Improved Understanding of Negative Stiffness in Filament Seals

Leaf seals have previously been proposed as an improved filament seal for gas turbine engines. Recently, a phenomenon known as negative stiffness has been reported from experimental testing. Good understanding of this phenomenon is required to ensure stable interaction between the seal and the rotor. In negative stiffness the displacement of the seal or rotor into an eccentric position causes a resultant force, which, rather than restoring the rotor to a central position, acts to amplify its displacement. The seal consists of a pack of thin planar leaves arranged around the rotor, with coverplates on either side of the leaf pack, offset from its surface. It is notable that negative stiffness only occurs when certain geometric configurations of the coverplates are employed. This paper gives insight into the fluid phenomena that contribute to the negative stiffness effect through the creation of a general 2-D model of the flow upstream of the leaf pack and between the leaves. These show that there is the capacity for the inertia force to be a significant contributor to the overall force acting on individual leaves depending on the coverplate configuration surrounding the leaf pack. The influence of a key parameter, coverplate height, is explored. Results from a test campaign with varying seal geometry are compared to the forces predicted by modeling to justify the proposed mechanisms for negative stiffness. The close agreement between the experimental and predicted data extends the previously published insight on negative stiffness to allow more general considerations for leaf seal design to be inferred.