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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.
41

Erosive wear failure of spool valves

Pomeroy, Paul E. January 1995 (has links)
No description available.
42

Development of an electro-hydraulic floating double-disc valve

Usman, Ayo January 1984 (has links)
There is a need for low-cost switching and proportional electro-hydraulic valves with low contamination sensitivity and good reliability. In an attempt to meet this need, a novel electro-hydraulic floating double-disc valve has been developed to the stage where it can be used to control hydraulic cylinders or motors directly. As the valve is significantly underlapped, problems still remain in achieving adequate hydraulic stiffness in the proportional mode of operation. The valve operation, which relies on the complex interaction between fluid and electro-magnetic forces acting on the valve discs, is described and a theoretical model of the fluid and electro-magnetic characteristics of the valve is presented. The theory shows satisfactory agreement with experimental data. A pre-production version of the double-disc valve has been designed and manufactured and it incorporates ideas for manufacturing cost reduction while at the same time conforming to CETOP 3 international valve port standards. This valve has been successfully tested as a switching or proportional device when controlling two different cylinders. Proportional control of the valve is achieved using Pulse-Width-Modulation technique. British Technology Group and University of Surrey have applied for a patent on the valve. The patented floating-disc valve has the following features: (a) 3 way or 4 way 2-position or proportional action with minor changes to produce the two types of action, (b) cartridge construction with interchangeable components, (c) low contamination sensitivity, (d) few critical dimensions, (e) no sliding surfaces, (f) CETOP valve port configuration and (g) potentially capable of operating with corrosive or non-lubricating fluids.
43

Non-Newtonian losses through diaphragm valves

Kazadi, Dieudonne Matang'a January 2005 (has links)
Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2005 / The prediction of head losses in a pipe system is very important because head losses affect the performance of fluid machinery such as pumps. In a pipe system, two kinds of losses are observed: major losses and minor losses. In Newtonian and non-Newtonian flow, major losses are those that are due to friction in straight pipes and minor losses are those that are due to pipe fittings such as contractions, expansions, bends and valves. Minor losses must be accurately predicted in a pipe system because they are not negligible and can sometimes outweigh major losses (Edwards et al., 1985). There is presently little data for the prediction of non-Newtonian head losses in pipe fittings in the literature and little consensus amongst researchers (Pienaar et al., 2004). In the case of diaphragm valves, usually, only one loss coefficient value is given in turbulent flow or in laminar flow with no reference to a specific size of the valve, assuming geometrical similarity that would lead to dynamic similarity. However, no one has done a systematic study of various sizes of diaphragm valves from the same manufacturer to establish if this is true. This could be the main reason for discrepancies found in the literature (Hooper, 1981; Perry & Chilton, 1973; Miller, 1978 and Pienaar et al., 2004). This work addresses this issue. A literature revIew on the flow of Newtonian and non-Newtonian fluids has been presented. The work of Hooper (1981) on diaphragm valves and the works of Edwards et al., (1985), BaneIjee et aI., (1994) and Turian et al., (1997) for non-Newtonian fluids in globe and gate valves were found to be relevant to this work.
44

Análise de manufatura e de produto para a utilização do ferro fundido austemperado ADI grau 3 em eixo comando de válvulas de motores Diesel / Product and manufacturing analysis of austempered ductile iron ADI grade 3 using for Diesel engine camshaft

Buchli, Marcelo 17 August 2018 (has links)
Orientador: Jefferson de Oliveira Gomes / Dissertação (mestrado profissional) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-17T00:45:15Z (GMT). No. of bitstreams: 1 Buchli_Marcelo_M.pdf: 3752727 bytes, checksum: 40d7b010cd2e281baaa3faf8554cb691 (MD5) Previous issue date: 2010 / Resumo: Neste trabalho foi estudado o ferro fundido austemperado (ADI) grau 3 para substituir o aço forjado SAE 1045 temperado e revenido na aplicação do eixo comando de válvulas para motores Diesel. Na comparação dos materiais foram verificadas as propriedades mecânicas através de ensaios de laboratório e na literatura. A resistência ao desgaste foi a principal propriedade mecânica avaliada. Com o objetivo de se eliminar a operação de acabamento em retífica, especificada para os eixos comando de válvulas em aço forjado, o acabamento na usinagem de torneamento do ADI grau 3, no estado beneficiado, também foi analisado. O ADI grau3 atendeu à solicitação de desgaste quando comparado ao aço forjado e obteve um acabamento na operação de usinagem em torno, muito próximo ao acabamento do aço forjado na operação de retífica. Os resultados deste trabalho servem como base de estudo para o desenvolvimento e a fabricação de eixo comando de válvulas em ferro fundido austemperado grau 3 para motores Diesel / Abstract: The austempered ductile iron grade 3 was investigated as a substitute to SAE 1045 forged steel quenched & tempered in Diesel engines camshaft application. The mechanical properties for both materials were compared in laboratory tests and in the literature. The main mechanical property studied was the wear resistance. The microstructure, chemical composition was analyzed and correlated to camshaft requirements. The finishing turning machining characteristic of heated treated ADI grade 3 also was tested to verify the possibility of eliminating the grinding operation to SAE 1045 forged steel. As a conclusion of this study the ADI grade 3 complied with wear requirements when compared to forged steel and reached a finishing turning machining very close to grinding operation on forged steel. The results of this study can be used as basis to new development and production of diesel engines camshaft with austempered ductile iron / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Automobilistica
45

Application of the Direct Oral Anticoagulants to Thromboprophylaxis of Mechanical Heart Valves

Jaffer, Iqbal H January 2017 (has links)
Patients with valvular pathology who require heart valve replacement surgery are faced with the option between a bioprosthetic heart valve (BHV) or a mechanical heart valve (MHV). BHVs have a limited lifespan and are prone to structural degeneration. In contrast, MHVs are free from structural deterioration, but necessitate lifelong anticoagulation due to a more robust activation of coagulation. This is usually achieved in the outpatient setting with the use of vitamin K antagonists (VKAs), such as warfarin. Due to the cumbersomeness of managing warfarin with regular monitoring and its numerous food- and drug-drug interactions, other oral anticoagulants were sought. Within the last decade, the advent of the direct oral anticoagulants (DOACs) has largely supplanted warfarin and other VKAs for numerous indications, including stroke prevention in atrial fibrillation and prevention and treatment of venous thromboembolism. When evaluated in a clinical trial in patients with MHVs however, use of the thrombin-directed DOAC, dabigatran, led to an increase in both bleeding and clotting; prompting a halt to the study and a black box warning against the use of dabigatran and other DOACs in patients with MHVs. The cause of this failure at the time was not understood and prompted the investigations detailed in this thesis. The work described within this thesis sought primarily to understand why dabigatran failed to adequately provide thrombo-protection for patients with MHVs. Firstly, the mechanism of thrombin generation on MHVs was identified as being initiated and propagated through the contact and intrinsic pathways, respectively. Within this same work, a mechanism for the failure of dabigatran and the efficacy of warfarin was presented. It was shown that dabigatran concentrations of greater than 250ng/mL were required to suppress thrombin generation, whereas the same effect could be achieved with warfarin at an INR of 1.5 or greater. Secondly, the effect of the other DOACs, rivaroxaban and apixaban, on MHV-induced thrombin generation was assessed, and we showed that individually, these two DOACs were unable to suppress thrombin generation within clinically-relevant dosing regimens. We also evaluated a combination of dabigatran and rivaroxaban to determine whether the combination would be superior to either agent alone at suppressing MHV-induced thrombin generation. Lastly, the capacity of two assays – the ecarin chromogenic assay and the dilute thrombin time – to detect both on treatment and low levels of dabigatran is presented as a tool for clinicians to determine whether patients taking dabigatran can safely undergo procedures or whether they necessitate rapid reversal in emergent situations. / Thesis / Doctor of Philosophy (PhD)
46

Discovering the link between bicuspid aortic valve and aortic aneurysms: genetic or hemodynamic?

Habchi, Karam 08 April 2016 (has links)
OBJECTIVES: The association between bicuspid aortic valves and aortic aneurysms has been well documented. In order to better understand this association, this study sought to accomplish two goals. The first was to determine if there was any correlation between specific bicuspid aortic valve phenotypes and aortic aneurysms. The second goal was to determine if the association between bicuspid aortic valve disease and aortic aneurysms has a genetic or hemodynamic cause. METHODS: For the non-genetic portion of the study, we used echocardiogram and surgical records to classify the phenotypes of the aortic valve and the aorta of 434 patients. We then evaluated the correlation between valve morphotype and aortic aneurysm phenotype. For the genetic portion, we used a genome wide association study on 452 patients to find genes that could potentially be responsible for aortic aneurysms. These were then compared with genes suspected of causing bicuspid aortic valve to determine if there is a common genetic link between the two disorders. RESULTS: We observed a significant association between bicuspid aortic valve and aortic aneurysms; however we did not find any significant association between the different bicuspid aortic valve phenotypes and aortic aneurysm phenotypes. For the genome wide association study, we identified genes that could potentially be responsible for causing aortic aneurysms; however, none of the suspected markers were considered statistically significant. Also none of the identified genes matched to the genes suspected of causing bicuspid aortic valve. CONCLUSION: While the results were not as expected, the study provided us with information to better understand the relationship between bicuspid aortic valves and aortic aneurysms. According to the results of the current study, patients with bicuspid aortic valve are more likely to develop an aortic aneurysm but specific phenotype has no effect on where the aneurysm occurs in the aorta. The increased frequency of aortic aneurysms in bicuspid valve patients is most probably due to a combination of altered hemodynamics and genetic effects. In order for this information to be useful in the clinical setting, the methods of this study should be repeated in a larger cohort to make sure the results are accurate.
47

Biomechanics and hydrodynamics of decellularised aortic valves for tissue engineering

Korossis, Sotirios Anastasios January 2002 (has links)
No description available.
48

Organic spin valves : hole injection from ferromagnetic materials into tris-8-hydroxyquinoline (Alq3) in the presence of interface states

Zhang, Hongtao January 2014 (has links)
This thesis presents the result of the charge carrier injection and the subsequent transport from ferromagnetic material into organic semiconductor in the Alq3 based organic spin valves. In order to study the dominant charge carrier polarity in the Alq3 based spin valves, a number of single (Alq3) layer and double (Alq3 and N,N’-bis(3-methylphenyl)-N,N’- diphenylbenzidine (TPD)) layer organic semiconductor diode devices are constructed using both conventional electrode materials as well as ferromagnetic electrodes. Single layer devices are characterised by time of flight (ToF) and dark injection (DI) transient techniques with or without ferromagnetic anodes. Double layer devices are characterised using current-voltage-luminescence (j-V-L) measurements with or without ferromagnetic cathodes. Despite Alq3 being considered an electron transport material, we measure long range hole transport within the devices with matched electron and hole mobility at large electric fields. The substitution of a conventional Al cathode with a ferromagnet drastically suppresses electroluminescence in double layer devices, due to poor electron injection from the large work function ferromagnet. DI measurements using a ferromagnetic anode display characteristic charge trapping consistent with the presence of hybridized interface states (HINTS) between anode and organic semiconductor. The temperature dependent DI and ToF measurements demonstrate a reduced hole injection barrier in the presence of the HINTS in the ferromagnetic/organic interface that enables Alq3 based organic spin valves operate at small bias. We conclude that the dominant charge carriers in Alq3 based spin valves are holes, contrary to conventional wisdom, and that hole injection under small bias conditions is aided by HINTS.
49

Anatomy of microvenous valves of normal and venous ulcerated lower limbs

Phillips, Mark N., n/a January 2005 (has links)
Venous disease is a very common disease that affects millions of people worldwide. While some of the factors that cause the development of varicose veins are well understood, the aetiology of venous ulceration is poorly understood. It has been demonstrated that venous valve failure in the large veins is an important factor leading to the development of varicose veins, however whether similar valves exist in the very small superficial veins of the human leg, and what role these valves may have in venous disease, is unknown. Therefore, the purpose of this study is to: 1. Identify whether venous valves are present in the very small superficial veins of the human leg, and if they are present, 2. Describe the density, size distribution, morphology, and regional distribution of these microvenous valves in �normal� cadaveric legs 3. Compare the �normal� microvenous valves from cadaveric tissue with microvenous valves from pathologic legs with chronic venous disease, to answer the hypothesis that individuals that develop venous ulceration have fewer microvenous valves than the normal population. In order to examine microvenous valves, two main methods have been utilised, E12 sheet plastination and vascular casting. These methods in combination provide valuable insights into the anatomy of microvenous channels, and allow examination and quantification of the venous valves. Using several techniques, this study has shown that microvenous valves are present within the very small veins of the superficial tissue of the human leg. These microvalves have been shown to be most prevalent in the smallest of the veins, down to 18[mu]m in diameter. Approximately 60% of the valves were found to be associated with tributaries. The gaiter region was demonstrated to contain the lowest number and density of microvenous valves, significantly less that the upper or mid calf regions. In addition, the gaiter region was found to have a much lower proportion of microvalves in the most superficial veins, when compared with the other regions examined. Contrary to our hypothesis, the number and density of microvalves in venous diseased legs was not different to that of normal legs. Similarly, the size and regional distributions were also not different. However, the microvalves from the venous diseased legs were significantly stretched and incompetent, allowing retrograde flow from the large veins through to the dermal capillaries. In conclusion, this study has shown that venous valves are present in the smallest of the superficial veins of the human leg, and that their density and distribution is not different between normal and venous diseased individuals. However, the microvalves from the diseased legs were incompetent and allowed retrograde flow. The role that these valves play in normal and pathological circulation is unclear, and warrants further examination.
50

Measurement and prediction of phase equilibrium properties at infinite dilution : alkanes in natural gases and organic solvents in aqueous solutions

Suleiman, David 12 1900 (has links)
No description available.

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