Modelling the wheel-space, rim seal and mainstream gas path interaction mechanisms

Gas turbine designers use end-wall contouring (EWC) to influence the static pressure field between vanes and rotating blades. Guiding the secondary end-wall flow and reducing the associated losses improves aerodynamic performance. This secondary flow field is affected by the egress of purge flow through rim-seals at the hub end-wall. And the ingress of hot gas is influenced by the EWC.

The interaction between the egress and mainstream, and its propagation through the downstream rotor, is complex and unsteady. Industry is moving towards a combined design of the rim-seal geometry, seal-clearance profile and mainstream end-wall contours. This is principally through the use of computational fluid dynamics (CFD).

Our project focuses on designing and building a rotating-disc rig. This will experimentally model the upstream wheel-space, rim seals and mainstream gas path of the first stage of a gas turbine. We have designed our test facility for optical access. It features interchangeable EWC profiles and rim-seal geometries on the stator and rotor.

Innovative experimental techniques

We will make the following measurements above the rim seal and through the blade passage in the annulus:

• Carbon dioxide planar laser-induced fluorescence (CO2-PLIF)
• Three-component volumetric velocimetry (V3V).

Using the CO2-PLIF measurements, we will be able to visualise the trajectory of rim-seal egress and its interaction with the main-annulus flow.

The V3V measurements will provide three-component velocities through the blade passage. This will allow us to observe secondary-flow structures. it is a world first to use both these techniques in gas turbine experiments. The data and insight we gain will provide unprecedented data for engine designers to validate CFD models.

We will also use the rig to measure the following in the rotor-stator wheel space cavity:

• The distribution of pressure and concentration with radius on the stator disc
• The variation of sealing effectiveness over a range of purge flow rates

We will combine all the CO2-PLIF, V3V, pressure and concentration measurements. This will allow us to address the impact of the rim-seal and EWC designs on:

• The purge-mainstream flow interaction
• The resulting effects on ingress to the wheel-space
• The formation of aerodynamically parasitic secondary flow structures in the blade passage

We will use CFD to design optimised rim-seals and EWC geometries that reduce ingress into the wheel-space. They will also maximise the efficiency of the turbine stage through reduction in secondary flow losses.