Fluid-Dynamics of Turbine Rim Seal Structures: A Physical Interpretation Using URANS

Unsteady Reynolds-Averaged Navier-Stokes modelling (URANS) is a valuable and cost-effective tool for Computational Fluid Dynamics (CFD), including the investigation of mainstream-cavity interaction in turbines. Despite the gap in accuracy with higher order CFD methodologies, URANS is among the few simulation strategies of industrial interest suitable for predicting ingress/egress over a wide range of conditions.

This paper presents a numerical study of the flow-field in the upstream double-radial seal of a 1.5 stage turbine. Various configurations are tested, including non-purged and purged conditions. Rigour of the approach is ensured by a set of sensitivity analyses, allowing the delineation of a best practice on the use of URANS in rim seal simulations: this includes an assessment of the effects of sector size, cavity domain size and blade count.

Time-averaged and time-resolved flow predictions capture coherent structures in the rim gap. An association between the 3D morphology of these structures and different ingress/egress mechanisms is proposed. Regions of enhanced radial activity are identified to correspond with the blade leading edges. A frequency analysis of unsteady pressure signals probed in the rim gap leads to a calculation of the structure number and speed.

The structures are synchronous with the disc rotation for non-purged cases but rotate at slower speed when purge is introduced. The relative number of blades and vanes directly influences the structure count and velocity. The configuration with no blades is characterised by the slowest structures. The calculations have been conducted at three different flow coefficient for the annulus flow. There is a reduction in radial activity and structure speed at lower flow coefficient, fundamentally related to the reduced pressure asymmetry and gradient of swirl across the rim seal.