3,451 research outputs found

    Engineering of freestanding films

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    The PhD work is focuses on the study, characterization and formation of freestanding films. A film is a system in which two dimensions prevail over the third one. In case of freestanding films both surfaces are free; they can be supported by a solid mount of any geometry or float on a liquid, even bubbles are freestanding films. Many innovations were achieved such as a new quantitative, self-reference and full-field thickness map measurement technique. New advances in understanding the phenomenon of leveling of freestanding thin liquid films were also achieved. A totally new device to handle and form freestanding films was designed and build, this new device has a very high number of applications which are not only related directly to freestanding films. This device is capable of overcome many limitations and problems of other devices that forms freestanding films. Moreover, a new protocol for particle deposition on liquid-air (or liquid-liquid) was design. Finally, two novel methods were invented to control the buckling instability of Graphene Sheets particles with many possible applications in stretchable electronics and much more

    3D Eulerian modeling of thin rectangular gas-solid fluidized beds: Estimation of the specularity coefficient and its effects on bubbling dynamics and circulation times

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    This study aims at investigating the influence of the wall boundary conditions and specifically the specularity coefficient on the fluidization behavior of a thin rectangular fluidized bed by means of 3D numerical simulation employing an Eulerian description of the gas and the solid phases. Thin rectangular fluidized beds have been extensively used in the research literature since it is assumed that the flow behaves like a simpler two-dimensional flow and hence they offer validation data for 2D simulations. However, the effects of the front and the back walls are significant, influencing the sensitivity of the fluidization hydrodynamics to the third dimension whose consideration is thus necessary. In order to investigate the influence of the specularity coefficient, Ď• (a parameter controlling the momentum transfer from the particles to the wall), on the fluidization hydrodynamics, a parametric analysis is conducted and the response of the bubble dynamics, reflecting the gasmotion, and the circulation fluxes, displaying the solids motion, are examined in detail. The computational results are compared with available experimental data in order to determine the values of Ď• that lead to the accurate description of the fluidization hydrodynamics via a two-fold validation strategy which involves the calculation of the circulation time and the solids concentration maps. It is observed that the appropriate value of the specularity coefficient depends rather strongly on the superficial gas velocity of the bed.BP (Firm

    Experimental characterisation of bubbly flow using MRI

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    This thesis describes the first application of ultra-fast magnetic resonance imaging (MRI) towards the characterisation of bubbly flow systems. The principle goal of this study is to provide a hydrodynamic characterisation of a model bubble column using drift-flux analysis by supplying experimental closure for those parameters which are considered difficult to measure by conventional means. The system studied consisted of a 31 mm diameter semi-batch bubble column, with 16.68 mM dysprosium chloride solution as the continuous phase. This dopant served the dual purpose of stabilising the system at higher voidages, and enabling the use of ultra-fast MRI by rendering the magnetic susceptibilities of the two phases equivalent. Spiral imaging was selected as the optimal MRI scan protocol for application to bubbly flow on the basis of its high temporal resolution, and robustness to fluid flow and shear. A velocimetric variant of this technique was developed, and demonstrated in application to unsteady, single-phase pipe flow up to a Reynolds number of 12,000. By employing a compressed sensing reconstruction, images were acquired at a rate of 188 fps. Images were then acquired of bubbly flow for the entire range of voidages for which bubbly flow was possible (up to 40.8%). Measurements of bubble size distribution and interfacial area were extracted from these data. Single component velocity fields were also acquired for the entire range of voidages examined. The terminal velocity of single bubbles in the present system was explored in detail with the goal of validating a bubble rise model for use in drift-flux analysis. In order to provide closure to the most sophisticated bubble rise models, a new experimental methodology for quantifying the 3D shape of rising single bubbles was described. When closed using shape information produced using this technique, the theory predicted bubble terminal velocities within 9% error for all bubble sizes examined. Drift-flux analysis was then used to provide a hydrodynamic model for the present system. Good predictions were produced for the voidage at all examined superficial gas velocities (within 5% error), however the transition of the system to slug flow was dramatically overpredicted. This is due to the stabilising influence of the paramagnetic dopant, and reflects that while drift-flux analysis is suitable for predicting liquid holdup in electrolyte stabilised systems, it does not provide an accurate representation of hydrodynamic stability. Finally, velocity encoded spiral imaging was applied to study the dynamics of single bubble wakes. Both freely rising bubbles and bubbles held static in a contraction were examined. Unstable transverse plane vortices were evident in the wake of the static bubble, which were seen to be coupled with both the path deviations and wake shedding of the bubble. These measurements demonstrate the great usefulness for spiral imaging in the study of transient multiphase flow phenomena.This work was supported by the Cambridge Australia Trust and Trinity College, Cambridg

    Optical Techniques for Experimental Tests in Microfluidics

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    This PhD dissertation deals with the use of optical, non-invasive measurement techniques for the investigation of single and two-phase flows in microchannels. Different experimental techniques are presented and the achieved results are critically discussed. Firstly, the inverse use of the micro Particle Image Velocimetry technique for the detection of the real shape of the inner cross-section of an optical accessible microchannel is shown by putting in evidence the capability of this technique to individuate the presence of singularities along the wetted perimeter of the microchannel. Then, the experimental measurement of the local fluid temperature using non-encapsulated Thermochromic Liquid Crystal particles is discussed. A deep analysis of the stability of the color of these particles when exposed to different levels of shear stress has been conducted by demonstrating that these particles can be used for simultaneous measurements of velocity and temperature in water laminar flows characterized by low Reynolds numbers (Re < 10). A preliminary experiment where the TLC thermography is coupled to the APTV method for the simultaneous measurement of the three-dimensional velocity and temperature distribution in a microchannel is shown. Finally, an experimental analysis of the different flow patterns observed for an adiabatic air-water mixture generated by means of a micro T-junction is discussed. The main air-water mixture features have been deeply observed in 195 different experimental conditions in which values of superficial velocity ranging between 0.01 m/s and 0.15 m/s for both the inlet flows (air and water) are imposed. The flow patterns of the air-water mixture are strongly influenced by the value of the water superficial velocity; on the contrary, the air superficial velocity plays a secondary role for the determination of the characteristics of the bubbles (i.e. length)

    Construction and execution of experiments at the multi-purpose thermal hydraulic test facility TOPFLOW for generic investigations of two-phase flows and the development and validation of CFD codes - Final report

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    The works aimed at the further development and validation of models for CFD codes. For this reason, the new thermal-hydraulic test facility TOPFLOW was erected and equipped with wire-mesh sensors with high spatial and time resolution. Vertical test sections with nominal diameters of DN50 and DN200 operating with air-water as well as steam-water two-phase flows provided results on the evaluation of flow patterns, on the be¬haviour of the interfacial area as well as on interfacial momentum and heat transfer. The validation of the CFD-code for complex geometries was carried out using 3D void fraction and velocity distributions obtained in an experiment with an asymmetric obstacle in the large DN200 test section. With respect to free surface flows, stratified co- and counter-current flows as well as slug flows were studied in two horizontal test channels made from acrylic glass. Post-test calculations of these experiments succeeded in predicting the slug formation process. Corresponding to the main goal of the project, the experimental data was used for the model development. For vertical flows, the emphasis was put on lateral bubble forces (e.g. lift force). Different constitutive laws were tested using a Multi Bubble Size Class Test Solver that has been developed for this purpose. Basing on the results a generalized inhomogeneous Multiple Size Group (MUSIG) Model has been proposed and implemented into the CFD code CFX (ANSYS). Validation calculations with the new code resulted in the conclusion that particularly the models for bubble coalescence and fragmentation need further optimisation. Studies of single effects, like the assessment of turbulent dissipation in a bubbly flow and the analysis of trajectories of single bubbles near the wall, supplied other important results of the project

    3D-flow measurement by stereo imaging

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    Ein neues Verfahren zur Messung des dreidimensionalen Strömungsfeldes mittels 'Particle Tracking Velocimetry' wird vorgestellt. Ein zweidimensionales Particle Tracking Velocimetry liegt dem Verfahren zugrunde. Es wurde auf die dritte Raumdimension erweitert, um den gesamten physikalischen Raum zu erfassen. Dieses Verfahren erlaubt, das Lagrange´sche Strömungsfeld zu bestimmen, woraus auch das von vielen anderen Verfahren erhaltene Euler´sche Geschwindigkeitsfeld berechnet werden kann. Das für den Einsatz am Wind-Wellen-Kanal entwickelte Kalibrierverfahren ermöglicht eine hohe räumliche Auflösung. Der Stereokamera-Aufbau und der Versuchsaufbau wurde für die Strömungsmessungen optimiert. Dabei wurde erstmals ein Flüssigkeitsprisma und eine Scheimpflug-Kamera-Anordnung eingesetzt. Numerische Rechnungen unter Verwendung der finite Elemente-Methode zeigen die Komplexität des Problems, freie Wasseroberflächen mit windinduzierten Scherkräften als Randbedingung zu behandeln. Sie machen deutlich, daß experimentelle Untersuchungen unerlässlich sind, um Phänomene wie das Abtauchen von oberflächennahen Flüssigkeitselementen in tiefere Schichten zu beschreiben. Strömungsmessungen wurden an einem neu konstruierten Wind-Wellen-Kanal (AEOLOTRON) und am kleineren Vorgängermodell mit dem neuentwickelten Bildverarbeitungsverfahren durchgeführt. Die Strömungsfelder von windinduzierten Wasserwellen konnten auf diese Weise Geschwindigkeitsfeld und 'Turbulenzzustand' charakterisiert werden

    Microscale investigation of complex liquids porous media

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    The interaction of Non-Newtonian fluid and porous media is a topic of great scientific interest and it finds application in a wide number of industrial fields: from the enhanced oil recovery to the drug delivery. In particular, a very interesting category of liquids are the so-called property called "yield stress fluids". These systems have the ability to behave as solids under this stress threshold and to flow as liquid above it. Numerous studies have been performed in this sense even if the mechanism on a microscale level still have not been completely elucidated. In this work, we identified some model systems such as polyacrylic acid (Carbopol)-water solutions and by the use of confocal microscopy we looked at the microstructure of the gel to understand the origin of the yield stress and how the confinement and the flow have an impact on this microstructure. It has been observed that the swollen particles of Carbopol build a 3D network whose connectivity causes the yield stress. Furthermore, the system can be described as two phases at equilibrium since the particle concentration does not influence the properties of the solvent phase. Changing the temperature, a phase diagram has been drafted finding analogies with typical polymeric systems, finding a miscibility gap. These learnings helped developing a novel experimental tool based on a torsion pendulum equipped with a magnetic dipole and a rotating cylinder immersed in the material, to measure yield stress in gels able to discriminate dynamic and static yield stress. At the end, an alternative system as detergent foams, has been studied focusing on the process of formation in sponges, in order to understand the effect of surfactant and sponge material on the foamability of the system. Our experimental data revealed that using a lower confinement in the foam formation allows the production of a drier foams (i.e. with lower liquid fraction, φL<0.3), more similar to the ones obtained in dish-washing applications. Our results are of potential interest for the optimization of foams in complex structures, such as in deformable porous media
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