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Title: Air-film behaviour in an air-riding face seal
Author: Ibrahim, Amir
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Increased cycle efficiency and reduced ownership costs of gas turbine engines can be facilitated by improvements in sealing between stationary and rotating engine components. Contacting seals have already improved performance but advanced non-contacting seals offer the possibility of high performance with near-infinite service life. This thesis investigates one potential style of non-contacting face seal which uses a thin film of air to maintain clearance between engine components at all engine operating conditions. Unlike conventional face seals, the seal must be capable of operating with moderate clearances in the order of tens to hundreds of microns and be deployable over large axial movements of several millimetres if it is to operate successfully in the jet engine environment where components are light, flexible and subject to extreme thermal growths. The level of leakage for this seal is intended to be similar to a brush seal at its end of life condition. The thesis reports experimental analytical and numerical studies of the behaviour of the air-film in the primary sealing gap over a wide range of engine representative conditions. The effect of rotor distortions causing converging and diverging coning are characterised. Experimental pressure distributions are compared to 2D modelling tools based on Reynolds flow equations, used widely to design conventional face seals. These were found to be largely inadequate. High fidelity 3D Commercial CFD however, provided highly accurate predictions of the seal face behaviour, particularly if coupled to mechanical prediction of the distortion of sealing faces under pressure loads. Detailed measurements and modelling of diverging gaps experienced during thermal transients are reported for the first time. On the basis of these results, a large axial movement prototype face seal was designed and tested. The seal was shown to be capable of running successfully without the addition of a thin, vulnerable to damage, front face typical of gas thrust bearings. A design strategy where passive hydrostatic balancing without the use of biasing springs was seen to be generally successful, although design for tolerance to circumferential distortions was shown to be vital, as optimisation for leakage with a flat face made the seal liable to touch-down under extreme axial offset conditions. Steady state RANS 3D CFD model provided sufficient resolution to model the seal behaviour showing excellent agreement with experimental testing.
Supervisor: Gillespie, David Sponsor: European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available