Une turbine diphasique est utilisée pour remplacer le détendeur classique des systèmes de réfrigération. Dans la turbine, une tuyère transforme l'enthalpie de l'écoulement en énergie cinétique. Remplacer la détente isenthalpique classique par une détente isentropique augmente le coefficient de performance d'un système de réfrigération d'une valeur qui peut aller jusqu'à 20 %, pour la même consommation électrique du compresseur. Pendant cette transformation, la pression statique de l'écoulement diminue et un changement de phase se produit au col de la tuyère. La détente d'un écoulement diphasique est étudiée analytiquement et simulée avec le logiciel de CFD Fluent 13. Un modèle d'évaporation adapté à la détente diphasique est ajouté à l'algorithme de dynamique de fluide dans Fluent 13, afin de créer et simuler le changement de phase à l'intérieur de la tuyère. Le modèle d'évaporation est basé sur la théorie de la nucléation et sur des résultats expérimentaux. Le modèle calcule le taux de nucléation hétérogène initiée par les cavités de la paroi intérieur de la tuyère, et le taux de nucléation hétérogène initiée par la présence de bulles de vapeur au cœur de l'écoulement. Outre les tuyères, la roue de la turbine est aussi conçue. La roue récupère l'énergie cinétique d'un écoulement diphasique et la transforme en un couple de moment. La géométrie des différents éléments de la roue est définie afin d'assurer l'intégrité mécanique et un bon rendement énergétique. La conception de la roue prend en considération le comportement de l'écoulement en contact avec les paliers de la roue, la perte de puissance due aux frottements aérauliques, ainsi que de nombreux autres facteurs. Fluent 13 est utilisé pour simuler l'écoulement diphasique à l'intérieur des augets de la roue, ainsi que pour calculer les pertes dues aux frottements aérauliques. Le rendement calculé de la turbine est vérifié avec des tests d'une turbine diphasique dans un groupe refroidisseur d'eau ayant une puissance de réfrigération de 700 kW. / A two-phase turbine used to replace the usual expander in a refrigeration system needs a nozzle/expander to transform the flow enthalpy into kinetic energy. Replacing the isenthalpic expansion by the isentropic one, increases the coefficient of performance of refrigeration system up to 20% for the same compressor input power. During this transformation, the static pressure of the flow decreases and a phase change occurs at the nozzle throat. The expanding and flashing flow in the convergent and divergent parts of a nozzle is analytically studied and then simulated using the CFD software Fluent 13. A separate evaporation model is added to the fluid dynamics algorithms in Fluent 13 in order to create the phase change inside the nozzle. The evaporation model is based on the classical theory of nucleation and on experimental results: it calculates the heterogeneous nucleation initiated by the wall cavities and the heterogeneous nucleation initiated by the presence of vapor bubbles in the flow. The turbine is composed of the designed nozzles and a rotor that recovers the kinetic energy of the two-phase flow and transforms it into torque. The geometry of the rotor components is designed to provide mechanical integrity and high efficiency. The design takes into consideration the flow behavior inside the rotor, the power loss due to drag force, and many other factors. Fluent 13 is used to study the flow inside the rotor buckets and to estimate the power loss due to drag force. The turbine calculated efficiency is verified by testing a two-phase turbine in a water chiller having a refrigeration capacity of 700 kW.

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Master of Science / Department of Mechanical and Nuclear Engineering / B. Terry Beck / Mohammad H. Hosni / Cavitation is the formation of vapor phase from the liquid phase by reduction in its absolute pressure below the saturation pressure. Unlike boiling, where the temperature of the liquid is increased to cause vaporization, the reduction in the pressure alone can cause the liquid to turn into vapor. Cavitation is undesirable in many engineering applications as it is associated with reduction in efficiency and is known to cause damage to pump and propeller components. However, the endothermic nature of cavitation could be utilized to create a region of low temperature that could be utilized to develop a new refrigeration cycle. The work presented in this thesis is part of ongoing research into the potential cooling capacity of cavitation phenomena, where the cavitation in a converging-diverging nozzle is being investigated. Due to the constricting nature of the throat of the converging-diverging nozzle, the liquid velocity at the throat is increased, obeying the continuity law. With an increase in velocity, a reduction in absolute pressure is accompanied at the throat of the nozzle according to the Bernoulli’s principle. The local absolute pressure at the throat can go lower than the saturation vapor pressure, thereby causing the fluid to cavitate. The effect of water temperature on the flowrates, the onset of cavitation within the nozzle, and the resulting length of the cavitation region within the nozzle are the subject of this thesis. Experimental results and analysis are presented which also show that near the onset of cavitation, the flowrate can go beyond the choked flowrate, causing the local pressure in the throat to go well below zero for an extended amount of time in the metastable state, before nucleating (cavitating) into a stable state. Flow visualization using a high speed digital camera under different operating conditions was aimed at investigating the region of cavitation onset, which appears to be associated with boundary layer separation just downstream of the nozzle throat. In order to delay the boundary layer separation point in the downstream section of the nozzle, the diffuser region of the nozzle was modified to enable two flow paths, where one path would suck the flow near the inner walls of the nozzle and the other would allow the bulk of the flow to pass through. This was achieved with the use of inserts. Various inserts were tested in an attempt to capture the effect of inserts on the cavitation phenomena. Their effect on the flowrates, length of two phase region, and cavitation onset are presented in this thesis.

Orientador: Niederauer Mastelari / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-14T13:06:41Z (GMT). No. of bitstreams: 1 Correa_FernandaCristina_M.pdf: 2774513 bytes, checksum: 26936aae7fde336d3017925056dfbe4e (MD5) Previous issue date: 2009 / Resumo: Escoamentos bifásicos gás-líquido são comumente encontrados na natureza e em diferentes atividades industriais. Neste tipo de escoamento, as fases presentes podem escoar dispostas em diferentes configurações espaciais no interior do duto, denominadas de regimes ou padrões de escoamento, cuja determinação é crítica para o projeto de sistemas de escoamento bifásico. No presente trabalho, a indicação dos padrões de escoamento é realizada a partir da medida de fração de gás utilizando um instrumento microcontrolado composto por um transdutor e um condicionador de sinais. Um sistema de reconhecimento de padrões é composto por diferentes módulos, que consistem em: adquirir o sinal; extrair informações relevantes do sinal; e por fim a classificação propriamente dita. Para a extração das informações relevantes do sinal, aplicou-se funções estatísticas de média e desvio padrão por ser interessante sob o ponto de vista computacional. Na etapa classificatória, a utilização de um conjunto de regras através de técnicas de Sistemas Especialistas é utilizado pelo desempenho deste tipo de classificação e também pouca necessidade computacional. Desta forma, com a junção das funções estatísticas de média e desvio padrão e um conjunto de regras com Sistemas Especialistas, é desenvolvido neste trabalho um sistema de reconhecimento de padrões de escoamento bifásico gás-líquido embarcado em um instrumento de medição de fração de gás microcontrolado. / Abstract: Two-phase flow gas-liquid are commonly found in nature and in various industrial activities. In this type of flow, the phases present can escape arranged in different spatial configurations in the duct, called regimes or flow patterns, the determination is critical for the design of two-phase flow systems. In this study, an indication of the flow pattern is achieved from the measurement of gas fraction microcontrolled using an instrument composed of a transducer and signal conditioner. A system of pattern recognition consists of different modules, which consist of: acquiring the signal, extracting relevant information from the signal, and finally the classification itself. Statistical functions of mean and standard deviation are applied to extract the relevant information from the signal, for computacional purposes. In the classification stage, a set of rules using techniques of expert systems is used by the performance of this type of classification and also little need computing. Thus, with the addition of the statistical functions of mean and standard deviation and a set of rules with expert systems is developed in this work a system of pattern recognition of two-phase flow gas-liquid loaded into an microcontrolled instrument for measuring gas fraction. / Mestrado / Mecanica dos Sólidos e Projeto Mecanico / Mestre em Engenharia Mecânica

Superhydrophobic surfaces have gained attention as a potential mechanism for increasing condensation heat transfer rates. Various aspects related to condensation heat transfer are explored. Adiabatic, air-water mixtures are used to explore the influence of hydrophobicity on two-phase flows and the hydrodynamics which might be present in flow condensation environments. Pressure drop measurements in a rectangular channel with one superhydrophobic wall (cross-section approximately 0.37 X 10 mm) are obtained, revealing a reduction in the pressure drop for two-phase flow compared to a control scenario. The observed reduction is approximately 10% greater than the reduction that is observed for single-phase flow (relative to a classical channel). Carbon nanotubes have been used to create superhydrophobic coatings due to their ability to offer a relatively uniform nanostructure. However, as-grown carbon nanotubes often require the addition of a thin-film hydrophobic coating to render them superhydrophobic, and fine control of the overall nanostructure is difficult. This work demonstrates the utility of using carbon infiltration to layer amorphous carbon on multi-walled nanotubes to achieve superhydrophobic behavior with tunable geometry. The native surface can be rendered superhydrophobic with a vacuum pyrolysis treatment, with contact angles as high as 160 degrees and contact angle hysteresis less than 2-3 degrees. Drop-size distribution is an important aspect of heat transfer modeling that is difficult to measure for small drop sizes. The present work uses a numerical simulation of condensation to explore the influence of nucleation site distribution approach, nucleation site density, contact angle, maximum drop size, heat transfer modeling to individual drops, and minimum jumping size on the distribution function and overall heat transfer rate. The simulation incorporates the possibility of coalescence-induced jumping over a range of sizes. Results of the simulation are compared with previous theoretical models and the impact of the assumptions used in those models is explored. Results from the simulation suggest that when the contact angle is large, as on superhydrophobic surfaces, the heat transfer may not be as sensitive to the maximum drop-size as previously supposed. Furthermore, previous drop-size distribution models may under-predict the heat transfer rate at high contact angles. Condensate drop behavior (jumping, non-jumping, and flooding) and size distribution are shown to be dependent on the degree of subcooling and nanostructure size. Drop-size distributions for surfaces experiencing coalescence-induced jumping are obtained experimentally. Understanding the drop-size distribution in the departure region is important since drops in this size are expected to contribute significantly to the overall heat transfer rate.

For the investigation of stratified two-phase flow, two horizontal channels with rectangular cross-section were built at Forschungszentrum Dresden-Rossendorf (FZD). The channels allow the investigation of air/water co-current flows, especially the slug behaviour, at atmospheric pressure and room temperature. The test-sections are made of acrylic glass, so that optical techniques, like high-speed video observation or particle image velocimetry (PIV), can be applied for measurements. The rectangular cross-section was chosen to provide better observation possibilities. Moreover, dynamic pressure measurements were performed and synchronised with the high-speed camera system. CFD post-test simulations of stratified flows were performed using the code ANSYS CFX. The Euler-Euler two fluid model with the free surface option was applied on grids of minimum 4∙105 control volumes. The turbulence was modelled separately for each phase using the k-ω based shear stress transport (SST) turbulence model. The results compare well in terms of slug formation, velocity, and breaking. The qualitative agreement between calculation and experiment is encouraging and shows that CFD can be a useful tool in studying horizontal two-phase flow. Furthermore, CFD pre-test calculations were done to show the possibility of slug flow generation in a real geometry and at relevant parameters for nuclear reactor safety. The simulation was performed on a flat model representing the hot-leg of the German Konvoi-reactor, with water and saturated steam at 50 bar and 263.9°C. The results of the CFD-calculation show wave generation in the horizontal part of the hot-leg which grow to slugs in the region of the bend.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely inter-facial heat transfer and combustionanalysis approach. After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities, a new solver, including the two developed models for phase change, was implemented under the name of interPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physical behavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties. It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advanced solvers within the phase change class.

Within the course of the master thesis project, two thermal phase change models for direct contact conden-sation were developed with different modeling approaches, namely interfacial heat transfer and combustionanalysis approach.After understanding the OpenFOAM framework for two phase flow solvers with phase change capabilities,a new solver, including the two developed models for phase change, was implemented under the name ofinterPhaseChangeCondenseTempFoam and analyzed in a series of 18 tests in order to determine the physicalbehavior and robustness of the developed models. The solvers use a volume-of-fluid (VOF) approach withmixed fluid properties.It has been shown that the approach with inter-facial heat transfer shows physical behavior, a strong timestep robustness and good grid convergence properties. The solver can be used as a basis for more advancedsolvers within the phase change class.

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No description available.