An investigation of pressure stiffness coupling in brush seals

In recent years, brush seals found common use in turbomachinery applications. There are a number of seal locations on gas turbines that have significant performance derivatives. These include the compressor discharge, bearing seals, turbine interstage packings, and the bucket tips. While brush seal applications keep expanding towards more challenging locations, the need for better understanding of seal dynamic behavior also increases. Inherent flexibility of brush seals allows fibers to compact under pressure load. Due to the frictional interaction between the fibers and the backing plate as well as within the fibers themselves, brush seals are known to exhibit pressure stiffening and hysteresis behavior. While hysteresis affects seal performance after a rotor excursion, pressure stiffening is critical in determining heat generation and seal wear during hard rubs. It is necessary to understand the physical behavior of a brush seal under the operating conditions, and to be capable of quantifying seal life and performance as functions of both operating parameters and seal design parameters. In this paper, a 3-D finite element model is used in order to explore pressure-stiffness coupling behavior. The analysis includes all the frictional effects to better calculate resulting seal stiffness and tip forces. The results indicate that rotor interference has some effect on seal tip forces in the absence of any pressure loading. However, upon application of small pressure loads, seal stiffness is generally dominated by pressure-stiffness coupling. Results also indicate presence of hysteresis when rotor excursion is removed under pressure load.