agreement with experimental static leakage test data collected on a high-speed test rig. Currently at GE Corporate Research and Development Center Professor of Mechanical Engineering INTRODUCTION The brush seal consists of a set of fine diameter metallic or ceramic fibers densely packed between retaining and backing plates. As illustrated in Figure 1, the backing plate is positioned downstream of the bristles to provide mechanical support for the differential pressure loads. The circular seal is installed in a static member with bristles touching the rotor with an angle in the direction of the rotor rotation. In the case of rotor excursions, this cant angle helps reduce the contact loads allowing bristles to bend rather than buckle. The inherent flexibility enables the seal to survive large rotor excursions without sustaining any appreciable permanent damage. With thousands of flexible bristles moving around under various loads, modeling the leakage through bristles is a real challenge. There are many difficult issues that need to be addressed in a realistic leakage model. To name a few: • compliance (individual bristles move around) • hysteresis and pressure closure • hydrodynamic lift and bristle flutter • flow in axial, radial and tangential (swirl) directions • change in bristle density with changing interference and pressure closure • change in tip clearance with pressure closure and wear. American Institute of Aeronautics and Astronautics 1 R E T A I N I N G PLATE-