Experimental and computational investigation into oil shedding from an aeroengine ball bearing

Hee, Jee Loong (2020) Experimental and computational investigation into oil shedding from an aeroengine ball bearing. PhD thesis, University of Nottingham.

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Aero-engines contain ball bearings that support the shafts and react to the loads. During the investigation of improvements and the pursuit for higher bypass ratio, propulsion and thermal efficiency, regimes are explored where an aero-engine’s core is subjected to higher mechanical and thermal loadings as it is required to operate at a higher temperature, pressure and speed. There is thus a need for continual development in aero-engine oil systems in order to meet the demands for optimised cooling and lubrication ie good cooling performance without increasing engine weight. Using computational fluid dynamics (CFD) to model aeroengine transmissions elements and systems is an attractive research approach but a pre-requisite for an accurate simulation lies within the ‘exactness’ of the boundary conditions. The research presented in this thesis is dedicated to understanding the oil shedding process of an aero-engine ball bearing operating at moderate speeds and to acquire the boundary conditions for CFD simulations. This thesis further presents a computational investigation that develops bearing chamber modelling techniques.

In the past, studies on rotating elements in aero-engines have largely been based on empiricism. Roller bearings have been experimentally investigated but this is not directly applicable to ball bearings because of key differences including: lubrication method, geometry, dynamic effects and axial loading. Prior data is limited for aero-engine ball bearings particularly with regard to how the oil sheds and its interaction with the air in the surrounding bearing chamber.

A test rig was built at the Gas Turbine Transmission Research Centre (G2TRC) to understand the bearing oil shedding process. It consists of an aero-engine ball bearing fitted onto a direct-drive electric motor and contained within two chambers. Oil is fed through under-race lubrication and the churning of oil from the mechanically rotating ball bearings results in shedding at the orbiting cage in the forms of droplets, ligaments and oil sheets. The oil impinges onto the outer periphery of the chamber wall, merging with the axially exiting oil from bearing gap and forms a layer of thin continuous film driven around by interfacial shear stress and gravity (at low speed). High speed imaging was used during the experimental investigation.

A new optical image processing technique was developed as part of this research and used to determine the oil film thickness at various angular positions of the bearing containment. Application was to the co-current and counter-current sides of the chamber wall where gravity acts with and opposing the interfacial shear stress respectively. The technique employed the use of the temporal-averaging scheme known as ‘median-stacking’ along with gradient detection and spatial calibration to determine the film thickness. High-speed imaging was also used in the assessment of modes of droplet disintegration from the orbiting cage and characterisations of the structure of the wave on the surfaces close to the bearing. An inversely proportional relation between oil film thickness and shaft rotational speed was established, with no significant effect of change of oil flow rate observed. Axial load was found to be axial-plane specific in affecting the oil film thickness. In particular at a plane very close to the ball bearing the film thickness was found to be proportional to the load. The effect is attributed to changes in dynamic behaviour of the ball bearing. A correlation for ligament number was also developed for aero-engine operating conditions and geometry for the first time. The ligament number was comparable with those from classical studies involving rotary disks and cups.

Image analysis revealed the existence of multiscale wave structures on the surface of the film. At low shaft speed, the waves exhibit gravity-capillary like structures with distinctive in-phase wave- fronts, long wavelength and low frequency. In contrast at higher rotational speed, small scale wave structures were observed known as capillary waves which contain steeper wave-fronts, shorter wavelength and higher frequency. These structures are of primary importance in the momentum transfer between air and oil, and are reported as difficult to model during published prior numerical studies. Further modelling challenges were highlighted by observed droplet impact outcomes which included splashing and jetting.

A computational modelling strategy was developed and validated against a published study. This strategy centres on the application of coupled Volume-of-Fluid, Discrete Phase Model and Eulerian Thin Film Model (VOF-DPM-ETFM) where different parts of the flow are modelled based on the cost and accuracy suitable for the regime. In particular, the uniform thickness shear- driven film at high rotational speed is a suitable candidate of ETFM with droplets tracked using DPM. An efficient computational guideline was established in the assessment of the fidelity and numerical stability of coupled DPM-ETFM to ensure that the model works as intended. This is based on the simulation of jet-breakup and thin film formation in cross-flow that is transferable to bearing chamber modelling. The studies indicated that the developed approach is capable of modelling the discrete-phase and thin film to a certain level of accuracy but instability remains when specialised terms for gas-film momentum, edge separation and surface tension are activated. It was found that the thin film model with inclusion of stripping criteria, yields the best result compared with the published experimental study at 28.9% difference.

The experimental research presented in this thesis represents a significant step forward in understanding how oil sheds from a bearing and what the influencing operational factors are. The computational study has made an important contribution to the development of an industrially relevant bearing chamber modelling framework.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Johnson, Kathy
Hann, David
Kakimpa, Bruce
Keywords: aero-engine; bearing chamber; ball bearing; computational fluid dynamics; experimental fluid mechanics; film thickness; optical technique; thin film; image analysis; two-phase flow; oil shedding
Subjects: T Technology > TJ Mechanical engineering and machinery
T Technology > TL Motor vehicles. Aeronautics. Astronautics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 60067
Depositing User: Hee, Jee
Date Deposited: 31 Jan 2023 08:36
Last Modified: 31 Jan 2023 08:37
URI: https://eprints.nottingham.ac.uk/id/eprint/60067

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