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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

CFD study of surface roughness effects on the thermo-elastohydrodynamic lubrication line contact problem

Srirattayawong, Sutthinan January 2014 (has links)
This research investigates the effect of surface roughness on Thermo- Elastohydrodynamic Lubrication (TEHL) by Computational Fluid Dynamics (CFD). Traditionally, the Reynolds equation has been used to describe the flow of a lubricant for the TEHL problem, but this approach has some limitations. To overcome these, CFD is used in this research, as an alternative to solving the Reynolds equation. The commercial software packages ANSYS ICEM CFD 13.0 and ANSYS FLUENT 13.0 are employed to solve the Navier-Stokes equations. User-defined functions (UDFs) for the heat generated in the lubricant film, the density and the viscosity of lubricant, and the elastic deformation of the cylindrical roller bearing are created for this particular research. For viscosity, the lubricant is modelled as a non-Newtonian fluid based on the Ree-Eyring model. A number of CFD models are created under different conditions to predict the flow characteristics in the TEHL line contact problem, including the pressure distribution, the temperature distribution, the film thickness, and the friction coefficient. The effect of surface roughness is considered in the CFD models. The predicted results from the CFD models and the Reynolds equation are compared. The pressure distribution and the film thickness of both models are found to be in agreement. The simulation results show that the surface roughness affects significantly for the behaviour of fluid film lubrication problems, especially in the thin film case. It is found that the pressure profile at the centre of the contact area directly relates to the roughness amplitude. Furthermore, the CFD models can model the elastic deformation of cylinders of different materials, which is another advantage of the CFD approach over the Reynolds equation.

Oxygen electrochemistry on inorganic/graphene hybrid materials for energy applications

Bikkarolla, Santosh Kumar January 2015 (has links)
Developing low cost oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts that perform with high efficiency is desirable for the commercial success of energy conversion devices, such as fuel cells and metal-air batteries. In this thesis, electrochemically reduced graphene oxide (ErGO) and Mn304 nanoflakes anchored on nitrogen doped reduced graphene oxide (NrGO) sheets synthesised by electrodeposition method were developed as ORR catalysts. CUC0204 nanoparticles were integrated with NrGO sheets through solvothermal method as a potential OER catalyst. A partially reduced graphene oxide electrocatalyst synthesised by electrochemical reduction of graphene oxide exhibited significantly enhanced catalytic activity towards the ORR in alkaline solutions compared to the starting GO. The resultant ErGO electrode also showed an enhanced capacitance and an ORR onset potential similar to that of NrGO electrode, produced by a solvothermal process. However, the ErGO exhibited considerably lower electron transfer numbers, indicating that although both catalysts operate under combined 4e- and 2e- ORR processes, ErGO followed a more predominant 2e- pathway. The ORR process in ErGO has been linked to the presence of quinone functional groups, which in turn favoured the 2e- ORR pathway. Also in this work, a three dimensional Mn304 hierarchical network was grown on NrGO by a facile and controllable electrodeposition process, and its electrocatalytic performance for ORR was assessed. The directly electrodeposited MnO. on the glassy carbon electrode (GCE) exhibited little electrocatalytic activity, whereas the integrated Mn304/NrGO catalyst was more ORR active than the NrGO. The resulting electrode architecture exhibited an "apparent" 4e-oxygen reduction pathway involving a dual site reduction mechanism due to a synergetic effect between Mn304 and NrGO. In addition, the 3D Mn304/NrGO hierarchical al'chitectur~ exhibited improved durability and methanol tolerance, far exceeding that of commercial ptlC. A composite material consisting of CUC020 4 nanoparticles anchored on NrGO sheets (CuCo204/NrGO) was prepared by a solvothermal method as a highly efficient OER electrocatalyst in both alkaline and neutral solutions. The CuCo204/NrGO exhibited high OER performance when compared to the other control materials, as well as good stability under strong alkaline condition. The enhanced OER performance of CuCo204/NrGO can be related to: (i) a reduction in the size of the CUC0204 nanoparticles as measured by the TEM, (ii) an enhancement of electrochemically active surface area (ECSA), (iii) a replacement of the least OER active C02+ ions with Cu2+ ions as confirmed by XPS and (iv) a synergetic effect between CuCo204 nanoparticles and NrGO sheets.

Detection and quantification of fibre waviness in composite laminates using ultrasonic arrays

Pain, Damien January 2014 (has links)
The work presented in this thesis shows that progress has been made in the inspection of composite materials and a methodology to improve imaging and detection of subtle defects such as waviness has been established. In 2D modelling of the Point Spread Function (PSF) is used to assess the p81'formance of various imaging algorithms. It is shown that for isotropic materials the directivity function removes the side lobes. For anisotropic materials, the simulations have shown that the size of the PSF is minimised with the addition of a velocity correction. The main factors influencing the ultrasonic wave propagation in composites material have been described experimentally and corrected for in new imaging algorithms. The experiments on carbon fibre composites have shown that by applying these corrections, side-drilled-holes (SDHs) at depths of 4mm, 10mm and 16mm can be clearly imaged with the SNR improved by 7dB, 16dB and 14 dB respectively.

A novel test rig to study the effects of elastic follow-up, long range residual stress and applied load on creep crack initiation

Shirahatti, Anilkumar January 2014 (has links)
One of the many challenges in the behaviour of structures is to understand if the presence of residual stress plays an important role in contributing to failure of a structure operating at high temperature. Structural integrity assessments of components operating at high temperature require an accurate prediction of the creep crack initiation. In general, assessments are based on experiments carried out using standard laboratory scale creep test specimens tested under either displacement or load controlled conditions. In practice, structures are subjected to combinations of residual and applied stresses which in turn lead to mixed boundary conditions. Conventional laboratory creep tests do not represent these circumstances. This dissertation considers the effects of elastic follow-up , long-range residual stress and applied load on creep crack initiation of Type 316H stainless steel. Novel test rigs are designed for the purpose of investigating. The concept of rig is based on a three bar structure with an initial misfit introduced into the central bar to represent a long range residual stress and could be characterised easily without using time consuming residual stress measurement techniques. Initial results demonstrated that the magnitude and the interaction of the residual stress with the applied loading is a function of the initial misfit displacements and the relative stiffness of the components of the system. Additionally, the subsequent behaviour of the system, with and without the application of additional loading, is governed by (a) the degree to which the misfit is accommodated by plastic and creep strain and (b) the elastic follow-up provided by the system.

Design, development and experiments to investigate the effect of elastic follow-up on creep stress relaxation in austenitic steels

Wang, Yiqiang January 2015 (has links)
Elastic follow-up represents a boundary condition that lies between constant load and constant strain control (stress relaxation). This condition is known to exist in many engineering components operating at high temperature. The subject of this dissertation is the study of creep stress relaxation with and without elastic follow-up in 316 Type austenitic stainless steel at 550°C, involving the design and development of a new experimental test system based on a three bar model. The mechanical response and elastic follow-up in the three bar model is fully described in order to provide the fundamental theory for the following experimental design and data analysis. Three test rigs (Rig 1, 2 and ENGIN-X rig 3) were designed and built to conduct conventional stress relaxation tests with different values of elastic follow-up factor. The ENGIN-X rig 3 can also be used in the EN GIN-X neutron beam line with the purpose of observing the evolution of the lattice strains at different crystallographic planes under different loading conditions. The experimental results illustrate that the presence of elastic follow-up decreases the stress relaxation rate and introduces additional strain accumulation in the specimens. Our short term neutron diffraction measurements show that the intergranular stresses between different grains families remain constant during the tests irrespective of the degree of elastic follow-up, and elastic follow-up has no effect on the redistribution of lattice strains. In addition, a group of creep laboratory and an in-situ neutron diffraction anelasticity tests were conducted to study the effect of applied stress and creep history on internal stress. The creep laboratory anelasticity tests show that the internal stresses are proportional to the applied stress, and are only slightly influenced by the creep deformation. The in-situ neutron diffraction anelasticity test shows that the intergranular stresses between different grains families may contributed to anelasticity partly. A number of creep stress relaxation models were developed and based on the RCCMR creep equation and average creep rate law in order to predict the stress relaxation and elastic follow-up behaviour. The corresponding constants in the models were obtained from constant load creep tests. The predictions show that the RCC-MR strain hardening or differential strain hardening models with considering of global creep strain in specimens gave the best prediction. It was found that the consideration of internal stress in a model did not improve the prediction due to the incomplete understanding of the internal state during creep.

The y-a transformation in iron-chromium alloys

Bee, J. V. January 1974 (has links)
No description available.

Templated and activated porous carbon and carbon nitride materials for gas storage applications

Almasoudi, Afaf January 2014 (has links)
This Thesis details the synthesis, characterisation, and appl ications of micro/mesoporous carbon materials with tunable porosity prepared via template carbonisation. The main focus is the development of carbonaceous porous materials via for energy related gas storage applications. The Thesis investigates three synthesis strategies, namely; (i) nanocasting via liquid impregnation using zeolitic imidazolate framework (ZIF-8) as template, (ii) combination of liquid impregnation and chemical vapour deposition (CVD) using ZIF -8 as template and (iii) use of zeolite 13X and Y as templates for porous N-doped (carbon nitride type) materials. All the templated carbon materials were additionally activated to enhance their porosity and evaluated for gas (hydrogen and C02) storage.

Computational modelling of electron irradiation induced effects in carbon nanomaterials

Santana Sanchez, Adriano January 2014 (has links)
A novel computational approach that takes into account knock-on e-beam effects of the deformation of sample structure during imaging in high resolution transmission electron microscopy (HRTEM) is presented. The proposed approach has been implemented in the in-house software CompuTEM in which the evolution of the sample structure is described as a sequence of externally initiated discrete damage events with a frequency determined by the cross section, which depends on the energy of the electron beam. A series of images showing structure evolution with time is obtained by coupling molecular dynamics with the image simulation. These simulation parts are linked by two experimental parameters: the energy of the electron beam and the electron dose rate. CompuTEM is used to simulate the recently observed in HRTEM process of structural transformation of a graphene flake into a fullerene cage by HRTEM. The simulated series of images showing the evolution of a graphene flake under the 80 keY electron beam closely reproduces experimental HRTEM images with regard to the structure transformation route, transformation rate, and signalto- noise ratio. The structure transformation process is found to depend on the position and subsequent behaviour of the vacancy created by the electron beam during sample imaging. The stability and dynamics of a monovacancy in graphene flakes is studied by means of density functional theory and molecular dynamics techniques. The obtained results explain the mechanisms driving structural transformations in graphene.

The effect of environment on the abrasion of silicates

Crimes, Geoffrey Michael January 1973 (has links)
No description available.

Influence of acoustic cavitation on the controlled ultrasonic dispersion of nanomaterials

Sesis, Achilleas January 2014 (has links)
Processing of nanoparticles is a core research area of nanotechnology. Carbon nanotubes (CNTs) are a prototype high aspect ratio nanomaterial and have been extensively studied due to their remarkable properties and the wide range of potential applications. Ultrasonication is the most widely used technique for the dispersion of a range of nanomaterials but the underlying mechanism is poorly understood, with the role of acoustic cavitation largely ignored by the materials science community despite its critical role in the dispersion process. As a consequence, many of the dispersion strategies in the literature are empirical in nature and typically specify only the solute concentrations, the electrical input power of the device and the exposure time. Having as an aim the need to clarify, standardise and optimise these processes, this thesis presents new in sights into the dispersion mechanism of CNTs in aqueous surfactant solutions using a novel sono-reactor and an in situ technique for the measurement of acoustic cavitation activity during sono-processing. Distinction is made between stable cavitation, which leads to chemical attack on the surface of the CNTs, and inertial cavitation, which favours CNT exfoliation and length reduction. These main conclusions are supported by a range of characterisation techniques, including measurement of sonochemically generated hydrogen peroxide, characterisation of CNT quality using Raman spectroscopy and of dispersion efficiency using absorption spectroscopy and atomic force microscopy. This work highlights that careful measurement and control of cavitation rather than blind application of input power is essential in the production of nanomaterial dispersions with tailored properties. The results have major implications for enhanced control and scale-up of nanoparticle dispersion using ultrasonic processing.

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