Numerical Simulation of the Unsteady Flow Field in an Axial Gas Turbine Rim Seal Configuration

Abstract

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.

Flow mechanisms in axial turbine rim sealing

“…[33], Savov et al [34], Schädler et al [35], Jakoby et al [24] and Town et.al. [36] have all indicated unsteady rim seal cavity flow features which are unrelated to blade passing.…”

“…For "short seals", such as those in figure 3, the surface areas in the seal may be sufficiently small for the surface drag on the rotor and stator to be negligible in the seal flow. Counter to this argument is the observation in, for example, references [11,24] that rotating cavity, seal and annulus flows may be subject to 3D rotating flow modes that significantly affect ingestion. For more extended seals involving overlapping rotor and stator parts the rotation and surface drag may also affect the flow in the seals as, for example, suggested by Graber et al and supported by computations for a chute rim seal in reference [25].…”

“…As indicated in figure 1 and table 1, axial clearance geometries have been studied experimentally by Cao et al [11], Jakoby et al [24] and Schädler et al [35]. The seal and cavity geometries were quite different in these three cases, but all these cases were tested within turbine stages with a high swirling annulus flow.…”

Effect of Bolts on Flow and Heat Transfer in a Rotor-Stator Disc Cavity

“…In this case the throughflow Reynolds number C w = 3x10 4 , the rotational Reynolds number Re ∅ = 7.4x10 6 , and the throughflow parameter = 0.09. CFD results for this case were reported and analysed in detail in reference [10].…”

“…For example, Cao et al [5] and Jakoby et al [6] have observed large scale flow structures in disc cavities with ingestion. These rotate at a lower speed than the disc.…”