The Effects of a Parametric Variation of the Rim Seal Geometry on the Interaction Between Hub Leakage and Mainstream Flows in HP Turbines

Abstract

The objective of this work was to assess and understand the effects of parametric variation performed on a typical overlapping rim seal geometry. The datum geometry has been focus of a detailed experimental investigation employing a large-scale linear cascade subjected to a range of the mass flow rates and swirl velocities of the leakage air. The parametric variations described in this paper were examined using validated CFD. As a part of the parametric studies, both the axial and the radial seal clearance between the rotor fin (angel wing) and stator platform were varied as well as the length of the overlap between stator and rotor platforms. In addition, the effects of forward and backward facing annulus steps were also investigated. It has been found that a backward facing annulus step was detrimental for all conditions considered, while a forward facing step offered improvements for smaller step heights and/or lower leakage fractions. Tightening of the seal clearances closer to the annulus line improved the sealing effectiveness, but often at the expense of increased losses. On the other hand, increasing the overlap length led to improvements in the sealing effectiveness with very small effects on the overall losses. Moving the rim seal away from the blade leading edges reduced the pressure asymmetry at the rim seal and increased the flow uniformity of the leakage air. However, this led to an increased cross-passage flow (more negative skew) and higher losses at all but lowest leakage fractions. The results presented in this paper highlight the fact that there may not be an optimum rim seal solution that would offer an improvement for the full range of leakage fractions and that for different rim sealing flows there may be a different optimum geometry. In addition, rotor disc movements in radial and axial directions at various off-design conditions should be considered as a part of the design process. Based on the presented results it may be of a benefit to the turbine designer to consider rotor disc designs which would be more biased towards the upstream and outward disc movements that would result in tightening of the seal clearances and avoidance of a backward facing annulus step.