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The Global ETD Search service is a free service for researchers
to find electronic theses and dissertations. This service is
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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.
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Showing 31 to 39 of 39 (0.032 seconds)| 31 |
Hydrodynamics of plane liquid jets aimed at applications in paper manufacturingSöderberg, Daniel January 1999 (has links)
Process industries are in general depending, in one way or the other, on fluid mechanics.Specifically, paper manufacturing, which probably is the dominant processindustry in Sweden, is depending on the flow of cellulose fibres suspended in water.As a part of the process the suspension, consisting of fibres in water, is spread out onor between two moving permeable weaves, i.e. wires. The speed of this is usually 10–30 m/s and the suspension is spread out by a plane jet issuing from a headbox nozzle.It has been show that the conditions in the headbox and jet have a large influence onthe quality of the final paper sheet. Primarily, streaks in the paper sheet are believedto be the result of streamwise streaks in the headbox jet.The thesis is aimed at the flow phenomena which occur in the headbox jet. Theinvestigations have been made with numerical calculations, stability theory and modelexperiments using water, as well as experiments with a real paper machine headboxand fibre suspension. In the thesis an introduction to the hydrodynamics of planeliquid jets is presented together with a description of the paper forming process andthe fluid mechanics of headbox flow.The basic flow and stability of a two-dimensional plane liquid jet has been investigatedby numerical calculations, stability theory and experiments. The calculationsof the laminar basic flow is successfully compared to pitot-tube measurements of thestreamwise velocity profile. By visualisations of the flow it is found that wave disturbanceson the jet has a severe effect on the flow. These waves can be predicted bylinear stability theory, which shows the presence of five convectively unstable modes.These can be divided into three types and by comparison with the experiments thetype of the visible waves is determined. These waves seem to initiate a break-up ofthe jet, which leads to strong streamwise streaks inside the jet.By flow visualisation of headbox flow of an experimental paper machine, togetherwith analysis of the resulting paper structure using the wavelet method the correspondencebetween flow disturbances and paper quality was investigated. It was shownthat the wave instability, which is present on the low Reynolds number water jet, alsocan be found in the real the headbox jet. It is shown that these waves play an importantrole in the dynamics of the headbox jet and also have an influence on the final papersheet. / QC 20100825
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Non-thermal processes on ice and liquid micro-jet surfacesOlanrewaju, Babajide O. 19 January 2011 (has links)
Processes at the air-water/ice interface are known to play a very important role in the release of reactive halogen species with atmospheric aerosols serving as catalysts. The ability to make different types of ice with various morphologies, hence, different adsorption and surface properties in vacuum, provide a useful way to probe the catalytic effect of ice in atmospheric reactions. Also, the use of the liquid jet technique provides the rare opportunity to probe liquid samples at the interface; hitherto impossible to investigate with traditional surface science techniques. Studies of reactions on both ice and liquid surfaces at ambient conditions are usually complicated by the rapid desorption and adsorption processes due to the high evaporation rates at the surface. To gain a better understanding and improve modeling of several atmospheric relevant reactions, it is therefore important to develop laboratory techniques that provide an opportunity to investigate non-thermal reactions on both ice and liquid surfaces. Detailed investigation of the interactions of atmospheric relevant molecules (methyl iodide and hydrogen chloride) on water ice at low temperature in UHV conditions has been carried out. These interactions were studied using different techniques such as temperature programmed desorption (TPD), electron stimulated desorption (ESD) and resonance enhanced multiphoton ionization (REMPI). Unlike probing reactions on ice surfaces, investigating air/liquid interfaces present several challenges. This is because traditional surface science techniques require an ultra high vacuum environment to prevent distortion of information due to interference from equilibrium vapor above the liquid surface during data acquisition. The liquid jet technique facilitates the direct study of continually renewed liquid surfaces in high vacuum, thereby preventing the constant changing of the properties and composition of the liquid surface due to the aging process (diffusion of impurities or liquid constituent). A linear time-of-flight mass spectrometer has been used to monitor ion ejection during laser irradiation of liquid jet containing aqueous solutions and pure water. Since these ions are ejected exclusively from the surface of the liquid and the cluster distributions observed are influenced by the local structure, these experiments provide a sensitive probe of the liquid vacuum interface of these solutions. Though the research is fundamental, the results obtained from these investigations indicate how the discontinuity of bulk properties on the surface of both ice and aqueous solutions affects interfacial reactions.
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Využití nekonvenčních paprskových technologií (LASER/vodní paprsek) ve strojírenství / Utilization unconventional beam technology (LASER/water jet) in machineHoráček, Lukáš January 2008 (has links)
This thesis deal with utilization unconventional beam technology (LASER/ water jet) in machine. The aim works is analysis technological possibility method laser and analysis technological possibility method water jet. Further aim of this work is mutual trade-off of the both methods unconventional beam technology and technical economic interpretation.
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Nekonvenční metoda obrábění vodním paprskem / The unconventional Method of Cutting Water JetPrax, Ondřej January 2011 (has links)
The master thesis contains a literature search dealing with the unconventional technology of water jet cutting. The first part is aimed to description of the water jet technology and shows the possibilities of controlling and awarding data for cutting. Follows the chapter about the influences of the input parameters to a surface quality and machining accuracy. The thesis also giving an account of ecological factors of the water jet cutting. The description of the whole technology for production of the sprocket is a part of the thesis. The last part of the thesis is solving the economics of the water jet cutting technology, especially for production of the sprocket with comparing dependency of the cutting conditions.
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Characterization and Control of an Electrospinning ProcessLiu, Kaiyi 18 June 2013 (has links)
No description available.
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Erzeugung und Untersuchung von schnellen Mikrotropfen für Reinigungsanwendungen / Generation and investigation of fast micro drops with respect to cleaning applicationsFrommhold, Philipp Erhard 20 May 2015 (has links)
Seit mehr als einem Jahrhundert ist ein wachsendes wissenschaftliches Interesse an Tropfen und den Vorgängen bei deren Aufprall auf die verschiedensten Substrate zu verzeichnen, wohl auch durch die Fotografien von Worthington (1908) ausgelöst. Inzwischen wurden viele Erkenntnisse durch große Fortschritte bei der experimentellen Untersuchung (z.B. mittels Hochgeschwindigkeitsaufnahmen) und durch theoretische und computergestützte Untersuchung (z.B. durch skalenfreie und numerische Modellierung) gewonnen. Trotzdem bleibt durch die Vielfältigkeit und Komplexität der Phänomene während des Tropfenaufpralls sowie wegen der ständig erweiterten Anwendungsbereiche dieses Forschungsgebiet hochaktuell. Insbesondere sehr kleine und gleichzeitig sehr schnelle Tropfen (Tropfendurchmesser 10µm bis 100µm, Tropfengeschwindigkeit 10m/s bis 100m/s) kommen in vielen modernen Anwendungen vor (z.B. Verbrennungsmotoren, Tintenstrahldrucker, Reinigung von Oberflächen). In diesem wichtigen, aber für Untersuchungen schwer zugänglichen Parameterbereich gibt es immer noch offene Fragen. Die vorliegende Arbeit beschäftigt sich daher mit diesen schnellen Mikrotropfen in Bezug auf ihre Herstellung und den Aufprallvorgang auf ein festes, trockenes oder benetztes Substrat.
Zunächst wird eine Methode zur Erzeugung eines Hochgeschwindigkeitssprays realisiert, welche auf dem durch Ultraschall gesteuerten Plateau-Rayleigh-Zerfall eines Flüssigkeitsstrahls beruht. Sie ermöglicht es, sowohl Tropfengröße als auch –geschwindigkeit präzise und mit hoher Reproduzierbarkeit über den gesamten oben angegebenen Parameterbereich einzustellen. Durch gezielte Manipulation eines Einzeltropfens durch elektrische Felder wird anschließend der Tropfenaufprall auf Substrate unterschiedlicher Benetzbarkeit mit sehr hoher zeitlicher Auflösung (ca. 100 Mio. Bilder pro Sekunde) bei gleichzeitig hoher räumlicher Auflösung (< 1µm) untersucht. Es zeigt sich, dass bekannte Modelle für langsamere und größere Tropfen im Millimeterbereich auch für schnelle Mikrotropfen Gültigkeit behalten. Somit ist bei gleichen dimensionslosen Kennzahlen (z.B. Reynolds-Zahl, Weber-Zahl, Ohnesorge-Zahl) eine skalenfreie Beschreibung des Tropfenaufpralls möglich. Schließlich wird die Methode zur Tropfenerzeugung auf einen für Anwendungen in der Reinigung relevanten Fall übertragen. Hierbei geht es um den Tropfenaufprall auf ein von einem Flüssigkeitsfilm überströmten Substrat. Es werden die während des Auftreffvorgangs auftretenden Geschwindigkeiten in der sich bildenden radialen Strömung in Abhängigkeit von verschiedenen Prozessparametern bestimmt. Aus den Ergebnissen lassen sich Aussagen über die zu erwartende Reinigungswirkung durch derartige Tropfen und den Einfluss der Prozessparameter treffen.
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Experimental Investigation of superheated liquid jet atomization due to flashing phenomenaYildiz, Dilek 19 September 2005 (has links)
The present research is an experimental investigation of the atomization of a superheated pressurized liquid jet that is exposed to the ambient pressure due to a sudden depressurization. This phenomena is called flashing and occurs in several industrial environments.<p><p>Liquid flashing phenomena holds an interest in many areas of science and engineering. Typical examples one can mention: a) the accidental release of flammable and toxic pressure-liquefied gases in chemical and nuclear industry; the failure of a vessel or pipe in the form of a small hole results in the formation of a two-phase jet containing a mixture of liquid droplets and vapor, b) atomisation improvement in the fuel injector technology, c) flashing mechanism occurrence in expansion devices of refrigerator cycles etc. The interest in flashing events is especially true in the safety field where any unexpected event is undesirable. In case of an accident, flammable or toxic gas clouds are anticipated in close regions of the release because of the sudden phase change .Due to the non-equilibrium nature of the flow in these near field regions, conducting accurate data measurements for droplet size and velocity is a challenging task resulting in scarce data in the very close area.<p><p>This research has been carried out at the von Karman Institute (VKI) within the 5th framework of European Commission to fulfill the goal of understanding of source processes in flashing liquids in accidental releases. The program is carried out under name of FLIE (Flashing Liquids in Industrial Environments)(Contract no: EVG1-CT-2000-00025). The specific issues that are presented in this thesis study are the following:a) a comprehensive state of art of the jet break up patterns, spray characteristics and studies related to flashing phenomena; b)flashing jet breakup patterns and accurate characterization of the atomized jet such as droplet diameter size, velocity and temperature evolution through carefully designed laboratory-scale experiments; c) the influence of the initial storage conditions on the final atomized jet; d) a physical model on the droplet transformation and rapid evaporation in aerosol jets.<p><p>In order to characterize the atomization of the superheated liquid jet, laser-based optical techniques like Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA) are used to obtain information for particle diameter and velocity evolution at various axial and radial distances. Moreover, a high-speed video photography presents the possibility to understand the break-up pattern changes of the simulating liquid namely R-134A jet in function of driving pressure, superheat and discharge nozzle characteristics. Global temperature measurements with an intrusive technique such as thermocouples, non-intrusive measurements with Infrared Thermography are performed. Cases for different initial pressures, temperatures, orifice diameters and length-to-diameter ratios are studied. The break-up patterns, the evolution of the mean droplet size, velocity, RMS, turbulence<p>intensity and temperature along the radial and axial directions are presented in function of initial parameters. Highly populated drop size and velocity count distributions are provided. Among the initial storage conditions, superheat effect is found to be very important in providing small droplets. A 1-D analytical rapid evaporation model is developed in order to explain the strong temperature decrease during the measurements. A sensitivity analysis of this model is provided.<p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished
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Bildgebung und chemische Analytik mit Laserdesorptions-Massenspektrometrie im Bereich Forensik und Astrophysik / Imaging and Chemical Analysis with Laser Desorption Mass Spectrometry in Forensics and AstrophysicsBeinsen, Alexander 21 June 2011 (has links)
No description available.
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Constructing and Commissioning HELIOS – A High Harmonic Generation Source for Pump-Probe Measurements with sub 50 fs Temporal Resolution : The Development of Experimental Equipment for Extreme Ultraviolet SpectroscopyTerschlüsen, Joachim A. January 2016 (has links)
This thesis presents HELIOS, an in-house laboratory for time-resolved pump-probe spectroscopy with extreme-ultraviolet (XUV) probe radiation. A wide span of pump wavelengths can be generated using commercial laser equipment while XUV probe radiation is generated via a high harmonic generation process in a noble gas delivering probe photons with energies between 20 eV and 72 eV. The XUV beam path features a time-preserving monochromator and was constructed and built in-house. HELIOS features an overall time resolution of about 50 fs when using 800 nm pump and 41 eV probe photons. An energy resolution of 110 meV at 41 eV photon energy can be achieved. HELIOS features two beamlines. One µ-focus beamline with an XUV focal size of about 20 µm can be used with experiments that require such a small XUV focal size as well as with different end stations. The other beamline features a semi-permanently mounted end station for angle-resolved photoelectron spectroscopy under ultra-high vacuum conditions. Experiments demonstrating the usability of HELIOS and the two beamlines are presented. A pump-probe measurement on graphene demonstrates the capability of determining a large part of the k-space in only one measurement due to the use of an ARTOF angle-resolved time-of-flight electron spectrometer. A non-angle-resolved pump-probe measurement on the conducting polymer PCPDTBT demonstrates the high signal-to-noise ratio achievable at this beamline in non-angle-resolved photoelectron-spectroscopy pump-probe measurements. The usability of the µ-focus beamline is demonstrated with time-resolved measurements on magnetic samples employing an in-house-designed spectrometer. These experiments allow the retrieval of element-specific information on the magnetization within a sample employing the transversal magneto-optical Kerr effect (T-MOKE). Additionally, a Fourier transform spectrometer for the XUV is presented, the concept was tested at a synchrotron and it was used to determine the longitudinal coherence of the XUV radiation at HELIOS.
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