Prediction of welding distortion in butt joint of thin plates

This paper investigates distortions and residual stresses induced in butt joint of thin plates using Metal Inert Gas welding. A moving distributed heat source model based on Goldak's double-ellipsoid heat flux distribution is implemented in Finite Element (FE) simulation of the welding process. Thermo-elastic-plastic FE methods are applied to modelling thermal and mechanical behaviour of the welded plate during the welding process. Prediction of temperature variations, fusion zone and heat affected zone as well as longitudinal and transverse shrinkage, angular distortion, and residual stress is obtained. FE analysis results of welding distortions are compared with existing experimental and empirical predictions. The welding speed and plate thickness are shown to have considerable effects on welding distortions and residual stresses. © 2009 Elsevier Ltd. All rights reserved.

Gas-metal reactions and porosity in the inert gas arc welding of copper

A study has been made of the effects of welding and material variables on the occurrence of porosity in tungsten inert gas arc welding of copper. The experiments were based on a statistical design and variables included, welding current, welding speed, arc atmosphere composition, inert gas flow rate, weld preparation, and base material. The extent of weld metal porosity was assessed by density measurement and its morphology by X-ray radiography and metallography. In conjunction with this the copper-steam reaction has been investigated under conditions of controlled atmosphere arc melting. The welding experiments have shown that the extent of steam porosity is increased by increased water vapour content of the arc atmosphere, increased oxygen content of the base material and decreased welding speed. The arc melting experiments have shown that the steam reaction occurs in the body of the weld pool and proceeds to an apparent equi1ibrium state appropriate to to its temperature, the hydrogen and oxygen being supplied by the dissociation of water vapour in the arc atmosphere. It has been shown conclusively that nitrogen porosity can occur in the tungsten inert gas arc welding of copper and that this porosity can be eliminated by using filler wires containing small amounts of aluminum and titanium. Since it has been shown to be much more difficult to produce sound butt welds than melt runs it has been concluded that the porosity associated with joint fit up is due to nitrogen entrained into tho arc atmosphere. Clearly atmospheric entrainment would also, to a much lesser extent, involve water vapour. From a practical welding point of view it has thus been postulated that use of a filler wire containing small amounts of aluminum and/or titanium would eliminate both forms of porosity since these elements are both strongJy deoxidising and denitriding.

Nanoengineering the active site in heterogeneous catalysis

Here we demonstrate the first application of time-resolved synchrotron X-ray absorption spectroscopy to simultaneously follow dynamic nanoparticle surface restructuring and the evolution of surface and gas-phase products during an organic reaction. Surface palladium oxide, and not metal, is identified as the catalytic species responsible for the selective oxidation (selox) of crotyl alcohol to crotonaldehyde. Elevated reaction temperatures facilitate reversible nanoparticle redox processes, and concomitant catalytic selectivity loss, in response to reaction conditions. These discoveries highlight the importance of stabilizing surface palladium oxide and minimizing catalyst reducibility in order to achieve high selox yields, and will aid the future design of Pd-derived selox catalysts. This discovery has important implications for the design of future liquid and vapor phase selox catalysts, and the thermochemical behavior of Pd nanostructures in general.

Risk-based maintenance model for offshore oil and gas pipelines:a case study

Offshore oil and gas pipelines are vulnerable to environment as any leak and burst in pipelines cause oil/gas spill resulting in huge negative Impacts on marine lives. Breakdown maintenance of these pipelines is also cost-intensive and time-consuming resulting in huge tangible and intangible loss to the pipeline operators. Pipelines health monitoring and integrity analysis have been researched a lot for successful pipeline operations and risk-based maintenance model is one of the outcomes of those researches. This study develops a risk-based maintenance model using a combined multiple-criteria decision-making and weight method for offshore oil and gas pipelines in Thailand with the active participation of experienced executives. The model's effectiveness has been demonstrated through real life application on oil and gas pipelines in the Gulf of Thailand. Practical implications. Risk-based inspection and maintenance methodology is particularly important for oil pipelines system, as any failure in the system will not only affect productivity negatively but also has tremendous negative environmental impact. The proposed model helps the pipelines operators to analyze the health of pipelines dynamically, to select specific inspection and maintenance method for specific section in line with its probability and severity of failure.

Optical fibre sensors with applications in gas and biological sensing

This thesis describes the study of various grating based optical fibre sensors for applications in refractive index sensing. The sensitivity of these sensors has been studied and in some cases enhanced using novel techniques. The major areas of development are as follows. The sensitivity of long period gratings (LPGs) to surrounding medium refractive index (SRI) for various periods was investigated. The most sensitive period of LPG was found to be around 160 µm and this was due to the core mode coupling to a single cladding mode but phase matching at two wavelength locations, creating two attenuation peaks, close to the waveguide dispersion turning point. Large angle tilted fibre gratings (TFGs) have similar behaviour to LPGs, in that they couple to the co-propagating cladding modes. The tilted structure of the index modulation within the core of the fibre gives rise to a polarisation dependency, differing the large angle TFG from a LPG. Since the large angle TFG couple to the cladding mode they are SRI sensitive, the sensitivity to SRI can be further increased through cladding etching using HF acid. The thinning of the cladding layer caused a reordering of the cladding modes and shifted to more SRI sensitive cladding modes as the investigation discovered. In a SRI range of 1.36 to 1.40 a sensitivity of 506.9 nm/URI was achieved for the etched large angle TFG, which is greater than the dual resonance LPG. UV inscribed LPGs were coated with sol-gel materials with high RIs. The high RI of the coating caused an increase in cladding mode effective index which in turn caused an increase in the LPG sensitivity to SRI. LPGs of various periods of LPG were coated with sol-gel TiO2 and the optimal thickness was found to vary for each period. By coating of the already highly SRI sensitive 160µm period LPG (which is a dual resonance) with a sol-gel TiO2, the SRI sensitivity was further increased with a peak value of 1458 nm/URI, which was an almost 3 fold increase compared to the uncoated LPG. LPGs were also inscribed using a femtosecond laser which produced a highly focused index change which was no uniform throughout the core of the optical fibre. The inscription technique gave rise to a large polarisation sensitivity and the ability to couple to multiple azimuthal cladding mode sets, not seen with uniform UV inscribed gratings. Through coupling of the core mode to multiple sets of cladding modes, attenuation peaks with opposite wavelength shifts for increasing SRI was observed. Through combining this opposite wavelength shifts, a SRI sensitivity was achieved greater than any single observed attenuations peak. The maximum SRI achieved was 1680 nm/URI for a femtosecond inscribed LPG of period 400 µm. Three different types of surface plasmon resonance (SPR) sensors with a multilayer metal top coating were investigated in D shape optical fibre. The sensors could be separated into two types, utilized a pre UV inscribed tilted Bragg grating and the other employed a post UV exposure to generate surface relief grating structure. This surface perturbation aided the out coupling of light from the core but also changed the sensing mechanism from SPR to localised surface plasmon resonance (LSPR). This greatly increased the SRI sensitivity, compared to the SPR sensors; with the gold coated top layer surface relief sensor producing the largest SRI sensitivity of 2111.5nm/URI was achieved. While, the platinum and silver coated top layer surface relief sensors also gave high SRI sensitivities but also the ability to produce resonances in air (not previously seen with the SPR sensors). These properties were employed in two applications. The silver and platinum surface relief devices were used as gas sensors and were shown to be capable of detecting the minute RI change of different gases. The calculated maximum sensitivities produced were 1882.1dB/URI and 1493.5nm/URI for silver and platinum, respectively. Using a DFB laser and power meter a cheap alternative approach was investigated which showed the ability of the sensors to distinguish between different gases and flow rates of those gases. The gold surface relief sensor was coated in a with a bio compound called an aptamer and it was able to detect various concentrations of a biological compound called Thrombin, ranging from 1mM to as low as 10fM. A solution of 2M NaCl was found to give the best stripping results for Thrombin from the aptamer and showed the reusability of the sensor. The association and disassociation constants were calculated to be 1.0638×106Ms-1 and 0.2482s-1, respectively, showing the high affinity of the Aptamer to thrombin. This supports existing working stating that aptamers could be alternative to enzymes for chemical detection and also helps to explain the low detection limit of the gold surface relief sensor.

Non-metallic cold-cathode electron emission from composite metal-insulator microstructures

A detailed investigation has been undertaken into a field-induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated, dielectric-coated and composite-coated metallic cathodes. An optical imaging technique has been used to observe and characterize the spatial and temporal behaviour of the populations of emission sites on these cathodes under various experimental conditions, e.g. pulsed-fields, gas environment etc. This study has shown that, for applied fields of 20MVm^-1, thin dielectric (750AA) and composite metal-insulator (MI) overlayers result in a dramatic increase in the total number of emission sites (typically 30cm^-2), and hence emission current. The emission process has been further investigated by a complementary electron spectroscopy technique which has revealed that the localised emission sites on these cathodes display field-dependent spectral shifts and half-widths, i.e. indicative of a `non-metallic' emission mechanism. Details are also given of a comprehensive investigation into the effects of the residual gas environment on the FIEE process from uncoated Cu-cathodes. This latter study has revealed that the well-known Gas Conditioning process can be performed with a wide range of gas species (e.g. O_2, N_2 etc), and furthermore, the degree of conditioning is influenced by both a `Voltage' and `Temperature' effect. These experimental findings have been shown to be particularly important to the technology of high-voltage vacuum-insulation and cold-cathode electron sources. The FIEE mechanism has been interpreted in terms of a hot-electron process that is associated with `electroformed' conducting channels in MI, MIM and MIMI surface microstructures.

Multiphase modelling of the characteristics of close coupled gas atomization

Not withstanding the high demand of metal powder for automotive and High Tech applications, there are still many unclear aspects of the production process. Only recentlyhas supercomputer performance made possible numerical investigation of such phenomena. This thesis focuses on the modelling aspects of primary and secondary atomization. Initially two-dimensional analysis is carried out to investigate the influence of flow parameters (reservoir pressure and gas temperature principally) and nozzle geometry on final powder yielding. Among the different types, close coupled atomizers have the best performance in terms of cost and narrow size distribution. An isentropic contoured nozzle is introduced to minimize the gas flow losses through shock cells: the results demonstrate that it outperformed the standard converging-diverging slit nozzle. Furthermore the utilization of hot gas gave a promising outcome: the powder size distribution is narrowed and the gas consumption reduced. In the second part of the thesis, the interaction of liquid metal and high speed gas near the feeding tube exit was studied. Both axisymmetric andnon-axisymmetric geometries were simulated using a 3D approach. The filming mechanism was detected only for very small metal flow rates (typically obtained in laboratory scale atomizers). When the melt flow increased, the liquid core overtook the adverse gas flow and entered in the high speed wake directly: in this case the disruption isdriven by sinusoidal surface waves. The process is characterized by fluctuating values of liquid volumes entering the domain that are monitored only as a time average rate: it is far from industrial robustness and capability concept. The non-axisymmetric geometry promoted the splitting of the initial stream into four cores, smaller in diameter and easier to atomize. Finally a new atomization design based on the lesson learned from previous cases simulation is presented.

Evaluation of remote sensing for the detection of landfill gas and leachate in an urban environment

The technique of remote sensing provides a unique view of the earth's surface and considerable areas can be surveyed in a short amount of time. The aim of this project was to evaluate whether remote sensing, particularly using the Airborne Thematic Mapper (ATM) with its wide spectral range, was capable of monitoring landfill sites within an urban environment with the aid of image processing and Geographical Information Systems (GIS) methods. The regions under study were in the West Midlands conurbation and consisted of a large area in what is locally known as the Black Country containing heavy industry intermingled with residential areas, and a large single active landfill in north Birmingham. When waste is collected in large volumes it decays and gives off pollutants. These pollutants, landfill gas and leachate (a liquid effluent), are known to be injurious to vegetation and can cause stress and death. Vegetation under stress can exhibit a physiological change, detectable by the remote sensing systems used. The chemical and biological reactions that create the pollutants are exothermic and the gas and leachate, if they leave the waste, can be warmer than their surroundings. Thermal imagery from the ATM (daylight and dawn) and thermal video were obtained and used to find thermal anomalies on the area under study. The results showed that vegetation stress is not a reliable indicator of landfill gas migration, as sites within an urban environment have a cover too complex for the effects to be identified. Gas emissions from two sites were successfully detected by all the thermal imagery with the thermal ATM being the best. Although the results were somewhat disappointing, recent technical advancements in the remote sensing systems used in this project would allow geo-registration of ATM imagery taken on different occasions and the elimination of the effects of solar insolation.

A study in the reduction of efficiency of nitrogen oxide removal catalysts for automobiles

Automotive catalysts are the most effective short-term answer to air pollution from automobiles. Since strict control of exhaust emissions is, or will be,covered by legislation in most developed countries in the world, catalytic devices will be increasingly fitted to cars. There is consequently an urgent need for the development of catalysts that will not compete for scarce precious metal resources. A number of problems have already been identified in connection with base metal catalysts but quantitative investigations are lacking. The base metal reduction catalysts developed by Imperial Chemical Industries Limited, catalysts and Chemical Group, in collaboration with the Air Pollution Control Laboratory, B L Cars Limited for automotive emission control, are susceptible to de-activation by three major mechanisms. These are: physical loss of the wash-coat (a high surface area coating which supports the active species), aggregation of the active species and poisoning by fuel and engine oil additives. This thesis is especially concerned with the first two of these and attempts to indicate the relative magnitude .of their effect on the activity of. the catalysts. Aggregation of the active species or sintering, as it is loosely called, was studied by using impregnated granules to overcome effects due to the loss of the wash-coat. Samples were aged in a synthetic exhaust gas, free from poisons, and metal crystallite sizes were measured by scanning-electron microscopy. The increase in particle size was correlated with the loss in catalytic activity. In order to maintain a link with the real conditions of service a number of monolithic catalysts were tested in an engine-dynamometer and several previously tested endurance catalysts were examined. A mechanism is proposed for the break-up and subsequent 10s.5 of the wash-coat and suggestions for improved resistance to loss of the' coating and active species are proposed.

In situ synthesis and catalytic activity in CO oxidation of metal nanoparticles supported on porous nanocrystalline silicon

Reactive surface of mesoporous nanocrystalline silicon was used to synthesise noble metal nanoparticles via in situ reduction of the precursor salt solutions. The synthetic methodology for metal nanoparticle formation was systematically developed, and reaction conditions of metal salts reduction were optimised to prepare nanoparticles of controlled size distribution in the order 5–10 nm inside the mesoporous silicon template. CO oxidation was used as a test reaction for the synthesised Pt/porous silicon catalysts. Sharp reaction light-off was observed at about 120 °C on the optimised catalysts. The catalysts were shown to be stable in the extended steady-state runs and in the catalysts re-use experiments. Metal nanoparticles were shown to be stable to sintering at elevated temperatures up to 1000 °C. However, after thermal treatment on air, Pt nanoparticles were covered by a SiOx layer and were less active in CO oxidation.

In-situ XPS study on the reducibility of Pd-promoted Cu/CeO2 catalysts for the oxygen-assisted water-gas-shift reaction

Cu/CeO2, Pd/CeO2, and CuPd/CeO2 catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO2 catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO2 components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO2 greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce3+ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO2 catalysts, possibly by helping to maintain Cu in a reduced state during reaction.

On the active site in heterogeneous palladium selox catalysts

The nature of the active site in the Pd-catalysed aerobic selective oxidation of cinnamyl and crotyl alcohols has been directly probed by bulk and surface X-ray techniques. The importance of high metal dispersions and the crucial role of surface palladium oxide have been identified. © The Royal Society of Chemistry 2006.

Single-phase active rectifier selection for fault tolerant machine

A fault tolerant, 5-phase PM generator has been developed for use on the low pressure (LP) shaft of an aircraft gas turbine engine. The machine operates at variable speed and therefore has a variable voltage, variable frequency electrical output (VVVF). The generator is to be used to provide a 350V DC bus for distribution throughout the aircraft, and a study has been carried out that identifies the most suitable AC-DC converter topology for this machine in terms of losses, electrical component ratings, filtering requirements and circuit complexity.

Tail gas catalyzed N2O decomposition over Fe-beta zeolite. On the promoting role of framework connected AlO6 sites in the vicinity of Fe by controlled dealumination during exchange

A novel route to prepare highly active and stable N2O decomposition catalysts is presented, based on Fe-exchanged beta zeolite. The procedure consists of liquid phase Fe(III) exchange at low pH. By varying the pH systematically from 3.5 to 0, using nitric acid during each Fe(III)-exchange procedure, the degree of dealumination was controlled, verified by ICP and NMR. Dealumination changes the presence of neighbouring octahedral Al sites of the Fe sites, improving the performance for this reaction. The so-obtained catalysts exhibit a remarkable enhancement in activity, for an optimal pH of 1. Further optimization by increasing the Fe content is possible. The optimal formulation showed good conversion levels, comparable to a benchmark Fe-ferrierite catalyst. The catalyst stability under tail gas conditions containing NO, O2 and H2O was excellent, without any appreciable activity decay during 70 h time on stream. Based on characterisation and data analysis from ICP, single pulse excitation NMR, MQ MAS NMR, N2 physisorption, TPR(H2) analysis and apparent activation energies, the improved catalytic performance is attributed to an increased concentration of active sites. Temperature programmed reduction experiments reveal significant changes in the Fe(III) reducibility pattern with the presence of two reduction peaks; tentatively attributed to the interaction of the Fe-oxo species with electron withdrawing extraframework AlO6 species, causing a delayed reduction. A low-temperature peak is attributed to Fe-species exchanged on zeolitic AlO4 sites, which are partially charged by the presence of the neighbouring extraframework AlO6 sites. Improved mass transport phenomena due to acid leaching is ruled out. The increased activity is rationalized by an active site model, whose concentration increases by selectively washing out the distorted extraframework AlO6 species under acidic (optimal) conditions, liberating active Fe species.