The fluid flow in gas turbine rim seals and the sealing effectiveness are influenced by the interaction of the rotor and the stator disk and by the external flow in the hot gas annulus. The resulting flow structure is fully 3-dimensional and time-dependant. The requirements to a sufficiently accurate numerical prediction for front and back cavity flows are discussed in this paper. The results of different numerical approaches are presented for an axial seal configuration. This covers a full simulation of the time-dependant flow field in a 1.5 stage experimental turbine including the main annulus and both rim cavities. This configuration is simplified in subsequent steps in order to identify a method providing the best compromise between a sufficient level of accuracy and the least computational effort. A comparison of the computed cavity pressures and the sealing effectiveness with rig test data shows the suitability of each numerical method. The numerical resolution of a large scale rotating structure that is found in the front cavity is a special focus of this study. The existence of this flow pattern was detected first by unsteady pressure measurements in test rig. It persists within a certain range of cooling air massflows and significantly affects the sealing behaviour and the cavity pressure distribution. This phenomenon is captured with an unsteady calculation using a 360 deg. computational domain. The description of the flow pattern is given together with a comparison to the measurements.

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