Experimental Modelling of Gas Turbine Rim Seals

Abstract

The gas turbine engine has been used in a variety of applications ranging from electric power generation to aerospace and marine propulsion. The strict environmental regulations and increasing fuel prices have encouraged the engine manufacturers to design sophisticated engines with high efficiencies. Increasing the turbine entrance temperature (TET) is a common practice to reduce the specific fuel consumption of a gas turbine and therefore temperatures above the melting point of materials may be reached. Bleeding air directly from the compressor stage can be used to protect the components from overheating and fouling, however, over purging will reduce significantly the efficiency and performance of the engine.

One important problem that gas turbine designers face is the ingestion of hot gases into the cavities formed between the stationary and rotating discs. Rim seals are fitted at the periphery of the discs to reduce the required amount of purge. Ingress is principally caused by the swirled mainstream flow and its interaction with the rotating blades and egress. This thesis aims to study ingress from the simplest possible configuration and gradually increase the complexity to reach engine scaled conditions. Additionally, this research remarks the importance of the external annulus swirl to the ingestion problem which has not been properly acknowledged yet in the literature.

Experimental measurements of ingress in the wheel-spaces upstream and downstream of a rotor disc is presented using an axial 1.5-stage turbine rig. Theoretical and computational results accompany some of the experimental findings in order to explain the physics of ingress. A series of generic, however, engine representative seals is tested to assess their performance and reduction of required purge. Gas concentration measurements of CO2 were used to quantify the amount of ingress in the cavity and derive sealing effectiveness. The flow structure in the cavity was revealed by measuring the time average pressure and swirl ratio. Monitoring the unsteady pressure in the rim seal cavity allowed to study the rotating large-scale structures. The flow field in the annulus was interrogated by a five hole-probe and the annulus swirl ratio was derived.

Ingress under rotationally induced conditions was studied for a range of seals using a rotor disc without blades and a stator platform without vanes. The distribution of effectiveness and pressure in the cavity was collected experimentally for many conditions and was validated with CFD data which demonstrated excellent agreement and revealed rotating regions of ingress and egress; furthermore, a predictive cavity model was capable to capture the pressure distribution.

The importance of the annulus swirl ratio to ingestion was demonstrated under off-design conditions. Ingress was measured in the downstream cavity using three rotors, one without blades and two with blades but with different degrees of reaction; that way the flow coefficient was decoupled from the annulus swirl ratio. The variation of effectiveness was plotted for the first time against the swirl ratio and demonstrated similar levels of ingress regardless of the blade profile. A theoretical model based on turbulent transport was successfully used to fit a curve to the experimental data capturing the characteristics.

The influence of flow coefficient on ingress was studied in the upstream and downstream cavities using a rotor with and without blades, and a stator platform with and without vanes respectively. The interaction of the mainstream flow with the blades changed the swirl ratio in the annulus leading to increased ingress; this was especially noticeable over a narrow range of flow coefficients corresponding to the leading edge of the blade. Similarly, the downstream vanes alter the swirl in annulus and increased ingress with the effect to be reflected in the swirl ratio of the rotating core. Measurements of the unsteady pressure in the upstream cavity revealed rotating large-scale structures which were linked to ingress. The structures were present for both rotors, but their effect was intensified by the rotor blades.

Date of Award	22 Feb 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	James Scobie (Supervisor), Nicola Bailey (Supervisor) & Gary Lock (Supervisor)