Negative Stiffness in Gas Turbine Leaf Seals

The stiffness of contacting shaft seals such as brush seals and leaf seals is a required characteristic to accurately predict their performance and life in the gas turbine engine. This paper describes the results of a test campaign in which a series of eccentric rotor excursions are applied at low rotational speed and engine representative pressure differences to characterise the behaviour of a prototype leaf seal. A phenomenon that may best be described as negative seal stiffness is reported. Here, the displacement of the seal rotor to an eccentric position causes a resultant force, which, rather than trying to return the rotor to a central position, acts to amplify its displacement. These data were used to develop an empirical model of the seal behaviour. It was possible to model the negative stiffness phenomenon and show that it is caused by a combination of two effects: the inherent mechanical stiffness of the leaf pack, and the aerodynamic stiffness of the seal. The latter is caused by the pressure distribution and changes in the flow field through the leaf pack as a result of the displacement of the rotor.