"The condensation of vapor onto a cooled surface is a phenomenon which can be difficult to quantify spatially and as a function of time; this thesis describes an ultrasonic system to measure this phenomenon. The theoretical basis for obtaining condensate film thickness measurements, which can be used to calculate growth rates and film surface features, from ultrasonic echoes will be discussed and the hardware and software will be described. The ultrasonic system utilizes a 5MHz planar piston transducer operated in pulse-echo mode to measure the thickness of a fluid film on a cooled copper block over the fluid thickness range of 50 microns to several centimeters; the signal processing algorithms and software developed to carry out this task are described in detail. The results of several experiments involving the measurement of both non-condensing and condensing films are given. In addition, numerical modeling of specific condensate film geometries was performed to support the experimental system; the results of modeling nonuniform fluid layers are discussed in the context of the effect of such layers on the measurement system."

The thesis describes the design of a fringe projection system to study the dynamics of condensation with potential application in a reduced gravity environment. The concept is that an optical system for imaging the condensation layer enables extraction of valuable data from the image because of the ability of the optical system to image the perturbations in the condensation films. By acquiring a sequence of images of the deformed fringe pattern, the change in the surface topology can be observed over time, giving greater understanding of condensation dynamics in reduced gravity.

Condensation of water vapor is an everyday phenomenon which plays an important role in power generation schemes, desalination applications and high-heat flux cooling of power electronic devices. Continuous dropwise condensation is a desirable mode of condensation in which small, highly-spherical droplets regularly form and shed off the surface before a thick liquid is formed, thereby minimizing the thermal resistance to heat transfer across the condensate layer. While difficult to induce and sustain, dropwise condensation has been shown to achieve heat and mass transfer coefficients over an order of magnitude higher than its filmwise counterpart. Superhydrophobic surfaces have been extensively studied to promote dropwise condensation with mixed results; often surfaces that are superhydrophobic to deposited droplets formed in the gas phase above the surface do not retain this behavior with condensed droplets nucleated and grown on the surface. Recently, nanostructured superhydrophobic surfaces have been developed that are robust to vapor condensation; however, these surfaces still are not ideal for condensation heat transfer due to the high thermal resistance of the vapor layer trapped underneath the droplets and the reduced footprint of direct contact between the highly-spherical droplets and the underlying substrate. This work has two main objectives. First, a comprehensive free energy based thermodynamic model is developed to better understand why traditional superhydrophobic surfaces often lose their properties when exposed to condensed droplets. The model is first validated using data from the existing literature and then extended to analyze the suitability of amphiphilic (e.g. part hydrophobic and part hydrophilic) nanostructured surfaces for condensation applications. Secondly, one of the promising amphiphilic surfaces identified by the thermodynamic model is fabricated and tested to observe condensation dynamic behavior. Two complementary visualization techniques, environmental scanning electron microscopy (ESEM) and optical (light) microscopy, are used to probe the condensation behavior and compare the performance to that of a traditional superhydrophobic surface. Observations from the condensation experiments are used to propose a new mechanism of coalescence that governs the temporal droplet size distribution on the amphiphilic nanostructured surface and continually generates fresh sites for the droplet nucleation and growth cycle that is most efficient at heat transfer.

DNA condensation is the process in which an anionic polymer in combination with condensing agents undergoes a drastic reduction in volume and collapses into ordered structures. Double-stranded DNA has a uniform helical secondary structure, whereas single-stranded DNA is complex and adopts numerous different conformations. Novel mixed-stranded DNA molecules, with defined regions of both single-stranded and double-stranded secondary structures attached to one another in the same molecule, were created in this body of work. Mixed-stranded DNA was designed to be intermediate between its parent secondary structures in order to discover if mixed-stranded DNA will find a balance in terms of condensation properties as well. Mixed-stranded DNA was found to condense into minimally aggregated, globular particles in the presence of low mM concentrations of divalent transition metals in aqueous solvent at room temperature, a property not observed for either pure dsDNA or ssDNA. A model is presented to describe how mixed-stranded DNA -Mn2+, -Ni2+, and -Cd2+ condensates with the observed properties are produced. Multivalent-induced condensation of mixed-stranded DNA is also characterized and found to involve an unusual rod-like morphology in order to accommodate the secondary structures condensing independent of one another at different concentrations of multivalent cations. The attachment of a ss region to an otherwise ds molecule was found to greatly influence condensation properties of the entire molecule.

An experimental and numerical investigation was conducted to study boiling and condensation - the two most important phenomena occurring in a dual-chamber thermosyphon. Boiling experiments were carried out using water at sub-atmospheric pressures of 9.7, 15 and 21 kPa with a three-dimensional porous boiling enhancement structure integrated in the evaporator. Sub-atmospheric pressure boiling achieved heat fluxes in excess of 100 W/cm2 with negligible incipience superheat, for wall temperatures below 85 oC. Reduced pressures resulted in reduction of heat transfer coefficient with decrease in saturation pressure. The boiling enhancement structure showed considerable heat transfer enhancement compared to boiling from plain surface. Increased height of the structure decreased the heat transfer coefficient and suggested the existence of an optimum structure height for a particular saturation pressure. A parametric study showed that a reduction in liquid level of water increased the CHF for boiling with plain surfaces. For boiling with enhanced structures, the liquid level for optimum heat transfer increased with increasing height of the enhanced structure. A numerical model was developed to study condensation of water in horizontal rectangular microchannels of hydraulic diameters 150-375 µm. The model incorporated surface tension, axial pressure gradient, liquid film curvature, liquid film thermal resistance, gravity and interfacial shear stress, and implemented successive solution of mass, momentum and energy balance equations for both liquid and vapor phases. Rectangular microchannels achieved significantly higher heat transfer coefficient compared to a circular channel of similar hydraulic diameter. Increasing the inlet mass flow rate resulted in a higher heat transfer coefficient. Increasing the inlet temperature difference between wall and vapor led to a thicker film and a gradually decreasing heat transfer coefficient. Increasing the channel dimensions led to higher heat transfer coefficient, with a reduction in the vapor pressure drop along the axial direction of the channel. The unique contributions of the study are: extending the knowledge base and contributing unique results on the thermal performance of thermosyphons, and development of a analytical model of condensation in rectangular microchannels, which identified the system parameters that affects the flow and thermal performance during condensation.

The objective of this study is to determine the mean heat transfer coefficient and heat transfer rate and to analyse the effect of inclination angles,the effect of subcooling temperatures and the effect of vapour velocity for laminar filmwise condensation of water vapour on a vertical tier of spheres experimentaly and analyticaly. For this purpose, the experimental aparatus were designed and manufactured. In the free condensation experimental study &Oslash / 50mm and &Oslash / 60 mm O.D. spheres were used to analyse the diameter effect . In the experimental studies of free and forced condensation &Oslash / 60mm O.D. spheres on which vapour flows at 2,75 bars were used to analyse the effect of vapour velocity. For the experimental study of the annular condensation in the concentric spheres the effect of vapour velocity was studied by forcing the vapour to flow in the area between two concentric spheres. In the free condensation experiments it is observed that at smaller diameters the heat flux and mean heat transfer coefficients for sphere is higher. In the free and forced condensation experiments increasing the velocity of vapour increases the mean heat transfer coefficient. At the experiments with annular condensation between the concentric spheres high mean heat transfer coefficient values have been obtained compared to the free and forced condansation over the surface of spheres experimental studies.

Direct contact condensation of steam injected into water is a special mode of condensation where condensation occurs on the interface between steam and water. This type of condensation forms an essential part of various industrial applications and correct prediction and modelling of the condensation behaviour is crucial to obtain an optimised design of such devices. While present prediction models for direct contact condensation are valid for a limited range of flow conditions only, the work presented in this thesis provides improved models for direct contact condensation. The models are developed in the form of diagrams and include: a condensation regime diagram, for predicting the condensation behaviour, a steam plume length diagram, for predicting the penetration distance of steam into water, and a heat transfer coefficient diagram. These models are derived using a wide range of data and therefore provide more accurate predictions compared with alternative models available in literature. In contrast to present models, the derived models presented in this work are constructed using an additional physical parameter to describe the process. The diagrams are validated against independent experiments and demonstrate close agreement. Furthermore, the predictions from the condensation regime diagram and steam plume length diagram are self-consistent. The models developed in this study are capable of predicting condensation behaviour for a wide range of initial conditions and can be used in conjunction with computational fluid dynamics techniques for direct contact condensation.

Microchannel heat exchangers are widely used in air-conditioning and refrigeration systems, high heat flux electronics cooling, and are also being considered for biological devices. Heat transfer and pressure drop in microchannels with single-phase flow have been studied in greater detail compared to two-phase flow. Heat transfer and pressure drop in two-phase flow inside tubes are closely related to the structure of the flow. In convective condensation, the fluid flows in a variety of flow regimes as it changes from vapor to liquid. The flow patterns formed in microchannels differ in type and extent from those seen in conventional tubes. Wavy and stratified flows are virtually absent at microchannel dimension, while intermittent and annular flows predominate. The subject research focuses on understanding the flow physics in microchannels during condensation. The extensive condensation flow-regime database of a previous study is employed for this purpose. This database comprises the flow-regime observations in tubes of hydraulic diameter ranging from 1-4.91 mm during condensation of refrigerant R-134a. The mass flux ranges from 150-750 kg/m2-s over a vapor quality range of 0 to 1. The results from this previous experimental study are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes. Using this understanding and data, criteria for transitions between the different regimes have been developed. These criteria developed in non-dimensional form can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing the applicability of these results. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the choice of the appropriate models for the evaluation of heat transfer and pressure drop, and therefore enable the development of optimum microchannel heat exchangers.

Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 53-54). Also available on the World Wide Web.

La réduction continue des dimensions des transistors depuis les années 60 est à l’origine de l’explosion des usages de l’électronique. Toutefois, la réduction des dimensions à l’échelle nanométrique s’accompagne de nouvelles difficultés qui tendent à limiter les gains des transistors en termes de performances et de consommation.Afin de surmonter ces obstacles et maintenir cette dynamique, des canaux à base de nouveaux matériaux à forte mobilité et de nouvelles architectures de transistors sont désormais utilisées ou à l’étude. L’intérêt de films SiGe contraint en compression sur isolant (SGOI: SiGe-On-Insulator) ultra-minces est double : ils bénéficient de la forte mobilité des trous du SiGe contraint en compression ainsi que du meilleur contrôle électrostatique des structures dites « sur isolant ». Des films SGOI présentant une forte concentration en Ge et une importante contrainte peuvent être fabriqués par une technique industrielle appelée condensation. Cette technique repose sur deux processus simultanés : l’oxydation thermique et sélective du SiGe (seul le Si est oxydé) et l’inter-diffusion du SiGe entre l’oxyde thermique et l’oxyde enterré qui se comporte comme une barrière à la diffusion.L’utilisation de cette technique dans un environnement industriel nécessite de relever deux défis : maîtriser les mécanismes et la cinétique d’oxydation, et atteindre les plus fortes contraintes et qualités cristallines pour lesfilms SGOI.La cinétique de plusieurs procédés d’oxydation industriels et pertinents au regard des besoins technologiques actuels est étudiée à l’aide d’une nouvelle méthodologie d’analyse quantitative. Nous établissons une corrélationentre le coefficient de diffusion de l’espèce oxydante, qui détermine la cinétique d’oxydation, la concentration en Ge à l’interface d’oxydation, et la densité de l’oxyde mesurée par réflectivité de rayons X sur une ligne desynchrotron.Puis, nous avons fabriqué des films SGOI présentant des concentrations en Ge jusqu’à 80%. Nous discutons l’évolution de la contrainte de ces films en fonction des paramètres du procédé et des niveaux de contrainte. Enfin,nous mettons en évidence les effets du procédé de condensation sur la qualité cristalline du film SiGe aux interfaces avec les oxydes grâce à l’effet de canalisation d’une technique de rétrodiffusion d’ions à moyenne énergie (MEIS : Medium Energy Ion Scattering) / The tremendous spread of electronic devices and networks into our day-to-day life has been enabled by the constant downscaling of transistors since the 60’s. However, downsizing transistors has become increasingly difficult in the past few years and going to the nanometer scale brings new detrimental effects that have put power consumption and performances on quasi-plateaux for a few years. To overcome these limitations, high mobility channels based on new materials and new transistor architectures are being introduced. Ultrathin compressivelystrained SiGe-On-Insulator (SGOI) films benefit from the advantages of both the higher hole mobility of compressively strained SiGe as well as of the better electrostatic control of On-Insulator structures. The condensation techniqueis a CMOS-compatible technique that allows fabrication of such films with possibly high Ge content and high strain levels. The technique is based on Si-selective thermal oxidation of SiGe and concurrent SiGe diffusion between the thermal oxide and the buried oxide layer that acts as a diffusion barrier.Two main challenges still need to be taken up for an efficient and optimized use of the condensation technique in an industrial environment: oxidation mechanisms and kinetics must be well controlled, and strain and crystal quality of the SGOI film must be as high as possible.Firstly, this work bridges the gap between previous studies by covering various oxidation processes relevant to today’s technological needs with a new and quantitative analysis methodology of oxidation kinetics. A correlation is established between the diffusivity of the oxidizing species that governs oxidation kinetics, the Ge concentration at the oxidation interface, and the oxide density measured by X-Ray Reflectivity on a synchrotron beamline.Secondly, SGOI films with Ge concentrations up to 80% were fabricated by the condensation technique. The evolution of strain of SGOI films is discussed as a function of process parameters and strain energy levels. How the condensation technique alters the crystal quality, both at interfaces with oxides and in the bulk of the SiGe crystal, is evaluated by the Medium Energy Ion Scattering (MEIS) technique by using the channeling effect.