Les travaux présentés dans cette thèse concernent l'étude de l'évolution des régimes d'écoulements diphasiques lors de l'évaporation progressive dans un canal horizontal. Le but est de mieux comprendre l'écoulement à l'intérieur d'un tube récepteur d'une centrale solaire à concentration à génération directe de vapeur. Cette technologie, présentée comme une amélioration des systèmes actuels pouvant permettre une réduction des coûts, consiste en la production de vapeur directement sous l'effet du rayonnement solaire concentré. La prévision de l'écoulement liquide-vapeur alors généré dans le tube est encore de nos jours difficile, c'est pourquoi le recours à la simulation numérique est intéressant. Pour cela un modèle a été développé permettant la simulation de ces écoulements, depuis le début de la création de la vapeur jusqu'à l'existence de larges poches. Basé sur le modèle diphasique VOF du code Fluent, par l'ajout de fonctions personnalisées et d'une phase dispersée supplémentaire, il permet de modéliser différents phénomènes liés au processus d'évaporation : création en paroi, transport, recondensation et création de larges structures. Ce développement a été mis en oeuvre pour simuler des écoulements en évaporation, permettant de reproduire l'évolution des régimes d'écoulement. La validation est faite grâce à une étude expérimentale de la littérature, en comparant les régimes d'écoulements obtenus pour différents débits de liquide et sous l'effet de différents flux de chaleur. Enfin, le modèle a été appliqué à la simulation de la génération de vapeur dans le tube récepteur d'une centrale solaire, mettant en évidence l'apparition et l'évolution des différents régimes d'écoulement. Au vu du peu d'installations expérimentales trouvées dans la littérature sur le sujet, et afin de valider au mieux les fonctions développées, une installation expérimentale a été conçue et dimensionnée. / This PhD thesis is about the study of two-phase flow patterns evolution during progressive evaporation in horizontal tubes. The goal is to better understand the flow regimes inside a receiver tube of a concentrated solar power plant with direct steam generation. This technological evolution allows vapor production directly inside the solar field, which can lead to coast reductions. A two-phase liquid-vapor flow occurs inside the tube, which is currently still difficult to predict. Numerical simulation is an interesting way to investigate these complex phenomena. A model has been developed in order to simulate the flow patterns, from first vapor generation to large vapor slugs. It is based on Fluent software's two-phase VOF model, to which are added user-defined functions and a new dispersed phase. Different phenomena linked to the evaporating process are taken into account: vapor creation at the wall, its transport, recondensation and large structures creation. The model is used to simulate evaporating flows, and retrieves well two-phase flow patterns evolution. Validation is made using experimental data from the literature, by comparing flow regimes obtained for different flow rates and heat fluxes. Finally numerical simulation of direct steam generation inside a concentrated solar plant receiver is conducted, clearly showing apparition and evolution of two-phase flow patterns. Because few experimental data where found in the literature concerning evaporating two-phase flows visualization, a new experimental apparatus has been conceived and sized in order to better validate our numerical results.

The development flow region of stepped chutes was studied experimentally. Three configuration of chute bed slopes 3.5H:1V, 5H:1V, and 10H:1V were used to study the flow characteristics. Each model had five horizontal steps and with constant step height of 15 cm. Constant temperature anemometry was used to investigate the velocity field characteristics as well as local void fraction. Pressure transducers were used to examine the pressure distribution. The conditions of aerated and non-aerated cavity were studied. It was found that the temperature anemometry is a valuable tool in the study of water flow problems due to its good spatial and temporal resolution. It is recommended that the constant overheat ratio procedure should be used in dealing with non-isothermal water flows. Flow conditions along the development flow region were found to be quite complex with abrupt changes between steps depending whether or not the flow jet has disintegrated. The flow on this region does not resemble a drop structure and after the first step, the step cavity condition does not affect the flow parameters. Pressure distribution was also found to be complex. It was found that there are no conclusive pressure profiles either on the step treads nor on step risers. No correlation was observed with the values of pool depth. The instantaneous characteristics of the velocity field along the jet of a drop structure were also studied. It was concluded that the cavity condition does not affect the velocity field of the sliding jet. The shear stress layer at the jet/pool interface was quantified.

Phase-redistribution and full-phase separation data were generated for two-phase (air-water) flow splitting at an equal-sided impacting tee junction with a horizontal inlet and inclined outlets. The flow loop incorporated a tee junction machined in an acrylic block with the three sided having an equal diameter of 13.5 ± 0.1 mm I.D. Both sets of experiments were conducted at a nominal pressure (Ps) of 200 kPa (abs) and near-ambient temperature (Ts). The operating conditions for the phase-redistribution experiments were as follows: inlet superficial liquid velocities (JL1) ranging from 0.01 to 0.18 m/s, inlet qualities (x1) ranging from 0.1 to 0.9, mass split ratios (W3/W1) from 0 to 1.0, and outlet inclination angles ranging from horizontal to vertical. These inlet conditions corresponded to inlet flow regimes of stratified, wavy, and annular. Phase-redistribution data revealed that the redistribution of phases depended on the inlet conditions, the mass split ratio at the junction, and the inclination angle of the outlets. The magnitude of the inclination effect was dependent on the inlet flow regime. The phase redistribution in stratified flow was very sensitive to the outlet angle and full separation could be achieved at angles as low as 0.7°. Wavy flow was less sensitive to the outlet angle and annular flow was even less sensitive to the outlet angle. The capability of a single impacting tee junction to perform as a full phase separator has been examined. Experimental data were obtained for the limiting inlet conditions under which full separation was attainable at various outlet inclinations (θ) of 2.5°, 7.5°, 15°, 30°, 60°, 75°, and 90°. Full separation data have shown that a single impacting tee junction can perform as a full-phase separator for some inlet conditions. Flow phenomena near the limiting conditions were observed and a simple correlation based on the similarity between these flow phenomena and the phenomenon of liquid entrainment in small upward branches was developed. This correlation was capable of accurate prediction of the data in terms of magnitude and trend.

The development flow region of stepped chutes was studied experimentally. Three configuration of chute bed slopes 3.5H:1V, 5H:1V, and 10H:1V were used to study the flow characteristics. Each model had five horizontal steps and with constant step height of 15 cm. Constant temperature anemometry was used to investigate the velocity field characteristics as well as local void fraction. Pressure transducers were used to examine the pressure distribution. The conditions of aerated and non-aerated cavity were studied. It was found that the temperature anemometry is a valuable tool in the study of water flow problems due to its good spatial and temporal resolution. It is recommended that the constant overheat ratio procedure should be used in dealing with non-isothermal water flows. Flow conditions along the development flow region were found to be quite complex with abrupt changes between steps depending whether or not the flow jet has disintegrated. The flow on this region does not resemble a drop structure and after the first step, the step cavity condition does not affect the flow parameters. Pressure distribution was also found to be complex. It was found that there are no conclusive pressure profiles either on the step treads nor on step risers. No correlation was observed with the values of pool depth. The instantaneous characteristics of the velocity field along the jet of a drop structure were also studied. It was concluded that the cavity condition does not affect the velocity field of the sliding jet. The shear stress layer at the jet/pool interface was quantified.

iv The primary objective of this study is to understand the mechanism, the hydraulics and the characteristics, of the two-phase flow in horizontal annuli. While achieving this goal, both theoretical and experimental works have been conducted extensively. The METU-PETE-CTMFL (Middle East Technical University, Petroleum and Natural Gas Engineering Department, Cuttings Transport and Multiphase Flow Laboratory) multiphase flow loop consists of 4.84 m long eccentric horizontal acrylic pipes having 0.1143m inner diameter (I.D) acrylic casing - 0.0571m outer diameter (O.D) drillpipe and 0.0932m I.D acrylic casing - 0.0488m O.D drillipipe geometric configurations. During each experiment, differential pressure loss data obtained from digital and analog pressure transmitters at a given liquid and gas flow rate were recorded. The flow patterns were identified visually. Meanwhile a mechanistic model has been developed. The flow pattern identification criteria proposed originally for twophase flow through pipes by Taitel and Dukler1 has been inherited and modified for the eccentric annular geometry. The complex geometry of eccentric annuli has been represented by a new single diameter definition, namely representative diameter dr. The representative diameter has been used while calculating the pressure losses. A computer code based on the algorithm of the proposed mechanistic model has been developed in Matlab 7.0.4. Both the flow pattern prediction and the frictional pressure loss estimation are compared with the gathered experimental data. Moreover, friction factor correlations have been developed for each flow pattern using experimental data and statistical methods. The performance of the proposed model and the friction factor correlations has been evaluated from experimental data. The mechanistic model developed in this study accurately predicts flow pattern transitions and frictional pressure losses. The model&rsquo / s pressure loss estimations are within &plusmn / 30% for two different annular flow geometries.

Pressure drop across miniature-scale flow disturbances, including abrupt flow area changes, is an important source of error and confusion in the literature. Such pressure drops are frequently encountered in experiments, where they are often estimated using methods and correlations that have been developed based on experimental data obtained in conventional systems. However, physical arguments as well as the relatively few available experimental observations indicate that such pressure drops in microchannel systems are likely to be different than what is known about similar phenomena in conventional flow systems. Experimental data dealing with pressure drop associated with two-phase flow across abrupt flow area changes in microchannels are scarce, however, and the available data are insufficient for the development of reliable predictive methods. In this investigation, experiments were conducted using a test section consisting of two capillaries, one with 0.84 mm, and the other with 1.6 mm inner diameters. A multitude of pressure transducer ports were installed along the two capillaries, and allowed for the measurement of the pressure gradients over the entire test section. The test section allowed for the measurement of frictional pressure gradients in the two straight channels, as well as pressure drops caused by the flow area expansion and contraction depending on the flow direction, for single-phase as well as two-phase flows. These measurements were performed over a wide range of parameters, using air as the gaseous phase, and room-temperature water as the liquid phase. The single-phase flow data were compared with existing conventional correlations, and with predictions of CFD simulations using the Fluent computer code.

This work has been aimed at developing a mechanistic, transient, 3-D numerical model to predict the behavior of an evaporating thin liquid film on a non-uniformly heated cylindrical rod with simultaneous parallel and cross flow of vapor. Interest in this problem has been motivated by the fact that the liquid film on a full-length boiling water reactor fuel rod may experience significant axial and azimuthal heat flux gradients and cross flow due to variations in the thermal-hydraulic conditions in surrounding subchannels caused by proximity to inserted control blade tip and/or the top of part-length fuel rods. Such heat flux gradients coupled with localized cross flow may cause the liquid film on the fuel rod surface to rupture, thereby forming a dry hot spot. These localized dryout phenomena can not be accurately predicted by traditional subchannel analysis methods in conjunction with empirical dryout correlations. To this end, a numerical model based on the Level Contour Reconstruction Method was developed. The Standard k- turbulence model is included. A cylindrical coordinate system has been used to enhance the resolution of the Level Contour Reconstruction Model. Satisfactory agreement has been achieved between the model predictions and experimental data. A model of this type is necessary to supplement current state-of-the-art BWR core thermal-hydraulic design methods based on subchannel analysis techniques coupled with empirical dry out correlations. In essence, such a model would provide the core designer with a "magnifying glass" by which the behavior of the liquid film at specific locations within the core (specific axial node on specific location within a specific bundle in the subchannel analysis model) can be closely examined. A tool of this type would allow the designer to examine the effectiveness of possible design changes and/or modified control strategies to prevent conditions leading to localized film instability and possible fuel failure.

ANP; FINEP; MCT / Misturas bifásicas líquido-gás são frequentemente transportadas em tubulações através de terrenos com relevos irregulares. Estas irregularidades ocasionam perturbações nos parâmetros do escoamento bifásico, portanto tornando-se necessário prever o comportamento destes escoamentos. Neste cenário, o presente trabalho estuda, numérica e experimentalmente, os efeitos causados por uma leve mudança de direção no escoamento bifásico. O estudo foi realizado para escoamentos bifásicos ar-água em padrão de golfadas em uma tubulação horizontal de 26 mm e 5,8 m de comprimento seguida de um trecho inclinado ascendente de 3,4 m de comprimento e mesmo diâmetro com uma leve mudança de direção de 7° com relação à horizontal. Na abordagem numérica utilizou-se o método de seguimento dos pistões (slug tracking), para prever o comportamento do escoamento quando se produz uma leve mudança de direção. O trabalho experimental foi realizado utilizando um circuito experimental projetado e instalado no LACIT/UTFPR. Nele, foram instalados sensores resistivos e transdutores de pressão em quatro pontos da seção de teste, chamadas estações de medida, com a finalidade de monitorar a evolução da fração do vazio, velocidade de translação da bolha, a pressão e os comprimentos do pistão de líquido e da bolha. Os resultados numéricos e experimentais foram comparados entre si como função dos resultados médios e de distribuições de frequência nas estações de medição, tendo sido observada uma boa concordância. / Two-phase, liquid-gas flow mixtures are usually transported through pipes stretching over irregular terrains. Those irregularities disturb the flow characteristics, thus predicting the behaviour of such disturbed flows becomes necessary. The present work introduces an experimental and numerical study on the effects of a slight direction change on the behaviour of a gas-liquid two-phase slug flow. The aforementioned study was carried out in a 1” (26 mm), 5.8-m long transparent pipe section followed by a +7o inclined pipe (3.4-m long) transporting air-water two-phase mixtures at different flow rates. This experimental rig was designed and assembled in the LACIT/UTFPR labs. Resistive sensors and pressure transducers were installed at four testing positions in the pipe, designated as metering stations. Void fractions, bubble translational velocities, in situ pressures and the lengths of the liquid slug and the bubble region – as well as the fluctuations of those flow parameters – were measured at these stations. A classical slug tracking approach was applied as the numerical scheme. That approach attempts at simultaneously solving the one-dimensional mass and momentum conservation equations for each instant of time, resulting in coupled system of pressure-velocity equations. A mathematical model to predict the flow behaviour at the pipe position where the change in direction occurred was developed and implemented. The equations of such mathematical model were implemented as source terms in the slug tracking model. The numerical results were compared to the experimental data using averaged results and frequency distribution of the data collected at the metering stations, and a good agreement was observed.

LACIT / Escoamentos bifásicos gás-líquido estão presentes em inúmeras aplicações de engenharia, como por exemplo, na produção de petróleo, onde óleo e gás escoam nas linhas de produção. A monitoração online do escoamento bifásico é de grande importância para operação segura e eficiente dos processos de produção. Assim, nos últimos anos tem-se buscando o desenvolvimento de técnicas simples e de baixo custo para monitoração do escoamento da produção. Das diversas formas que um escoamento bifásico se apresenta, o padrão de escoamento em golfadas é o mais comum nas linhas de produção. Este trabalho apresenta um sistema óptico infravermelho que mede especificamente a velocidade das bolhas de gás em um escoamento em golfadas. Esse sistema emprega componentes optoeletrônicos operando a 950 nm juntamente com uma peça adutora utilizada para manter estáticos o emissor e receptor de luz. O circuito eletrônico responsável pelo funcionamento do sensor é composto por três canais de medição modulados em frequências diferentes. Dessa forma pelo tempo de trânsito que uma bolha leva no percurso entre os canais, pode-se calcular a velocidade da bolha. Essa técnica óptica traz a vantagem de ser um sistema de medição não invasivo e pode ser utilizado para outros fluidos, mas é necessário adaptar o comprimento de onda de trabalho de acordo com as características do fluido a ser detectado. O sistema desenvolvido foi testado em uma planta piloto sob diversas condições operacionais e comparado com sensores de referência (wire-mesh e câmera de alta velocidade). Resultados obtidos mostram o bom desempenho do sistema com relação aos sensores de referência. / Gas-liquid two-phase flows are present in many engineering applications such as in petroleum production, where oil and gas stream in the production lines. The online monitoring of two-phase flow is of great importance for safe and efficient operation of production processes. Thus, in recent years there has been an attempt to develop simple and low cost techniques for flow production monitoring. Among the various ways a two-phase flow may occur in a pipe, the flow pattern known as slug flow is the most common in production lines. This work presents an infrared optical system that specifically measures the translational speed of gas bubbles in slugs flow. This system employs optoelectronic components operating at 950 nm along with a mechanical assembly used to assure optical alignment of light emitters and receivers. The electronic circuitry responsible for operating the sensor consists of three measuring channels modulated at different frequencies, thus by measuring the time lag a gas bubble needs to travel from one channel to another, one can calculate speed of gas bubble. This optical technique has the advantage of being a noninvasive measurement system and can be used for other fluids, but it is necessary to adapt the working wavelength according to the characteristics of the fluid to be detected. The developed system was tested in a pilot plant under various operating conditions and compared with reference sensors (wire-mesh and high-speed camera). Results show the good performance of the system with respect to the reference sensors.

Les travaux présentés dans cette thèse concernent l'étude de l'évolution des régimes d'écoulements diphasiques lors de l'évaporation progressive dans un canal horizontal. Le but est de mieux comprendre l'écoulement à l'intérieur d'un tube récepteur d'une centrale solaire à concentration à génération directe de vapeur. Cette technologie, présentée comme une amélioration des systèmes actuels pouvant permettre une réduction des coûts, consiste en la production de vapeur directement sous l'effet du rayonnement solaire concentré. La prévision de l'écoulement liquide-vapeur alors généré dans le tube est encore de nos jours difficile, c'est pourquoi le recours à la simulation numérique est intéressant. Pour cela un modèle a été développé permettant la simulation de ces écoulements, depuis le début de la création de la vapeur jusqu'à l'existence de larges poches. Basé sur le modèle diphasique VOF du code Fluent, par l'ajout de fonctions personnalisées et d'une phase dispersée supplémentaire, il permet de modéliser différents phénomènes liés au processus d'évaporation : création en paroi, transport, recondensation et création de larges structures. Ce développement a été mis en oeuvre pour simuler des écoulements en évaporation, permettant de reproduire l'évolution des régimes d'écoulement. La validation est faite grâce à une étude expérimentale de la littérature, en comparant les régimes d'écoulements obtenus pour différents débits de liquide et sous l'effet de différents flux de chaleur. Enfin, le modèle a été appliqué à la simulation de la génération de vapeur dans le tube récepteur d'une centrale solaire, mettant en évidence l'apparition et l'évolution des différents régimes d'écoulement. Au vu du peu d'installations expérimentales trouvées dans la littérature sur le sujet, et afin de valider au mieux les fonctions développées, une installation expérimentale a été conçue et dimensionnée. / This PhD thesis is about the study of two-phase flow patterns evolution during progressive evaporation in horizontal tubes. The goal is to better understand the flow regimes inside a receiver tube of a concentrated solar power plant with direct steam generation. This technological evolution allows vapor production directly inside the solar field, which can lead to coast reductions. A two-phase liquid-vapor flow occurs inside the tube, which is currently still difficult to predict. Numerical simulation is an interesting way to investigate these complex phenomena. A model has been developed in order to simulate the flow patterns, from first vapor generation to large vapor slugs. It is based on Fluent software's two-phase VOF model, to which are added user-defined functions and a new dispersed phase. Different phenomena linked to the evaporating process are taken into account: vapor creation at the wall, its transport, recondensation and large structures creation. The model is used to simulate evaporating flows, and retrieves well two-phase flow patterns evolution. Validation is made using experimental data from the literature, by comparing flow regimes obtained for different flow rates and heat fluxes. Finally numerical simulation of direct steam generation inside a concentrated solar plant receiver is conducted, clearly showing apparition and evolution of two-phase flow patterns. Because few experimental data where found in the literature concerning evaporating two-phase flows visualization, a new experimental apparatus has been conceived and sized in order to better validate our numerical results.