3-D analysis of high density brush stiffness with friction-pressure coupling

Achieving efficient sealing around high speed rotating bodies poses real engineering challenges. In recent years, dense brush structures found common use in turbomachinery sealing applications. As they maintain their flexibility at elevated temperatures, which are typical in gas turbines, high density brush seals made of super-alloy bristles found popularity among engine designers. 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. Typically brush-rotor contact occurs at very high surface speeds. If not managed properly, high contact loads may result in extreme wear and damage to rotor. In order to ensure engine operational safety brush seal stiffness should be controlled through seal design and detail analysis. In addition to the physical complexity of these dense brush structures, frictional contacts among the bristles themselves, between the bristles and the support plates, and between the bristles tips and the high speed rotor further increase the analysis complexity, and make it a major undertaking if not impossible. The complicated nature of bristle behavior under various combinations of pressure load and rotor interference requires computer analysis to study details that may not be available through analytical formulations. This work presents a 3-D computational brush seal structural FE model that can be used to calculate bristle forces. The analysis includes a representative brush segment with bristles formed by 3-D beam elements. Bristle interlocking and frictional interactions (interbristle, bristle-backing plate and bristle-rotor) are included to better simulate pressure-stiffness coupling. 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.