Development and application of a fully coupled eulerian thin film/discrete phase approach to a simplified aeroengine bearing chamber

Nicoli, Andrew (2020) Development and application of a fully coupled eulerian thin film/discrete phase approach to a simplified aeroengine bearing chamber. PhD thesis, University of Nottingham.

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Abstract

With a global drive to reduce emissions and improve fuel efficiency, aeroengine manufacturers are challenged with the significant task of developing more efficient engines. In order to achieve improved propulsion and thermal efficiencies, the desire to operate at both higher temperatures and shaft speeds requires sufficient understanding of the engine systems for design optimisation.

In aeroengines, the shafts are supported by bearings and these bearings are supplied with oil for lubrication and cooling. The bearings are housed in bearing chambers that are sealed, typically with labyrinth seals at present, creating a highly rotating two-phase environment. It is important that oil from the bearings is collected and returned to the tank to avoid unnecessary working of the oil, as this increases heat to the oil and may lead to its decomposition. Traditionally, to prevent over-heating, a surplus of oil is supplied to these rotating elements, therefore, through proper dimensioning of the oil system, a more light-weight and efficient design can be achieved.

The use of Computational Fluid Dynamics (CFD) to accurately model components of the aeroengine is seen as key to improving efficiencies and optimising design. Previous work at the Gas Turbine and Transmissions Research Centre (G2TRC) has highlighted the need for a sophisticated computational model including the capability to appropriately capture the oil shedding behaviour from bearings. Oil can breakup, generating droplets and ligaments, subsequently forming thin and thick films driven by both gravity and shear. At the G2TRC, experimental investigations into the oil shedding behaviour from a ball bearing within a simplified aeroengine bearing chamber have been previously published. This work has generated flow field images obtained using a high-speed camera configuration and derived film thickness data is presented. The experimental data provides vital validation resources for this current research.

To date, for aeroengine bearing chamber applications, there is still considerable uncertainty regarding the best practice computational modelling approach and, more specifically, the selection of a suitable turbulence model. Therefore, an objective of the work is to provide closure to this problem and recommendations are made for a turbulence model that is applicable to aeroengine bearing chamber simulations. However, to begin with, the capability to accurately model fluid flow within a rotating Taylor-Couette systems is seen as a necessary first step for informing the subsequent bearing chamber computational investigations carried out here.

Within recent aeroengine bearing chamber investigations, one of the areas yet to be properly explored is the behaviour of oil shed from the bearing into the bearing chamber; moreover, up to now, minimal progress has been made in order to quantify these droplets and the disintegration process. Historically, attempts to model this phenomenon has been demonstrated utilising a Eulerian Thin Film Model (ETFM) coupled with a Discrete Phase Model (DPM). However, these prior computational modelling approaches have been limited by the state-of-the-art capability and many challenges still need to be overcome. As such, within the scope of this thesis, the requirement for a more appropriate computational methodology, has led to the development of sprayParcelFilmFoam, a fully coupled two-way ETFM-DPM solver within OpenFOAM. In addition, two new thin film sub-models are included, based on empirical models for both a film stripping and an edge separation criteria. The newly developed sprayParcelFilmFoam solver is validated against an experimental study for a liquid jet injected into a high velocity crossflow. The fidelity of both the solver and the novel film sub-models are evaluated against both experimental film thickness measurements as well as particle diameters and velocities. Compared to the existing state-of-the-art techniques available, the results achieved within sprayParcelFilmFoam demonstrate a significant improvement and, overall, represents an advance in bearing chamber modelling capability.

Subsequently, sprayParcelFilmFoam is applied to a simplified aeroengine bearing chamber and has, for the first time, demonstrated a fully coupled two-way ETFM-DPM investigation into the droplet generation process. Numerical investigations are conducted for three different shaft speeds namely 5,000, 7,000 and 12,000 rpm, at two different oil flow rates: 5.2 and 7.3 l/min. CFD results are validated against the G2TRC experimental data for the two lower shaft speeds. Evaluation of computed mean film thickness results shows excellent agreement with the experimental measurements. Overall, there is a diminishing reduction in film thickness with each increase in shaft speed. Good qualitative agreement of the film development is found, whereby, upstream of the separation edge a fully shear driven flow regime is observed. However, immediately before the separation edge, a gravity dominated flow regime persists, even at 12,000 rpm. A comprehensive investigation into the oil disintegration process is performed. A detailed analysis of droplet statistics is presented, including: particle trajectories, diameters, velocities and residence times. For these droplets, an understanding of solution sensitivity to both operational parameters and initial conditions is provided. Predominately, the most notable quantities that influence the droplet shedding behaviour are the shaft speed and angular location.

The work presented in this thesis demonstrates a significant improvement in bearing chamber modelling capability through the use of the developed coupled ETFM-DPM approach. The results obtained within the simplified bearing chamber have provided a greater understanding of the underlying mechanisms and new flow physics. Overall, sprayParcelFilmFoam, will support future research activities, helping to improve bearing chamber design optimisation and, ultimately, better inform the heat and oil management process based on required functionality.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Johnson, Kathy
Jefferson-Loveday, Richard
Keywords: Aeroengine, Bearing chamber, Computational fluid dynamics, Turbulence modelling, Multiphase flow, ETFM, DPM, OpenFOAM
Subjects: T Technology > TA Engineering (General). Civil engineering (General) > TA 357 Fluid mechanics
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 61498
Depositing User: Nicoli, Andrew
Date Deposited: 07 Jan 2021 10:20
Last Modified: 07 Jan 2021 10:20
URI: http://eprints.nottingham.ac.uk/id/eprint/61498

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