An experimental study of ingress through gas-turbine rim seals

The gas turbine has been widely used for mechanical drive, electric power generation and jet propulsion. Challenged by strict CO2 emission regulations and competition between manufacturers, designers push the boundaries for an ever more efficient gas turbine. The cycle efficiency of the gas turbine is a crucial parameter that drives the engine performance and fuel burn. Improved cycle efficiency can be achieved by raising the turbine entry temperature, as well as the pressure ratio across the compressor. Today’s gas turbine engines use bypass cooling air from the high-pressure compressor, to cool down the seal disc cavities and extend the life cycle of critical components in the stage. The cooling air (secondary system) can account for 25% of the total bypass air, approximately 5% of the total bypass air is used to seal the disc cavities. To further enhance the benefits of the sealing flow (cooling air), rim seals are fitted at the periphery of both, stationary and rotating discs. Rim seals help to further reduce the ingestion of hot mainstream gases (combustion chamber gases) that can cause damage to the discs and blade roots. The cause of hot gas ingestion is principally due to the circumferential pressure asymmetry in the mainstream flow, furthermore, the mixing between the sealing flow and mainstream gas, results in a deterioration of aerodynamic performance. The first part of this thesis concerns experiments using a 1.5-stage turbine rig, from which the flow physics associated with ingestion could be studied. This thesis presents experimental results using the turbine test rig with wheel-spaces, upstream and downstream of a rotor disc. Ingress and egress were quantified using a CO2 concentration probe. The probe measurements have identified an outer region in the wheel-space and showed a flow structure consistent with Batchelor-type flow. This is the first time asymmetric variations of concentration have been shown to penetrate through the seal clearance and the outer portion of the wheel-space. For a given flow coefficient in the annulus, the concentration profiles were invariant with rotational Reynolds number. The measurements reveal that egress flow provides a film-cooling benefit, on the vane and rotor platforms. The second part of this thesis investigates the effects of ingress through a double radial rim seal. The effect of the vanes and blades on ingress was investigated by a series of carefully controlled experiments: firstly, the position of the vane relative to the rim seal was varied; secondly, the effect of the rotor blades was isolated using a disc with and without blades. Measurements of steady pressure in the annulus show a strong influence of the vane position. The relationship between sealing effectiveness and purge flow rate, exhibited a pronounced inflexion for intermediate levels of purge; an inflexion did not occur for the experiments with a bladeless rotor. Shifting the vane closer to the rim seal, and therefore the blade, caused a local increase in ingress, at the inflexion region; again, this effect was not observed in the bladeless experiments. Unsteady pressure measurements revealed the existence of large-scale flow structures (flow instabilities) which depended weakly on the vane position but strongly on the sealing flow rate. Unsteady pressure was measured with and without the blades on the rotor disc. In all cases, the flow structures rotated close to the disc speed. The third part of this thesis involves, experiments and computations of flow through a gas turbine chute seal. The study investigates ingress and the phenomena of flow instabilities. The aim of this study is to investigate the steady and unsteady flow features, near the rim seal mixing plane, for a geometrically scaled stage with a chute seal. The work presented here forms part of a future partnership with the KTH Royal Institute of Technology, to study the effect of engine scaling on ingress. Experiments and computations for; pressure, swirl and sealing effectiveness, were done at an engine representative turbulent flow (lT) condition. Under this condition, an engine representative wheel-space flow structure exists. CFD results suggest that flow instabilities influence ingress, instabilities caused by a shear interaction between the egress and mainstream. This supports the hypothesis of flow instabilities being driven by shear gradients. The final part of this thesis investigates re-ingestion. A mixture of upstream egress flow and mainstream, gets re-ingested in the downstream wheel-space. Re-ingestion was experimentally measured and evaluated using a model developed by Prof. Mike Owen. To the author’s knowledge, it is the first time this methodology has been applied to quantify re-ingested fluid in downstream wheel-space. The results showed that the upstream egress does not influence the fluid dynamics, downstream of the rotor blades. The experiments were conducted at incompressible flow conditions. Probe concentration measurements demonstrated that, an interaction occurs between the re-ingested fluid and the downstream egress flow, at the rim seal mixing region. Re-ingestion was evaluated for a range of sealing flow rates. It was shown that the mass fraction of re-ingestion increases with increasing downstream seeded sealing flow rate.