No description available.

The present methods of ascertaining the probable sludging of transformer oils are laborious. A simple test, comparable to that used in obtaining the dielectric strength, would be of considerable value in reducing test manpower, equipment and expense. Present investigators measure such properties as oxygen absorption, colour change, increase in acidity, carbon dioxide evolved, water formed, the power factor, and the lnterfacial-tension value. Numerous dielectric materials exhibit anomalous dispersions. The attendant rise in dissipation factor encountered at the points of dispersion is helpful in determining the amounts and types of polarization present. Transformer oils, although initially non-polar, are subject to oxidation. This oxidation produces materials of polar molecular structure which govern the power loss in alternating current electric fields. The problem is one of correlating such losses to the state of the oil under test. This thesis deals with the investigations carried out on the above problem. Absorption bands for transformer oils have been found. The present work describes the presence and location of such absorption bands, and illustrates how the dipolar state of the oil affects the position of the loss peaks. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

No description available.

No description available.

Several thousand tests throughout the world have been conducted on the shear strength of screw connections in cold-formed steel, however, little to no research has been conducted on how various thicknesses of insulation placed between two sheets of steel, such as a steel panel and structural supporting member, affects a screw's shear strength. Elemental tests were conducted as part of this study at Virginia Tech where rolled fiberglass insulation was placed between two pieces of steel connected by self-drilling screws and tested to failure. The results were compared to the North American Specification for the Design of Cold-Formed Steel Structural Members to determine if the presence of insulation affected the shear and tensile strengths of screw connections involving insulation. A series of diaphragm tests were also preformed to confirm the elemental tests. While the presence of insulation between two steel sheets connected by screws reduces the shear strength of the connection, the current equations for predicting this strength in the North American Specification are adequate. When the data acquired from this study and the screw shear data obtained in past research were combined, it was clear that the data collected during this study fell within the scatter of the data used to develop Section E4.3 of the North American Specification neglecting the need for modification. / Master of Science

The thesis constitutes a systematic theoretical and experimental investigation into impulse breakdown voltage-time curves of coaxial-cylinder gaps in SF(,6) and a 50% SF(,6) - 50% N(,2) mixture. The breakdown time delay probability is treated as a bivariate distribution in statistical and formative time lags. A method is developed to deduce the marginal breakdown voltage probability curve from statistics of time delay. The experimental work was performed on 2.54/7.0-cm diameter, 60-cm long coaxial-cylinder electrodes using SF(,6) or SF(,6)-N(,2) mixtures at pressures of 0.1 - 0.3 MPa subjected to 0.5/50-(mu)s and 1.2/50-(mu)s impulses of both polarities. The effect of a spherical conducting particle attached to the central electrode of the coaxial-cylinder gap on the voltage-time characteristics is also investigated.

A need was identified to test multi–layer vacuum super insulation (MLVSI) used in cryogenic applications for hydrogen storage. The study focuses on the application of commercially available MLVSI to a locally patented liquid hydrogen cryogenic storage system. This led to an investigation of different types of multi–layer vacuum insulation configurations, as well as further research on tank inlet coupling configurations. It includes the manufacturing of a liquid nitrogen testing cryostat to be able to test and evaluate the system performance. The first set of tests was based on the development of an inlet coupling configuration to limit heat transfer through the inner tank inlet, of a double cryogenic tank system in order to reduce gas boil–off. The couplings were manufactured in the form of a bellow to handle cryogenic vacuum levels, while ensuring low heat transfer rates between inner and outer tanks. It was found that various coupling designs can be considered to limit gas boil–off. The second set of tests was conducted on a specific MLVSI configuration to determine its effectiveness to insulate the spherical header surface of a typical hydrogen storage vessel. The installation procedure, to limit heat transfer and boil–off due to edge effects in this configuration was investigated. It was found that insulation–overlap–edge effects will always have an impact on insulation performance when a spherical header of a storage vessel is insulated, due to its specific geometry. A time efficient way to install MLVSI on such a spherical header is presented and evaluated. Further investigations were carried out by combining findings into one single system to determine the performance of an optimised insulated cryogenic system. It was found that copper plate discs installed between the vanes of a bellowed inlet/outlet nozzle is the most promising to limit heat transfer to the cryogenic fluid. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2012.

A need was identified to test multi–layer vacuum super insulation (MLVSI) used in cryogenic applications for hydrogen storage. The study focuses on the application of commercially available MLVSI to a locally patented liquid hydrogen cryogenic storage system. This led to an investigation of different types of multi–layer vacuum insulation configurations, as well as further research on tank inlet coupling configurations. It includes the manufacturing of a liquid nitrogen testing cryostat to be able to test and evaluate the system performance. The first set of tests was based on the development of an inlet coupling configuration to limit heat transfer through the inner tank inlet, of a double cryogenic tank system in order to reduce gas boil–off. The couplings were manufactured in the form of a bellow to handle cryogenic vacuum levels, while ensuring low heat transfer rates between inner and outer tanks. It was found that various coupling designs can be considered to limit gas boil–off. The second set of tests was conducted on a specific MLVSI configuration to determine its effectiveness to insulate the spherical header surface of a typical hydrogen storage vessel. The installation procedure, to limit heat transfer and boil–off due to edge effects in this configuration was investigated. It was found that insulation–overlap–edge effects will always have an impact on insulation performance when a spherical header of a storage vessel is insulated, due to its specific geometry. A time efficient way to install MLVSI on such a spherical header is presented and evaluated. Further investigations were carried out by combining findings into one single system to determine the performance of an optimised insulated cryogenic system. It was found that copper plate discs installed between the vanes of a bellowed inlet/outlet nozzle is the most promising to limit heat transfer to the cryogenic fluid. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2012.

No description available.

No description available.