Tesi sul tema "Valves"
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1
Anstine, Lindsey J. "Valve cell dynamics in developing, mature, and aging heart valves". The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1478692972995079.
Testo completo
2
Золотова, Світлана Григорівна, Светлана Григорьевна Золотова, Svitlana Hryhorivna Zolotova e O. I. Sidorets. "Control ball valves". Thesis, Видавництво СумДУ, 2008. http://essuir.sumdu.edu.ua/handle/123456789/16055.
Testo completo
3
Iredale, Peter David. "Investigation into unsteady valve flow in steam turbine inlet governing valves". Thesis, University of Leicester, 2000. http://hdl.handle.net/2381/30179.
Testo completoAbstract (sommario):
When partially closed, steam turbine governing valves rely on flow separation from the valve head and seat to generate loss and throttle the flow. The aim of this type of valve is to avoid separation and therefore eliminate loss when the valve is fully open, and to have stable and controllable separations at all other valve lifts. Any significant unsteadiness in the valve flow can result in unacceptable mechanical vibration of the valve, which in extreme cases can lead to failure. Results will be discussed from work that has been undertaken into valve flow instabilities at Leicester University Engineering Department in collaboration with Alstom Energy Ltd. At high lifts, the Mach number of the steam flow between the head and the seat is sufficiently low for the fluid to be considered as incompressible. Water was therefore used as the working fluid in the tests at Leicester to model accurately the flow in a fifth scale acrylic model valve under high lift conditions. Results from laser light sheet visualisation, Particle Image Velocimetry and transient pressure measurements of the valve flow are presented. Laser light sheet illumination and high-speed Cine photography have been used to visualise the highly three dimensional valve flow. A range of valve head geometries has been tested. The results of the flow visualisation show the presence of stable and unstable separation zones and their influence on the valve flow. Particle Image Velocimetry has provided quantitative information on these features. Methods for stabilising the separation zones by modifying the valve head and seat have also been investigated and the results from these tests have shown improvements in reducing valve exit pipe unsteadiness.
4
Bishop, Winona F. "Hydrodynamic performance of mechanical and biological prosthetic heart valves". Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/29461.
Testo completoAbstract (sommario):
One of the major achievements in cardiac surgery over the past 30 years has been the ability to replace severely diseased heart valves with prosthetic ones. The option of using prosthetic heart valves for the treatment of valvular diseases has improved and prolonged many lives. This is reflected in around 120,000 heart valve replacement operations carried out every year in North America alone to correct the cardiovascular problems of stenosis, insufficiency, regurgitation, etc.
The development of artificial heart valves depends on reliable knowledge of the hemodynamic performance and physiology of the cardiovascular system in addition to a sound understanding, at the fundamental level, of the associated fluid mechanics.
It is evident from the literature review that noninvasive measurements in a confined area of complex transient geometry, providing critical information relating to valve performance, are indeed scarce.
This thesis presents results of an extensive test program aimed at measuring turbulence
stresses, steady and transient velocity profiles and their decay downstream of the mitral valve. Three mechanical tilting disc-type heart valves (Björk-Shiley convexo- concave, Björk-Shiley monostrut, and Bicer-Val) and two biological tissue valves (Hancock II and Carpentier-Edwards supraannular) are studied. The investigation
was carried out using a sophisticated and versatile cardiac simulator in conjunction with a highly sensitive, noninvasive, two-component three-beam laser doppler anemometer system. The study covers both the steady (valve fully open) and pulsatile (resting heart rate) flow conditions. The continuous monitoring of the parametric time histories revealed useful details of the complex flow as well as helped establish location and timing of the peak parameter values. In addition,
orientation experiments are conducted on the mechanical valves in an attempt to reduce
stresses by altering the position of the major orifice. The experiments suggest correlation between high stresses and orientation.
Based on the the data, the following general conclusions can be made:
(i) Hemodynamic test results should be presented in nondimensional form to render them independent of test facilities, flow velocities, size of models, etc. This would facilitate comparison of results by different investigators, using different facilities and test conditions.
(ii) The valves tested showed very disturbed flow fields which generated high turbulent stresses presenting a possibility of thromboembolism and, perhaps,
haemolysis.
(iii) Implantation orientation of the valve significantly affect the mechanical prostheses
flow field. The single vortex formation in the posterior orientation results in a reduction in stresses compared to the anterior configuration.
(iv) The present results together with the earlier information on pressure drop and regurgitation provide a comprehensive and organized picture of the valve performance.
(v) The information is fundamental to the improvement in valve design, and development of guidelines for test methodology and acceptable performance criteria for marketing of the valves.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
5
Chan, Gene Yel. "Cryopreservation of porcine heart valves". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/MQ60420.pdf.
Testo completo
6
Huff, Michael Allan. "Silicon micromachined wafer-bonded valves". Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/12727.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1993.Includes bibliographical references (v. 2, leaves 429-436).
by Michael Allan Huff.
Ph.D.
7
Heinrich, Russell Shawn. "Assessment of the fluid mechanics of aortic valve stenosis with in vitro modeling and control volume analysis". Diss., Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/16664.
Testo completo
8
Cole, Matthew. "Feasibility of miniature polypyrrole actuated valves". Thesis, University of British Columbia, 2006. http://hdl.handle.net/2429/32239.
Testo completoAbstract (sommario):
Polypyrrole is a conducting polymer that can change in volume as its oxidation state is
altered. This change in volume, operable at low voltages, can be used to create small
actuating devices. The goal of this thesis is to evaluate the use of polypyrrole for the
creation of a low voltage polymer valve and to demonstrate the mechanisms that could
be used to create these valves. There are a number of challenges in using polypyrrole in
a commercially viable valve; it must be able to withstand large temperature variations,
have a high strain (to minimize mechanical amplification), have high work density (to
minimize the amount of polypyrrole required), have a long lifetime and be assemblable
into a compact valve.
To evaluate and meet these requirements: 1) The effect of modifying synthesis and
actuation conditions on the electrochemical actuation of polypyrrole is investigated to
find the conditions that give the highest electrochemical strain and strongest polypyrrole
films. Stable and fast strains of 6% at up to 2 MPa for films grown in propylene
carbonate and actuated in NaPF6(aq) are achieved. These films were stored for up to 3
months before use with no losses in strain but showed a loss of 0.06% of their maximum
strain per electrochemical cycle. 2) Polypyrrole is exposed to temperature variations,
showing that high temperature (up to 80°C) exposure has a deleterious effect on
polypyrrole actuation. To try and minimize losses, the effect of temperature in both
aqueous and organic electrolytes and the mechanism for degradation is investigated.
-PF6 grown films in aqueous electrolytes show the least degradation. 3) A linear valve
mechanism is built and demonstrates that it should be possible to achieve the force and
displacement required to open and close a sliding plate valve. Empirical models suggest
that it should be possible to use polypyrrole sliding oil sealed valves. 4) An
encapsulatable trilayer is built that seals holes in a perforated plate and could also be
used to make a valve.Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate
9
Sun, Yongbin. "Development of electromagnetic fluid disc valves". Thesis, University of Surrey, 1993. http://epubs.surrey.ac.uk/843323/.
Testo completoAbstract (sommario):
The development of electrohydraulic floating-disc valves at the University of Surrey started in the early 1980's. The progress in the last ten years since then has shown that floating-disc valves have the advantages of fast response time, reliable operation, simple configurations, few critical dimensions with no precision sliding surfaces, leading to low cost design and manufacture. They have great potential to fill the gap between conventional solenoid valves and high precision servo valves. However, limitations existed in previous designs hindering further development; for instance relatively large moving mass, low hydraulic stiffness, difficulty of installing springs and poor null position when operating in proportional control mode. The work presented in this thesis concentrates on improving the disc valve electromagnetic characteristics, hydraulic stiffness, electric power consumption, operating reliability, valve size and cost. A novel diaphragm-disc force motor has been successfully developed through this research. The theoretical study and experimental work has shown that the force motor has the features of high spring stiffness, fast response, improved accuracy and linearity, and miniaturised size. By implementing a pair of permanent ring magnets, the diaphragm-disc force motor also has the advantages of lower electric power consumption, dual-lane for fail safety operation, and higher control accuracy. Due to the use of conventional mild steel instead of Remco B soft iron as the coil magnetic conductor material, the valve manufacturing cost has been further reduced. Above all, this novel configuration shows good prospects of competing with the existing torque motor due to its low cost and simple construction. The research described also involves designing and testing two prototype disc valves for specific applications. A single disc pilot valve associated with the diaphragm configuration and permanent magnet arrangement has been built for use in an aviation engine fuel supply system. It has a dual-lane operating mode with a valve size of 58x50x50 millimetres, which is the smallest valve yet made in the disc valve family. The initial test results showed that the valve has good linearity and a bandwidth of 60 Hz in a blocked-load condition. Another successfully built valve is an improved version of a position controlled double-disc valve for use in vehicle semi-active suspension systems. It has been demonstrated that using proportional plus derivative electronic network compensation, the valve can operate continually in the whole damper control domain with the characteristics of balanced fluid forces and low electric power consumption.
10
Dana, Seresht Mahmoudreza. "Material Selection for Deepwater Gate Valves". Thesis, KTH, Materialvetenskap, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-170023.
Testo completoAbstract (sommario):
Material selection is an important step during the design process of an object. The goal is to produce an object to meet the requirement with minimum cost. During the recent years with discovery of oil and gas in deep water, oil and gas industry facing new challenges of handling corrosive material in seabed that gives more importance and criticality to material selection of equipment for this kind of application. Hydrogen sulfide (H2S), chloride and carbon dioxide (CO2) have made the big challenges for material that handle corrosive fluids in the seabed.This report presents a brief review of material selection for two parts of deepwater gate valve, Body and Gate. It is mostly focused on mechanical properties and required corrosion resistance. Ferritic alloys with low PRE numbers and low mechanical properties and also austenitic alloys with low yield strength are not a proper option for this case. Alloy 2205 is the most common stainless steel which is used in deep water gate valve production. There are other alloys in duplex group that show better mechanical and chemical properties than alloy 2205 but because of their high expense are not used by industries.
11
Stefanou, Georgios Konstantinos. "Transport properties of lateral spin valves". Thesis, University of Leeds, 2017. http://etheses.whiterose.ac.uk/20697/.
Testo completoAbstract (sommario):
The current study is focused on the investigation of the spin and heat properties of a Permalloy/silver/Permalloy, Permalloy/silver (doped with Iron)/Permalloy and Vanadium/silver/Vanadium spin valves in a lateral geometry. The presence of a downturn in the spin-signal of these lateral spin valves at low temperatures, below 30 K, can be avoided by depositing the Ag at a faster rate. This will promote a better microstructure that seems to have a crucial role in systems with very low or no magnetic impurities. A small part of the report is dedicated to test the two different behaviours of the spin signal that have been suggested in the literature, in the case of spin transport through an oxidised normal metal channel. According to literature, it would either remove the downturn of the spin signal at low temperatures, or form one. Our devices exhibited no downturn below 30 K, but by oxidising them, did not lead to the creation of it. By doping the normal metal channel with a dust layer of magnetic impurities (MIL) one would expect that by increasing scattering centers the spin signal and spin diffusion length would decrease no matter where the MIL would be. Interestingly that was not confirmed, as not only the spin signal and diffusion length vary depending on the position of the MIL in the NM channel, but also in the occasion of the device where the MIL was place right in the middle of the NM channel. At temperatures above 70 K the spin signal presented was bigger than that of the reference device. Moreover, the spin diffusion length presented in the particular set of devices, exceeded that of the reference set of devices for temperatures higher than 140 K. Finally, thermal effects were also expected to get picked up during the measurements as in order to create the spin accumulation, a charge current of 500 μ A is driven through the resistive Permalloy injector. That would give rise to Joule heating and Peltier effect once it reaches the Permalloy/silver junction due to the mismatch of the Peltier coefficients. Heat would then diffuse along the silver channel and through the substrate and would be detected in the second silver/Permalloy junction. The latter acted as a thermocouple and due to the difference in their Seebeck coefficients the temperature difference that was picked up, was converted to potential difference. The Scanning Thermal Microscopy technique was employed to image these effects. Based on the results of the technique, an analytical model that can predict the injector and detector junction temperature increase from that of the substrate was developed. In addition to that, since the voltage output is proportional to the product of the effective Seebeck coefficient with the temperature difference, the detector voltage could be predicted. To test the model a simple system of V/Ag/V device was used to deconvolute the thermal effects avoiding having a spin accumulation in the detector signal. After that, the thermal effects arising in a Py/Ag/Py lateral spin valve were investigated. Finally, since the Peltier effect was present in the measurements, the non-local resistance, spin signal and baseline resistance were analysed as a function of current direction to find a so far not studied effect. There was a splitting in the non-local and baseline resistance across all different devices that had temperature and current dependence. To confirm that the effect was not an artefact, three cryostats, three transport sticks, two sample holders and two more systems (Py/Cu/Py and CoFe/Cu/CoFe) were studied.
12
Gadyuchko, Andrey, e Sören Rosenbaum. "Nondestructive quality inspection of solenoid valves". Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-200756.
Testo completoAbstract (sommario):
The presented innovative magnetic testing method utilises the fact, that each commercially available electromagnet can not only be used as an actuator, but also comprises internal sensor functions. This allows a huge application variety in the fields of non-destructive testing and condition monitoring of electromagnetic systems during production and within the application in the field.
13
Kirkwood, A. D. "Aerdynamic noise generation in control valves". Thesis, University of Manchester, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.517180.
Testo completoAbstract (sommario):
An experimental study into the phenomenon of aerodynamic noise generation in control valves has been performed. Several model representations of control valves have been studied, in addition to making measurements on several full size control valves. A high speed computer based data acquisition system, coupled with miniature high frequency surface mounted pressure transducers, has been used to obtain measurements of both the mean and fluctuating wall static pressures at several locations throughout the length of the flow apparatus. For known flow conditions, it has been possible to determine the relative performance of each particular model or commercial valve design by examining the various recorded levels of the wall static pressure fluctuations. A series of flow visualisation experiments, in which both wall static pressures and photographs of the flow through a rectangular duct have been obtained simultaneously, has provided a valuable insight into the mechanisms that determine the performance of multi-plate models. These findings have led to the design and commissioning of a new working section constructed from lengths of circular pipe. This apparatus has been used to investigate the performance of models constructed from two circular multi-holed test plates placed normal to the flow, with the downstream test plate being significantly more porous. Relationships between parameters such as test plate hole diameter, test plate separation, hole pitch to diameter ratio and pressure ratio have been examined extensively for many combinations of model configuration. Subsequently, the vast amounts of experimental data produced by this systematic testing have been reduced to identify the clear links between the various parameters. This, in turn, has enabled the author to determine optimum values for the non-dimensional parameters which govern the design of such multi-plate systems. As a consequence of this study, the author has proposed an alternative approach to the design of 'Low Noise' control valves which offers the prospect of improved overall performance. Thus, it is envisaged that the findings from this investigation will have implications for the future design of 'Low Noise' control valves.
14
Ouchet, L. "Reliability modelling of subsea gate valves". Thesis, Cranfield University, 1993. http://dspace.lib.cranfield.ac.uk/handle/1826/11137.
Testo completoAbstract (sommario):
Offshore Installations
comprise complex, expensive and potentially hazardous facilities which
need to be maintained and
operated in a proper and safe manner. Valves are one of the most
widely used components on offshore installations and their reliability is important to both to
safety and to the minimisation of down time. During discussions with offshore operators and
valve manufacturers on the
significance of valve failure to safety, it has been found that valve
failures are
frequent occurrences and not restricted to a few isolated incidents. The problem is
wide
spread in the petroleum industry. This industry experience can be regarded as
unacceptable _
The success of a
reliability and risk analysis exercise is ultimately dependent on the availability
of historical
reliability data. This study conmed the findings of a number of recent surveys
which have revealed a dramatic lack of
knowledge and understanding of valve mechanical
failure mechanisms
amongst petroleum related industry. Furthermore it was found that
historical
reliability data was generally scarce and inadequate for thorough studies. As a result
of this situation, it was felt necessary to develop a approach to mechanical valve reliability
modelling which was predictive i nature and which would take into account design and
operational factors.
This research
project concentrates on the modelling and prediction of reliability of valves and
includes a case
study of subsea hydraulically operated gate valves. The modelling process
includes
procedures for the identification of vulnerable areas in valve design and its relation to
the
operating and environmental conditions. A number of statistical models have been
developed based on stress strength interference concepts. These models enable the probability
of failure to be described in terms of the
physical characteristics of mechanical valve
deterioration for
specific failure modes such a internal leakage, fail to close and fail to open.
The models
require information both on design and on deterioration performance
the latter of which is
poorly understood. Provided judgement can be utilised to describe
the
ageing parameters governing failures, the models enable engineers to predict the time dependent
reliability of this type of gate valve together with uncertainty at the design stage.
I addition to the fundamental
modelling work, the implications of reliability management
systems have been investigated. The role of condition monitoring and the use of monitoring
data for both for
anticipation of failure and for updating the predicted reliability has been
discussed. The role of failure
reporting systems to provide improved understanding of failure
mechanisms if valves was considered to be a
important element of a reliability management
system and vital to the future improvement in valve reliability. No evidence of any systematic
failure
reporting system was found in the offshore industry.
15
Winroth, Marcus. "On Gas Dynamics of Exhaust Valves". Licentiate thesis, KTH, Strömningsfysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-202719.
Testo completoAbstract (sommario):
With increasing effects of global warming, efforts are made to make transportation in general more fuel efficient. When it comes to internal combustion engines, the most common way to improve fuel efficiency is through ‘downsizing’. Downsizing means that a smaller engine (with lower losses and less weight) performs the task of a larger engine. This is accomplished by fitting the smaller engine with a turbocharger, to recover some of the energy in the hot exhaust gases. Such engine systems need careful optimization and when designing an engine system it is common to use simplified flow models of the complex geometries involved. The exhaust valves and ports are usually modelled as straight pipe flows with a corresponding discharge or loss coefficient, typically determined through steady-flow experiments with a fixed valve and at low pressure ratios across the valve. This means that the flow is assumed to be independent of pressure ratio and quasi-steady. In the present work these two assumptions have been experimentally tested by comparing measurements of discharge coefficient under steady and dynamic conditions. The steady flow experiments were performed in a flow bench, with a maximum mass flow of 0.5 kg/s at pressures up to 500 kPa. The dynamic measurements were performed on a pressurized, 2 litre, fixed volume cylinder with one or two moving valves. Since the volume of the cylinder is fixed, the experiments were only concerned with the blowdown phase, i.e. the initial part of the exhaustion process. Initially in the experiments the valve was closed and the cylinder was pressurized. Once the desired initial pressure (typically in the range 300-500 kPa) was reached, the valve was opened using an electromagnetic linear motor, with a lift profile corresponding to different equivalent engine speeds (in the range 800-1350 rpm). The results of this investigation show that neither the quasi-steady assumption nor the assumption of pressure-ratio independence holds. This means that if simulations of the exhaustion process is made, the discharge coefficient needs to be determined using dynamic experiments with realistic pressure ratios. Also a measure of the quasi-steadiness has been defined, relating the change in upstream conditions to the valve motion, i.e. the change in flow restriction, and this measure has been used to explain why the process cannot be regarded as quasi-steady.QC 20170306
16
Kazadi, Dieudonne Matang'a. "Non-Newtonian losses through diaphragm valves". Thesis, Cape Peninsula University of Technology, 2005. http://hdl.handle.net/20.500.11838/904.
Testo completoAbstract (sommario):
Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2005The 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.
17
Reynolds, Karen Jane. "Acoustic monitoring of prosthetic heart valves". Thesis, University of Leicester, 1994. http://hdl.handle.net/2381/34209.
Testo completoAbstract (sommario):
The aim of the work presented in this thesis was to examine the possibility of detecting structural changes to an implanted prosthetic heart valve by spectral analysis of the sounds produced by the valve. On closure, mechanical heart valves produce a distinct sound as the occluder strikes the metal frame of the valve. Any change in the mechanical state of the valve will produce changes in the modes of vibration of the entire structure, causing the spectrum of the closing sounds to change. Initial recordings were made in a large tank of water providing ideal valve actuation and recording conditions. Results showed that all valves produce a stable averaged spectrum, and that each valve has a unique averaged spectrum. A digital filtering technique was developed whereby a baseline spectrum recorded from each valve is used for comparison with all subsequently recorded spectra from that valve. Using this technique, averaged spectra from individual valves were found to be highly reproducible. However, a minor structural alteration to a valve (added mass, or strut fracture) caused significant spectral changes, readily detected by digital filtering. To investigate the effect of a finite recording volume, recordings were made in a tank with dimensions approximating those of a human thorax. Standing waves generated by reverberations were clearly visible in the results. Structural changes to a valve were still detectable. Recordings were also made from prosthetic valves implanted in patients. To reduce sound distortion at the thoracic surface, recordings were made with the patient submerged in water. Results showed that reproducible averaged spectra could be obtained from implanted valves provided recording conditions were kept constant. The technique has not yet been developed to the point where it can be applied clinically. Nevertheless the technique shows promise as a method of screening patients at risk.
18
Sitte, André, Oliver Koch, Jianbin Liu, Ralf Tautenhahn e Jürgen Weber. "Multidimensional flow mapping for proportional valves". Technische Universität Dresden, 2020. https://tud.qucosa.de/id/qucosa%3A71093.
Testo completoAbstract (sommario):
Inverse, multidimensional input-output flow mapping is very important for use of valves in precision motion control applications. Due to the highly nonlinear characteristic and uncertain model structure of the cartridge valves, it is hard to formulate the modelling of their flow mappings into simple parameter estimation problems. This contribution conducts a comprehensive analysis and validation of three- and four-dimensional input-output-mapping approaches for a proportional pilot operated seat valves. Therefore, a virtual and a physical test-rig setup are utilized for initial measurement, implementation and assessment. After modeling and validating the valve under consideration, as a function of flow, pressure and temperature different mapping methods are investigated. More specifically, state of the art approaches, deep-learning methods and a newly developed approach (extPoly) are examined. Especially ANNs and Polynomials show reasonable approximation results even for more than two inputs. However, the results are strongly dependent on the structure and distribution of the input data points. Besides identification effort, the invertibility was investigated.
19
Zhang, Yinxing. "Bioprosthetic heart valves : ultrastructure and calcification". Master's thesis, University of Cape Town, 1998. http://hdl.handle.net/11427/26921.
Testo completoAbstract (sommario):
Sumaary in English.Includes bibliographical references.
Background: Due to the geographic distance between abattoirs and commercial valve plants delays between harvest and fixation usually range from 48 to 72 hours. In order to assess the pre-fixation tissue damage arising from the hypoxic period and the resulting calcific degeneration after implantation, we used an ultrastructural damage score and transmission electron microscopy. Materials and Methods: In a step by step manner, three major issues were clarified: 1) The degree of pre-fixation tissue damage was determined in the four most widely used commercially produced tissue heart valves. Since stentless bioprostheses represent the latest promising trend in the development of biological heart valves, stentless models of the following makes were compared: Baxter, Medtronic, St. Jude and Biocor. Due to the fact that the aortic wall component of these valves proved most resistant to all anticalcification treatments, aortic wall tissue stood in the centre of our analyses. 2) Subsequently, three main determinants of the fixation process namely: delay, temperature and fixative-concentration were varied with the goal of significantly improving the ultrastructural preservation of the bioprosthetic tissue. 3) Eventually, the influence of improved ultrastructural preservation on calcific degeneration was evaluated under in vivo conditions in the non-human primate and the rat model. Results: The comparison of the four most commonly used stentless bioprosthetic heart valves revealed a disturbing degree of tissue damage in all valves. Using a damage score from 1 to 21 (21 being the worst), aortic wall tissue of commercial valves ranged from 10 to 18 and that of leaflet tissue from 12 to 20. When fixation conditions were permutated, tissue damage could almost be abolished by immediate fixation (within 30 minutes of slaughter), low-temperature fixation(4°C) and high glutaraldehyde concentrations (> 1 %). Our in vivo experiments confirmed that commercially used fixation (delayed fixation, room-temperature and I ow concentrations of glutaraldehyde) with its concomitant high degree of tissue damage results in high levels of calcification. Apart from a distinctly improved calcification potential in ultrastructurally well preserved tissue, there was also an inverse correlation between tissue calcification and the concentration of glutaraldehyde used for fixation. Conclusion: We could demonstrate that commercially produced bioprosthetic heart valves uniformly show badly damaged tissue and that tissue damage contributes to the calcific degeneration of these valves. We were also able to determine ideal fixation conditions which in turn significantly reduced tissue calcification.
20
Carroll, Turhan Kendall. "Radiation Damage in GMR Spin Valves". The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1281633368.
Testo completo
21
Batley, Joseph Thomas. "Spin transport in lateral spin valves". Thesis, University of Leeds, 2015. http://etheses.whiterose.ac.uk/12176/.
Testo completoAbstract (sommario):
This thesis outlines the construction of an ultra-high-vacuum angle-deposition system, developed specifically for the fabrication of lateral spin valves (LSVs). The thesis then proceeds to answer two important questions: what causes the loss of spin accumulation at low temperatures seen in LSVs? and how do spin currents interact in multi-terminal circuits? Through a double-dose electron beam lithography and angle-deposition technique, Cu/Py LSVs are fabricated and shown to have transparent contacts. By means of a DC injection method, the nonlocal voltage is measured as a function of injection current, magnetic field and temperature, enabling contributions from heat and spin currents to be isolated. The spin diffusion length is obtained from LSVs fabricated with Cu containing magnetic impurities $ < $1 ppm and $\sim$4 ppm. Temperature dependent charge and spin transport measurements provide evidence linking the presence of the Kondo effect in Cu to the suppression of the spin diffusion length below 30 K. The spin-flip probability for magnetic impurities is found to be 34\%, orders of magnitude larger than other scattering mechanisms. This is extended to explain similar observations in the spin accumulation. These measurements establish the dominant role of Kondo scattering in spin-relaxation, even in low concentrations of order 1 ppm. Finally, a new multi-terminal LSV (MTLSV) is fabricated and the interaction between two spin currents is investigated. Fan-out and fan-in measurements are performed, demonstrating that spin currents separate and combine at junctions in a circuit with magnitudes dictated by the spin resistance of the conduction channels. It is also shown that two spin currents of opposite polarity will cancel out. Whether Kirchhoff's law holds for spin currents is discussed and this chapter helps lay the ground work for spin current based circuits and computation.
22
Pomeroy, Paul E. "Erosive wear failure of spool valves". Thesis, University of Bath, 1995. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261073.
Testo completo
23
Davis, R. Ted. "Aerating Butterfly Valves to Suppress Cavitation". DigitalCommons@USU, 1986. https://digitalcommons.usu.edu/etd/3952.
Testo completoAbstract (sommario):
Proper aeration of cavitating hydraulic equipment can greatly reduce cavitation intensity, noise, and damage. This thesis quantifies the benefit, in terms of damage and noise, from aerating six inch butterfly valve. The incipient damage level of cavitation was obtained for both aerated and non -ae ra ted conditions. The level is defined as one pit per square inch of a soft aluminum test specimen per one minute of operation. A description of the cavitation pits that occurred plus where they appeared is presented. A graph showing the aerated and non-aerated limits of incipient damage is given along with a table showing the percent reduct ion of damage from aeration. A graph and table are also given depicting the reduction in noise. The proper location of aeration ports to allow natural aeration is outlined .
24
Muller, Matthew. "Modeling, design and control of forced-feedback metering poppet valve system". Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/4644.
Testo completoAbstract (sommario):
Thesis (M.S.)--University of Missouri-Columbia, 2006.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on April 17, 2009) Includes bibliographical references.
25
Simpson, Michael S. "An in vitro investigation of systolic anterior motion of the mitral valve". Thesis, Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/33615.
Testo completo
26
Williams, Franklin Pierce. "The numerical simulation of flow through an axisymmetric aortic heart valve". Diss., Georgia Institute of Technology, 1987. http://hdl.handle.net/1853/9378.
Testo completo
27
Gieseking, Elizabeth Robinson. "Control mechanism for the papillary muscles of the mitral valve : an In Vitro study". Thesis, Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/10912.
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28
Juthier, Francis. "Ingénierie tissulaire de valves cardiaques : apport des techniques de thérapie cellulaire". Phd thesis, Université du Droit et de la Santé - Lille II, 2009. http://tel.archives-ouvertes.fr/tel-00433512.
Testo completoAbstract (sommario):
Les prothèses valvulaires cardiaques actuellement commercialisées présentent des inconvénients importants. Les prothèses biologiques (xéno ou allo greffes) sont soumises à une dégénérescence progressive, et les prothèses mécaniques nécessitent un traitement anticoagulant à vie. Des perspectives nouvelles sont apparues avec l'ingénierie tissulaire, dont le but est de créer à partir d'un support non cellularisé (matrice biodégradable de polymère de glycogène, ou xénogreffe décellularisée) un substitut valvulaire « vivant » offrant une meilleur longévité et des performances hémodynamiques optimales, ainsi que des capacités de remodelage et de croissance. Nous avons élaboré un modèle de xénogreffe porcine décellularise implanté en position systémique puis pulmonaire chez un agneau et testé plusieurs modes de recolonisation de cette matrice, par différents types de cellules. La spécificité de notre projet était de tester in vivo la possibilité de recolonisation de matrices valvulaires par des cellules autologues de moelle osseuse, les valves obtenues étant soumises à des forces mécaniques supposées être responsables de la recolonisation cellulaire et de la régénération de la matrice extra-cellulaire. La première étape a été l'optimisation d'un processus de décellularisation. Nous avons retenu un protocole combinant les effets de solutions hypotoniques et d'un détergent anionique, le sodium dodecyl sulfate. L'étape suivante a été de tester les propriétés mécaniques d'une valve xénogénique décellularisée dans un modèle ovin (qui est le modèle de référence en pathologie valvulaire cardiaque), en flux artériel systémique. Une matrice valvulaire porcine a été implantée dans une aorte thoracique descendante chez 6 agneaux. Ses propriétés mécaniques en contrainte systémique étaient bonnes, sans rupture ni dilatation anévrismale, 2 à 16 semaines après l'implantation. Nous avons ensuite testé la possibilité de colonisation de cette valve xénogénique décellularisée porcine par des cellules souches endogènes stimulées par l'injection de granulocyte-Colony Stimulating Factor (G-CSF). Ce montage a été implanté chez 6 agneaux puis une évaluation hémodynamique, macroscopique et histologique a été réalisée à 3, 6, 8 et 16 semaines. Alors que le G-CSF recombinant humain induisait une croissance significative du nombre de globules blancs périphériques chez l'agneau, l'effet sur la valve était délétère, et induisait des réactions inflammatoires et de fibrose ressemblant à un rejet de greffe après 16 semaines. Nous avons ensuite testé les propriétés mécaniques de cette valve après son implantation dans l'artère pulmonaire sous circulation extra-corporelle, ainsi que l'effet de l'injection in situ de cellules autologues mononucléées médullaires (CMM) juste avant l'implantation chirurgicale. Ces valves pulmonaires porcines décellularisées ont été implantées chez 15 agneaux. Chez 9 animaux, des CMM autologues ont été injectées dans la valve immédiatement avant son implantation (groupe CMM), et chez 6 animaux, aucune injection cellulaire n'était réalisée (groupe contrôle). Un suivi de 16 semaines était réalisé comprenant l'étude fonctionnelle de la valve (échographique et angiographique) ainsi qu'une étude histologique. Les évaluations échographiques ne montraient aucune régurgitation dans chacun des 2 groupes et les index de perméabilité valvulaire étaient similaires. Les études histologiques montraient une prolifération myointimale plus importante et des dépôts calciques étaient mis en évidence dans le groupe contrôle par rapport au groupe CMM. Toutes les valves étaient recouvertes d'une monocouche de cellules endothéliales. Cependant chez 2 animaux une infiltration inflammatoire importante était retrouvée dans l'adventice. Ces résultats suggéraient que l'injection in situ de cellules autologues médullaires pouvait réduire la détérioration de valves xénogéniques décellularisées, mais également dans des circonstances inconnues favoriser des réactions inflammatoires locales Nous avons alors supposé que l'injection in situ d'une sous-population de CMM, les cellules souches mésenchymateuses (CSM) pouvaient promouvoir in vivo la recolonisation de nos substituts valvulaire sans entraîner de risque d'augmentation des réactions inflammatoires locales. Nous avons prélevé des CCM autologues 7 jours avant l'implantation chirurgicale. De cet échantillon, des cellules souches mésenchymateuses (CSM) ont été isolées par une culture cellulaire spécifique. Les valves porcines décellularisées ont été implantées dans l'artère pulmonaire sous circulation extra-corporelle chez l'agneau, ayant été au préalable injectées soit de CMM (n=7), soit de CSM (n=7). L'évaluation fonctionnelle des valves a été réalisée après 16 semaines par échographie, les valves ont été explantées en vue de leur analyse macroscopique et histologique. Après 4 mois, les gradients moyens transvalvulaires et distaux étaient significativement plus bas dans le groupe avec CSM que dans le groupe CMM. L'index de perméabilité était également significativement plus élevé dans le groupe CSM que dans le groupe CMM.. L'examen histologique montrait également que la disposition de la matrice et la colonisation cellulaire était plus proche de celle de valve native dans le groupe CSM que dans le groupe CMM.
29
Damen, Bas Stefaan, e bsdamen@hotmail com. "Design, Development, and Optimisation of a Culture Vessel System for Tissue Engineering Applications". Swinburne University of Technology. n/a, 2003. http://adt.lib.swin.edu.au./public/adt-VSWT20040512.125051.
Testo completoAbstract (sommario):
A Tissue Engineering (TE) approach to heart valve replacement has the aim of producing an implant that is identical to healthy tissue in morphology, function and immune recognition. The aim is to harvest tissue from a patient, establish cells in culture from this tissue and then use these cells to grow a new tissue in a desired shape for the implant. The scaffold material that supports the growth of cells into a desired shape may be composed of a biodegradable polymer that degrades over time, so that the final engineered implant is composed entirely of living tissue. The approach used at Swinburne University was to induce the desired mechanical and functional properties of tissue and is to be developed in an environment subjected to flow stresses that mimicked the haemodynamic forces that natural tissue experiences. The full attainment of natural biomechanical and morphological properties of a TE structure has not as yet been demonstrated.
In this thesis a review of Tissue Engineering of Heart Valves (TEHVs) is presented followed by an assessment of biocompatible materials currently used for TEHVs. The thrust of the work was the design and development of a Bioreactor (BR) system, capable of simulating the corresponding haemodynamic forces in vitro so that long-term cultivation of TEHVs and/or other structures can be mimicked. A full description of the developed BR and the verification of its functionality under various physiological conditions using Laser Doppler Anemometry (LDA) are given. An analysis of the fluid flow and shear stress forces in and around a heart valve scaffold is also provided.
Finally, preliminary results related to a fabricated aortic TEHV-scaffold and the developed cell culture systems are presented and discussed. Attempts to establish viable cell lines from ovine cardiac tissue are also reported.
30
Gallocher, Siobhain Lynn. "Durability Assessment of Polymer Trileaflet Heart Valves". FIU Digital Commons, 2007. http://digitalcommons.fiu.edu/etd/54.
Testo completoAbstract (sommario):
The durability of a polymer trileaflet valve is dependent on leaflet stress concentrations, so valve designs that reduce stress can, hypothetically, increase durability. Design aspects that are believed to contribute to reduced leaflet stress include stent flexibility, parabolic coaptation curvature, and leaflet anisotropy. With this in mind, the purpose of this investigation was to elucidate what specific combinations of these parameters promote optimal acute and long-term valve function. A combination of four stent designs, seven leaflet reinforcement materials, and three coaptation geometries were evaluated through a combination of experimentation and modeling. Static tensile and Poisson’s ratio tests and dynamic tensile fatigue testing were used to evaluate the individual leaflet components; and hydrodynamic testing and accelerated valve fatigue was used to assess complete valve prototypes. The two most successful designs included a 0.40 mm thick knit-reinforced valve with a fatigue life of 10.35 years, and a 0.20 mm thick knit-reinforced valve with a 28.9 mmHg decrease in pressure drop over the former. A finite element model was incorporated to verify the impact of the above-mentioned parameters on leaflet stress concentrations. Leaflet anisotropy had a large impact on stress concentrations, and matching the circumferential modulus to that of the natural valve showed the greatest benefit. Varying the radial modulus had minimal impact. Varying coaptation geometry had no impact, but stent flexibility did have a marked effect on the stress at the top of the commissure, where a completely rigid stent resulted in a higher peak stress than a flexible stent (E = 385 MPa). In conclusion, stent flexibility and leaflet anisotropy do effect stress concentrations in the SIBS trileaflet valve, but coaptation geometry does not. Regions of high stress concentrations were linked to failure locations in vitro, so a fatigue prediction model was developed from the S/N curves generated during dynamic tensile testing of the 0.20 mm knit-reinforced leaflets. Failure was predicted at approximately 400 million cycles (10 years) at the top of the commissure. In vitro fatigue of this valve showed failure initiation after approximately 167 million cycles (4.18 years), but it was related to a design defect that is subsequently being changed.
31
Weind, Kirsten L. "Potential oxygenation routes of aortic heart valves". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/NQ58247.pdf.
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32
Popinciuc, Mihăiţă. "Hybrid organic spin valves interfaces and transport /". [S.l. : Groningen : s.n. ; University Library Groningen] [Host], 2007. http://irs.ub.rug.nl/ppn/304590207.
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33
Goodman, Andrew M. "Magnetisation reversal in exchange biased spin-valves". Thesis, Bangor University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.322900.
Testo completo
34
Ellwein, Christian. "Vibration based fault detection for Solenoid valves". Thesis, Northumbria University, 2002. http://nrl.northumbria.ac.uk/236/.
Testo completoAbstract (sommario):
Solenoid valves play a vital role in many machines and systems. If one of these devices breaks down the whole system can be affected. Because of this importance of valves, it is desirable to observe these parts to detect faults, both when they are occurring and before they can cause serious damage. Among several possible methods of observation (monitoring actuation time, electrical current, fluidic parameters and others) the observation of mechanical vibrations is a well known method of observing mechanical systems which is commonly used for observation of rotating machinery, but which includes several challenges for diagnosis of solenoid valves. This thesis investigates the possibilities and advantages of vibration analysis of fault detection for solenoid valves. New algorithms are developed to automatically segment the overall non- stationary raw data to smaller sections with a higher degree of stationarity. These new segments are interpretable in a mechanical sense and they separate different sources of vibration. Furthermore a new method to detect regions of interest in a spectrum for classification without "a priori knowledge" about the process has been developed. The experiments presented in this thesis give the evidence that these new methods of pre¬processing and feature extraction enable reliable classification results for transient signals as they occur in the vibration of a switching valve.
35
Bien, D. C. S. "Micromachined valves and pump for microfluidic applications". Thesis, Queen's University Belfast, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269168.
Testo completo
36
Banerjee, Niladri. "Nonlinear giant magnetoresistance in dual spin valves". Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610064.
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37
Bergqvist, Per, e Linus Kemmler. "Voice Coil Controlled Inspiration and Expiration Valves". Thesis, KTH, Maskinkonstruktion (Inst.), 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-156855.
Testo completoAbstract (sommario):
This master thesis was performed at Maquet Critical Care located in Solna, Stockholm. Maquet Critical Care is a market leader in high performance medical ventilators. A ventilator is a medical device that helps patients to breathe. Two of the most vital components of a ventilator are the valves that are closest to the patient. These are the inspiration valve and the expiration valve. The main purpose with this thesis is to get, theoretical as well as practical insights into the inspiration and expiration valves to be developed. On top of this a ventilator using the developed valves was to be developed. On the base of theoretical knowledge, the aim of the thesis is to achieve a set of valve and ventilator requirements on developed prototypes. On top of this the thesis should be able to deliver advice and guidelines for possible future research on these valves. In order to use valves in a ventilator there were many requirement posed on the valves. These requirements included flow characteristics, power consumption, dynamic performance and cost. The methodology used included a pre study of fluid dynamics, valves and ventilators. Initial valve designs were developed with the use of flow simulations. The valve models were manufactured and tested as well as being compared to the flow simulations. The actuators used for the valves were voice coils. In this thesis a standard actuator was used to control the valves but in order to get the maximum performance an investigation into optimized voice coils were performed. In order to get insight into the control of the valves, a dynamic model of the voice coil were developed. This model was verified to have the same bandwidth as the measured voice coils and valve assembly. In order to realize a pressure controlled ventilation mode the inspiration valve and expiration valve were synchronized. The control algorithms were implemented in Matlab/Simulink with dSPACE. The tested requirements were fulfilled, resulting in valves that have a maximum power consumption of 2 Watts. The pressure controlled ventilation was executed on the developed demonstration ventilator. The performances of this ventilator were desirable and fulfilled the ventilator requirements. Two specializations topics have also been investigated, in the areas of flexure based voice coil suspension and gas modeling.Masteruppsatsen utfördes på Maquet Critical Care i Solna, Stockholm. Maquet Critical Care är marknadsledande inom högpresterande ventilatorer. En ventilator är en maskin som hjälper patienter att andas. Två av de viktigaste komponenterna i en ventilator är de två ventilerna som är placerade närmast patienten, nämligen inspirations- och exspirationsventilen. Huvudsyftet med denna masteruppsats är att få både en teoretisk och praktisk insikt av de inspirations och exspirationsventilerna som skulle utvecklas. Utöver detta har en ventilatorprototyp utvecklats med hjälp av dessa ventiler. Utifrån teoretisk kunskap är målet med uppsatsen att uppfylla ett antal krav på ventilerna och ventilatorn. Dessutom kommer uppsatsen att leda till riktlinjer för fortsatt forskning och utveckling av de framtagna ventilerna. För att möjliggöra användandet av ventilerna i en ventilator kom Maquet med många olika krav. Kraven var fördelade inom flödeskaraktäristik, effektförbrukning, dynamisk prestanda och kostnad. I arbetsmetoden som användes ingick en förstudie av flödesdynamik, ventiler och ventilatorer. Ventilernas design utvecklades med hjälp av flödessimuleringar. De tillverkade ventilmodellerna testades, dels för att verifiera krav och dels för att få en insikt om flödesimuleringarnas tillförlitlighet. De aktuatorer som skulle användas var talspolar. I denna uppsats vi har använt oss av en standardaktuator men för att få maximal prestanda har en undersökning angående en optimerad talspole gjorts. För att få insikt i ventilernas dynamiska prestanda, utvecklades en dynamisk modell av talspolen och ventilsätet. Bandbredden för denna modell har verifierats med bandbredden på uppmätta ventiler. För att realisera en tryckstyrd ventilation behövde inspirations- och exspirationsventilerna synkroniseras. Denna synkronisering och reglering implementerades i Matlab/Simulink och exekverades med dSPACE. Alla verifierade krav har uppfyllts och har resulterat i två ventiler som maximat förbrukar 2 Watt. Den tryckstyrda ventilationen exekverades på den framtagna ventilatorprototypen och dess prestanda uppfyller ställda krav. Två specialiseringar har också blivit undersökta. Dessa är inom flexure baserad upphängning av talspole samt hur gasen genom ventilator kan modelleras på lämpligast sätt.
38
Malakhatka, Elena. "Diagnostics of valves on the gas pipeline". Thesis, KTH, Kraft- och värmeteknologi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-101377.
Testo completoAbstract (sommario):
The work of the gas transmission pipeline system is regulated by the thousands of valveses and other elements of the shut-off valves, located in different places. Information about the current status of each valve, and confidence in its technical serviceability is an important element in the control of pipeline system. There are a number of problems that adversely affect the valving operation. Principal among them - is the lack of siystematical approach in valving diagnostics. To solve this problem, developed a new approach to the valving diagnostics – multi-level diagnostics. Depending on the level of diagnosis, we get a different amount of information about the object. The focus is on the 3rd level of diagnosis, which allows to determine the leackages of valves, and identify type of defects quantitatively. This level of diagnosis is regarded as an example of Method «Micropuls». The method «Micropuls» based on the theory of mechanical vibrations and forced with high accuracy to set the time, frequency and spectral characteristics of noise in the details of the valve, and their spatial location and intensity of that in the presence of a system of analysis allows to determine the state of the object, its faults and defects. Micropulse technology is based on the impact on the measured object micropower impulses, records the response and subsequent filtering, decoding and analyzing the data. Specially designed calibrated pulses can effectively influence the measured objects (valve) over a wide range of structural dimensions and mounting schemes, regardless of physical location of the valve.
39
Alaya, Oussama, e Maik Fiedler. "Optimal pressure control using switching solenoid valves". Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-200545.
Testo completoAbstract (sommario):
This paper presents the mathematical modeling and the design of an optimal pressure tracking controller for an often used setup in pneumatic applications. Two pneumatic chambers are connected with a pneumatic tube. The pressure in the second chamber is to be controlled using two switching valves connected to the first chamber and based on the pressure measurement in the first chamber. The optimal control problem is formulated and solved using the MPC framework. The designed controller shows good tracking quality, while fulfilling hard constraints, like maintaining the pressure below a given upper bound.
40
Yap, Cheng-Hon. "Factors influencing cryopreserved allograft heart valve degeneration". Connect to thesis, 2006. http://repository.unimelb.edu.au/10187/2120.
Testo completoAbstract (sommario):
Heart valve replacement is becoming more commonplace in developed nations. Despite this the ideal valve prosthesis has not been found. The allograft valve has been used for over 40 years and remains an important prosthesis with many advantages. However, like other biological valve prosthesis, they have a finite durability. The causes of allograft valve degeneration are still unknown. The study aims to identify factors associated with cryopreserved allograft valve degeneration. Knowledge of such factors will improve our understanding of the potential causes and mechanisms of allograft heart valve degeneration. (For complete abstract open document)
41
Perez, Thierry. "Traitement fibrinolytique des thromboses de valves cardiaques mécaniques chez l'enfant". Bordeaux 2, 1998. http://www.theses.fr/1998BOR23092.
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42
Nakatsu, Taro. "Intermediate-term outcomes of aortic valve replacement with bioprosthetic or mechanical valves in patients on hemodialysis". Kyoto University, 2020. http://hdl.handle.net/2433/252979.
Testo completo
43
Peacock, Jacqueline D. "The Role of Sox9 in Heart Valve Development and Disease". Scholarly Repository, 2011. http://scholarlyrepository.miami.edu/oa_dissertations/543.
Testo completoAbstract (sommario):
Heart valve structures open and close during the cardiac cycle to provide unidirectional blood flow through the heart, critical for efficient cardiovascular function. Valve dysfunction results in either incomplete opening or incomplete closure of the valve. Both types of valve dysfunction decrease efficiency of blood flow, increasing the load on the myocardium and leading to secondary heart disease such as pathological hypertrophy and heart failure. There are currently no effective treatments to prevent or slow the progression of valve disease, and there are no pharmacological treatments for advanced valve disease. Although most valve disease is associated with aging, increasing evidence suggests that valve disease often has origins in development. Congenital valvuloseptal defects affect many newborns, ranging from life-threatening malformations requiring immediate repair to more subtle, often undiagnosed defects that increase susceptibility to valve disease later in life. Therefore, an improved understanding of the mechanisms of heart valve formation and maintenance of adult valves may serve as an important step in improving valve disease treatment options. In this work, the mechanisms of normal valve development and the role of Sox9 in developing and mature valves are further studied. The temporal and spatial expression of extracellular matrix genes and proteins are examined throughout normal murine valve development. Sox9 function in the processes of valve development and valve maintenance is examined using mouse models of conditional Sox9 loss-of-function. Heart valve phenotypes in mice with reduced Sox9 function are examined throughout development and in adult mice with resultant calcific valve disease. The possible causative mechanisms of calcific valve disease in mice with reduced Sox9 function are further investigated by identification of novel possible targets of Sox9 transcriptional regulation. Together these studies improve our understanding of heart valve development, characterize a model of heart valve calcification with genetic etiology, and identify and characterize novel targets of Sox9.
44
Pope, Larry G. "Effect of Engineered Surfaces on Valve Performance". Thesis, University of North Texas, 2000. https://digital.library.unt.edu/ark:/67531/metadc2670/.
Testo completoAbstract (sommario):
Performance of air operated valves is a major maintenance concern in process industries. Anecdotal information indicates that reliability of some high maintenance valves has been improved by using an ion deposition process to achieve engineered surfaces on selected components. This project compared friction for various surface treatments of selected valve components. Results indicate valve performance may be slightly more consistent when an engineered surface is applied in the valve packing area; however surface treatment in this area does not appear to have a dominant affect on reducing valve friction. Results indicate a linear relation between stem friction and torque applied to packing flange nuts, and even after a valve is in service, controlled packing adjustments can be made without significantly changing valve stroke time.
45
Simon, Hélène A. "Influence of the implant location on the hinge and leakage flow fields through bileaflet mechanical heart valves". Available online, Georgia Institute of Technology, 2004:, 2003. http://etd.gatech.edu/theses/available/etd-04012004-192539/unrestricted/helene%5Fsimon%5Fa%5F200405%5Fmast.pdf.
Testo completoAbstract (sommario):
Thesis (M.S.)--Chemical Engineering, Georgia Institute of Technology, 2003.Sambanis Athanassios, Committee Member ; Sotiropoulos Fotis, Committee Member ; Yoganathan Ajit, Committee Chair. Includes bibliographical references (leaves 239-243).
46
Luo, Yamin. "System modeling and control design of a two-stage metering poppet-valve system". Diss., Columbia, Mo. : University of Missouri-Columbia, 2006. http://hdl.handle.net/10355/4453.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--University of Missouri-Columbia, 2006.The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on July 18, 2008) Vita. Includes bibliographical references.
47
Zappe, Ann-Katrin Reglinde [Verfasser]. "Follow-Up bei Patienten nach perkutaner Valve-in-Valve Aortenklappenimplantation : Patients with transcatheter valve-in-valve implantation for failed surgical aortic valves / Ann-Katrin Reglinde Zappe". Berlin : Medizinische Fakultät Charité - Universitätsmedizin Berlin, 2020. http://d-nb.info/1223928322/34.
Testo completo
48
Leung, Wing-ki Vikki, e 梁頴琪. "The implications of transcatheter aortic valve implantation (TAVI) adoption". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2012. http://hub.hku.hk/bib/B48424031.
Testo completoAbstract (sommario):
Aortic stenosis is a life-threatening valvular heart disease. At the onset of symptoms, a patient’s prognosis becomes poor and the risk of death rapidly increases. Aortic valve replacement surgery remains the gold standard in treatment for aortic stenosis. However, in the total population of patients with severe aortic stenosis, about one third are deemed inoperable due to their high surgical risk. In recent years, the development of transcatheter aortic valve implantation (TAVI), a non-invasive heart valve replacement procedure brought hope for the elderly, high-risk and inoperable aortic stenosis patient population pool. A literature review was performed to examine the safety, efficacy and effectiveness evidence for transcatheter aortic valve treatment option. The results showed that TAVI is a safe treatment option, however the effectiveness for the whole patient population is unknown. The adoption of this alternative treatment option is certainly coupled with multiple dimension of impact from a public health perspective. It remains inconclusive whether TAVI is an effective treatment option to be adopted.published_or_final_version
Public Health
Master
Master of Public Health
49
Southern, Lisa Jane. "Identification of glutaraldehyde induced structures in bioprosthetic heart valves using mass spectrometry : an insight into valve failure". Thesis, University of Sheffield, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366113.
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50
Kontz, Matthew Edward. "Haptic Control of Hydraulic Machinery Using Proportional Valves". Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19876.
Testo completoAbstract (sommario):
Supplying haptic or force feedback to operators using hydraulic machinery such as excavators has the potential to increase operator capabilities. Haptic, robotic, human-machine interfaces enable several enhancing features including coordinated motion control and programmable haptic feedback. Coordinated or resolved motion control supplies a more intuitive means of specifying the equipment's motion. Haptic feedback is used to relay meaningful information back to the user in the form of force signals about digging force acting on the bucket, programmable virtual constraints and system limitations imposed by the mechanism, maximum pressure or maximum flow. In order to make this technology economically viable, the benefits must offset the additional cost associated with implementation. One way to minimize this cost is to not use high-end hydraulic components. For smaller backhoes and mini-excavators this means that the hydraulic systems are comprised of a constant displacement pump and proportional direction control valves. Hydraulic and haptic control techniques suitable for backhoes/excavators are developed and tested on a small backhoe test-bed. A virtual backhoe simulator is created for controller design and human evaluation. Not only is the virtual simulator modeled after the test-bed, but the control algorithm used in the simulator is the same as the actual backhoe test-bed. Data from human subject tests are presented that evaluate the control strategies on both the real and virtual backhoe. The end goal of this project is to incorporate coordinated haptic control algorithms that work with low-cost systems and maximize the enhancement of operator capabilities.