Experimental Investigation and Mathematical Modelling of Clearance Brush Seals

In this paper an experimental programme and a CFD based mathematical model using a brush seal at two bristle to rotor clearances (0,27 mm. and 0,75 mm.), are presented. The experimental programme examined the radial pressure distributions along the backing ring, the axial pressure distribution along the rotor, and the mass flow through the seal, through a range of pressure ratios while exhausting to atmosphere. The results from this experimental programme have been used to further calibrate a CFD based model. This model treats the bristle pack as an axisymmetric, anisotropic porous region, and is calibrated by the definition of non-linear resistance coefficients in three orthogonal directions. The CFD analysis calculates the aerodynamic forces on the bristles, which are subsequently used in a separate program to estimate the bristle movements, stresses and bristle and rotor loads. The analysis shows that a brush seal with a build clearance produces a very different flow field within the bristle pack to one with an interference, and the need to understand the bulk movements of the bristles. These are shown to be affected by the level of friction between the bristles and the backing ring, which has an important effect on the bristles wear and seal leakage characteristics.