Passive porous coatings have been proposed in literature as a means of delaying transition to turbulence in hypersonic boundary layers. The nonlinear stability of hypersonic viscous flow over a sharp slender cone with passive porous walls is investigated in this study. Hypersonic flows are unstable to viscous and inviscid disturbances, and following Mack (1984) these have been called the first and second Mack modes. A weakly nonlinear analysis of the instability of the flow to axisymmetric and non-axisymmetric viscous (first Mack mode) disturbances is performed here. The attached shock and effect of curvature are taken into account. Asymptotic methods are used at large Reynolds number and large Mach number to examine the viscous modes of instability, which may be described by a triple-deck structure. Various porous wall models have been incorporated into the stability analysis. The eigenrelations governing the linear stability of the problem are derived. Neutral and spatial instability results show the presence of multiple unstable modes and the destabilising effect of the porous wall models on them. The weakly nonlinear stability analysis carried out allows an equation for the amplitude of disturbances to be derived. The stabilising or destabilising effect of nonlinearity is found to depend on the cone radius. It is shown that porous walls significantly influences the effect of nonlinearity. They allow nonlinear effects to destabilise linearly unstable lower frequency modes and stabilise linearly unstable higher frequency modes.

Total disc arthroplasty (TDA) can be used to replace a degenerated intervertebral disc in the spine. There are different designs of TDAs, but one of the most common is a ball-and-socket combination. Contact between the bearing surfaces of such designs can result in high frictional torque, which can then result in wear and implant loosening. This study was designed to determine the effects of change in design factors, such as dimensions and material combinations, on friction and wear of ball-and-socket TDAs. Friction tests were carried out on generic models with ball radii 10, 12, 14 and 16 mm. Three material combinations were investigated; metal-on-metal, metal-on-polymer and for the first time polymer-on-metal. Wear tests were performed on metal-on-polymer Charité® TDAs and generic metal-on-metal models to compare the wear rate under the same conditions. Friction test results showed that polymer-on-metal TDAs create less friction than metal-on-polymer and metal-on-metal TDAs. Wear test results showed that under the same conditions, metal-on-metal TDAs create 23 times less wear debris than metal-on-polymer. The results were in agreement with studies on total hip arthroplasty (THA). The results of this work suggest possible alternatives for future TDA designs.

Pit propagation in stainless steels under electrochemical control was investigated using in situ synchrotron X-ray microtomography, which was used to confirm that the lacy covers commonly found for pits in stainless steels grow through perforation of the metal surface by upward growth of rapidly dissolving lobes from the main pit. In situ synchrotron X-ray radiography has been used to observe the evolution of 2D pits growing at the edge of stainless steel foils under electrochemical control in chloride solutions. Pit growth shape, kinetics and stability under current and potential control at various bulk chloride concentrations have been studied. It was found that the pit depth tends to grow under diffusion control, whereas lateral development is influenced by solution conductivity. The impact of the perforated cover on the pit growth and stability was examined and its formation was found to be similar to the observations from 3D by X-ray microtomography. A method for extracting the key dissolution kinetic parameters from radiographs has been developed. The local anodic current density along the boundary of a pit was directly measured from the rate of advance of the pit into the metal. Then the local metal ion concentration and potential drop inside the pit cavity was back-calculated using transport equations and the requirement to maintain charge neutrality, establishing the relationship between local current density, interfacial potential and metal ion concentration in the solution. The predictive model for pit propagation in stainless steel developed by Laycock and co-workers was examined, its sensitivity to key growth parameters was evaluated, and a modified version of the model was developed based on the kinetic parameters extracted from the radiographic measurements.

The oxidation resistance of four prototype single crystal nickel-based superalloys for industrial gas turbine applications is studied. All contain greater quantities of Cr than in most existing single crystal superalloys; two are alloyed with Si, one with Re. To explain the results, the factors known to influence the rate of Al\(_2\)O\(_3\) scale formation are considered. Models are developed to predict whether any given alloy composition will form a continuous Al\(_2\)O\(_3\) scale. These are used to rationalise the dependence of Al\(_2\)O\(_3\) scale formation on alloy composition in these systems. The mechanical behaviour of a new single crystal nickel-based superalloy for industrial gas turbine applications is also studied under creep and out-of-phase thermomechanical fatigue (TMF) conditions. Neutron diffraction methods and thermodynamic modelling are used to quantify the variation of the gamma prime (\(\gamma\)’) strengthening phase around the \( \gamma\)’ solvus temperature; these aid the design of primary ageing heat treatments to develop either uniform or bimodal microstructures of the \( \gamma\)’ phase. During TMF, localised shear banding occurs with the \( \gamma\)’ phase penetrated by dislocations; however during creep the dislocation activity is restricted to the matrix phase. The factors controlling TMF resistance are rationalised.

The need for jet engines to burn fuel more effectively at higher temperatures requires the development of nickel-based superalloys containing increasing amounts of the main strengthening, stable, ordered L1\(_2\) intermetallic (Ni\(_3\)(Al, Ti)) \( \gamma\)' phase, with RR1000 being a candidate. Welding of this alloy by conventional methods has been found difficult due to a high susceptibility to heat affected zone (HAZ) liquation cracking. In order to produce welds with good joint integrity, inertia friction welding (IFW), a nominally solid state welding process, has been used to join gas turbine parts made from this alloy, based on the premise that the joining occurs below the melting point of the bulk material. The failure rate, however, is not zero. Detailed microstructural characterisation of the actual weld and of a thermo-mechanically simulated HAZ has revealed for the first time that non-equilibrium constitutional liquation of some strengthening precipitates occurs at the grain boundaries and within the grains of this alloy during IFW, with attendant formation of liquation microvoids within the HAZ. The temperature gradient across the HAZ is predicted to be 1150 \(^o\)C-1286 \(^o\)C. Hafnium-rich oxides were also found to coalesce and become smeared by friction along the weld interface, forming brittle hafnium oxide flakes. However, IFW has been found to be more effective than conventional welding techniques, always producing liquation crack-free welds within 150 \( \mu\)m of the bond line (not common in conventionally welded alloys). Micro tensile testing was used to characterise the local strength within the weld and to rationalise it with the microstructure.

This PhD project was set out to improve the performance of silica gel / water adsorption cooling systems using a number of investigation tools. A novel global lumped analytical simulation model has been constructed for a commercialised two-bed silica gel / water 450kW adsorption chiller. It was integrated with a genetic algorithm (GA) optimisation toolbox to determine the optimum operating conditions to obtain the optimum chiller performance. The model was used to investigate the effect of physical and operating on the chiller performance. The model was also used to investigate the effect of various adsorbent bed enhancement techniques that are presented in published literature. An experimental test facility has been designed, constructed and commissioned to study the performance of scaled down adsorbent bed modules. It has been constructed to understand the effect of the operating conditions on adsorbent-bed heat and mass transfer performance. A dynamic vapour sorption (DVS) gravimetric analyser has been used to characterise a new species of adsorbents (MOFs). These adsorbents have strong potential towards water sorption and could replace the currently applied silica gel. MOFs adsorbents have been characterised in terms of adsorption isotherms and kinetics, in addition to its cyclic analysis. It was found that HKUST-1 (copper based MOF) outperforms silica gel with 95.7% increase in the water uptake. Iron based MOF (MIL-100) was found to outperform silica gel for high evaporating temperature application.

This study involved the characterisation of a newly developed 9%CrMoCoVNbNB (FB2) martensitic creep resistant steel for use in large industrial turbine disc and shaft components. A major part of the work involved characterisation of FB2 with incorporation of material, thermal and physical data into a finite element-based model for the simulation of coupled thermo-mechanical working with consideration of grain size evolution based on the implementation of a user sub-routine within the FEM code (QForm). The main focus of the research was to gain a comprehensive understanding of the non-uniform temperature, strain rate, strain and grain size distributions apparent during hot open-die forging.

Due to some environmental problems for using the common cooling system which works based on the compressor air and cooling gas (Freon), researchers have focused on another type of it. One of the newest methods is using electrocaloric materials to make a cooling system. Electrocaloric effect (ECE) is the reverse of the pyroelectricity which means that the ability of the dielectric materials to change their temperature under electric field. Two ferroelectric materials were selected for this project: BCZT (Ba0.85Ca0.15Ti0.9Zr0.1O3) as a normal ferroelectric and BNT – BT (94 mol% Bi0.5Na0.5TiO3 – 6 mol% BaTiO3) as a relaxor ferroelectric. The results for BCZT shows the best properties with a relative density of 96.1%, the grain size of 32μm, d33 of 410 pC/N and kp of 46% for sintered sample at 1450°C. The highest ECE (∆T= 0.97) was achieved for BCZT sintered at 1450°C (∆T= 0.97). The best properties were founded at BNT-BT sintered at 1125°C with a relative density of 97%, the grain size of 3.2μm, d33 of 165 pC/N and kp of 47%. BNT-BT shows high ECE ∆T= -2.91 and -2.1°C for sintered sample at 1125 and 1150°C respectively under 50 kV/cm which due to two step calcination process is higher than previous researchers.

Global value chain (GVC) analysis serves an important role in the global economy and development. Whereas research on governance is well documented in GVC research, only recently has the institutional context emerged as a new dimension of GVC analysis. This thesis integrates critical frameworks of governance, including the five types of governance of Gereffi et al. (2005) and institutional theory with the four mechanism of Beckert (2010), to develop further analysis of GVCs. These frameworks are fundamental for exploring the nature of problematic issues, such as tensions, in re-shaping GVC structures. The research employs method triangulation in both data collection (semi-structured interviews, observations and documents) and analysis (thematic analysis, discursive devices and institutional logics) to achieve the research objective of solving the institutional governance tensions within GVCs in the research setting of the Vietnam Textile and Garment Industry. The findings show that tensions stem mainly from two resources, symmetrical power and new circumstances. Although tension is often neglected or considered a negative factor in management, it can motivate creative responses and effectively serve as a motional factor. In any form, the influence of tension on GVC structures occurs in both convergent and divergent trends but creates a new balance of power between actors in the chain.

Manufacturing continues to be the fabric of the UK economy, as firms have transformed and restructured themselves in response to new forms of competition, alterations in consumer demand and new technological developments. This thesis applies Evolutionary Economic Geography (EEG) approaches to the UK Technical Textile (TT) sector to investigate the adaptation and evolution of UK TT firms. TT is a higher value-added and advanced segment of the textiles industry. The thesis blends EEG approaches with the Resource-Based View (RBV) of the firm into a new conceptual framework to develop a novel firm-centered approach to understanding firm-based competitiveness and evolving economic geographies of manufacturing. This theoretical blend highlights how a focus on routines (EEG) must be combined with an understanding of resources and capabilities (RBV) to theorise firm behavior. A detailed comparative firm-level analysis of 40 TT firms located across the UK identifies the adaptation processes and drivers behind firms diversifying into the manufacture of TT. This thesis makes an empirical and theoretical contribution to understanding how the adaptation processes of firm’s routines are an evolutionary process to firm-based competitiveness, in response to changes in their external economic environment.