Flexible Seal Strip Design for Advanced Labyrinth Seals in Turbines

Abstract

The paper discusses the potential benefit of flexible seal strips in labyrinth seals for turbines. By reducing the radial stiffness compared to a standard straight and stiff knife, seal clearance could be reduced without significantly reducing the seal durability and long-term performance. As contact between the seal strips and the rotor can occur especially during transient operating phases, a more flexible design of the seal strips can prevent damage and wear, keeping the discharge rates constantly low. However, the pressure difference across the fin will cause a deflection of the seal strip due to the increased flexibility and thus creating an additional possible risk for an unwanted contact. Pressure balanced designs and supports on the low pressure side are used on the investigated seal designs to eliminate that risk. To give evidence of possible performance gain a standard labyrinth seal configuration is compared to two configurations with segmented and curved seal strips. In a first step, the discharge coefficient and the leakage rates for the nominal seal design are calculated using two-dimensional CFD. In order to investigate the impact of a worn seal tip on the leakage flow, the geometry change due to a rubbing event is simulated with FEA tools. Therefore, a specific high-speed wear model is implemented and calibrated by experimental data, enabling the correct cooling effects and plastic deformation. The discharge coefficient and the leakage mass flow rates of the worn geometry are then again modeled with CFD for the various seal configurations and compared to the unworn state. The study shows that a wise combination of the advantages of flexible curved seal strips can be used to reduce the leakage rates significantly, improving the life time of seal elements at the same time.