Hydrodynamic Air-Riding in Leaf Seals

Leaf seals are filament seals for use at static to rotating interfaces in rotating machinery. They are capable of withstanding significant pressure differences while minimising leakage. One of their advantages over comparable filament seals is the ability of the leaves to generate significant hydrodynamic lift at their tips. If this force is sufficient to lift the leaf tip away from the rotor, leaf wear is eliminated and an infinite life seal is created. In order to design seals that are capable of operating in this mode, a good understanding of the hydrodynamic effect and how it interacts with the seal is required. This paper presents a detailed theoretical and experimental investigation into hydrodynamic air-riding in leaf seals. First the hydrodynamic lift is investigated by analysing the flow field and forces generated between a static structure resembling the leaf tip geometry and a moving surface resembling the rotor. This allows the fundamental effects behind air-riding to be identified and quantified. Next a coupled model is presented, which captures the interactions between the lift force and the leaf tip movements. This gives a full picture of the steady-state fluid-structure interactions controlling air-riding in leaf seals. Based on these results several guidelines for obtaining air-riding are extracted. Finally the predictions from the coupled model are compared to results from a high speed test campaign using a prototype leaf seal. Good agreement is found, confirming the presence of hydrodynamic air-riding in leaf seals and demonstrating the accuracy of the presented coupled model.