Heat transfer and pressure drop during condensation of propane flowing through minichannels is investigated in this study. Studies of condensation of hydrocarbons are important for applications in the petrochemical industry. Insights into the mechanisms of propane condensation are required for accurate design of heat transfer equipment for use in hydrocarbon processing. At present, there is very little research on vertical condensation, especially of hydrocarbons, for the tube sizes and flow conditions of interest to the present study. An experimental facility was designed and fabricated to measure the frictional pressure drop and heat transfer coefficients during condensation of propane in plain tubes with an inner diameter of 1.93 mm. Measurements were taken across the vapor-liquid dome in nominal quality increments of 0.25 for two saturation temperatures (47°C and 74°C) and four mass flux conditions (75 – 150 kg m‾² s‾¹). The data were compared to the predictions of relevant correlations in the literature. The data from this study were also used to develop models for the frictional pressure drop and heat transfer coefficient based on the measurements and the underlying condensation mechanisms. These results and the corresponding correlations contribute to the understanding of condensation of hydrocarbons in vertical minichannels.

The research presented in this thesis is a contribution to the modeling and understanding of the dynamic behavior of proton exchange membrane fuel cells (PEMFCs). A time-dependent, two-phase non-isothermal model of the membrane electrode assembly was developed and implemented using the finite element method. In addition to solving a phenomenological transport equation for water in the membrane, the model takes into consideration the non-equilibrium water sorption to better capture some of the dynamic characteristics of water transport in the MEA. Mass transfer using Fickian diffusion is implemented in the model. Two different models describing the electrochemical reactions in the catalyst layer including a macro-homogeneous model and an agglomerate model, are also implemented. Conservation of energy is included in the solution procedure in order to assess the impact of thermal effects on the dynamics of the transport in the MEA. For the purpose of model and concept validation, the model was first solved in a steady two-dimensional mode for a through- plane computational domain using a commercial software package, COMSOL Multiphysics version 3.2b. The impact of using a single- and two-phase modeling approaches was evaluated, and the predicted current-voltage performance characteristic are found in good agreement with the experimental data available in the literature. In addition, the developed model was benchmarked against a finite element-based in-house code for further validation and to evaluate numerical accuracy and computational performance. Transient simulations of operation under dynamic voltage sweeps are presented, and parametric studies are conducted to investigate the impact of various model, operation and transport properties on the predicted dynamic cell performance. In particular, the rate of load change, the difference in water content between the anode and cathode, and the water sorptions rate are shown to have significant impact on cell performance in unsteady operation, especially at higher current densities. Parametric studies also address the sensitivity of the model results to physical properties, highlighting the importance of accurately determining certain physical properties of the fuel cell components. Finally, the application of the model to air-breathing fuel cells provides further insight into the dynamic performance characteristic of such type of fuel cells.

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The extensive experimental results presented in this report provide a high-quality database for air-/water flows in a vertical pipe with a nominal diameter of 200 mm. This database can be used for the development and validation of CFD-like models for two-phase flows, e.g. for bubble coalescence and fragmentation. In particular, the investigations aim on the evolution of the two-phase flow along the pipe height. Therefore, up to 18 single measurements with varying distances between the gas injection and measurement plane were realised for each of the 92 combinations of gas and water flow rates. The pressure at the position of the activated gas injection was kept constant at 0.25 MPa(a). This boundary condition has the advantage that the measured data represent exactly the evolution of the flow along the pipe, i.e. they reflect a configuration at which the gas injection is at a fixed height position, while the measurement plane varies. Important results of this test series are time averaged radial profiles of the gas fraction, and the gas velocity, as well as the time and cross-section averaged bubble size distributions. Furthermore, gas fraction data resolved regarding the bubble size and spatial distribution are presented. As in previous test series, flow patterns were analysed, whereby the classification results from the bubble size. A substantial part of these new air/water experiments were quality and plausibility checks of the measured data. In the result, a clear and consistent trend regarding their evolution with increasing distance from the position of the gas injection was found. Comparisons of the trend of time and cross section averaged gas volume fraction along the pipe height with the theoretically expected values were carried out. The influence of the orifice diameter of the gas injection on flow patterns is also discussed in the report.

Die im Rahmen dieser Versuchsserie erzielten umfangreichen experimentellen Ergebnisse bilden eine hochwertige Datenbasis für Luft-Wasser-Strömungen in einem vertikalen DN200-Rohr, die für die Entwicklung und Validierung von CFD-Modellen, beispielweise bzgl. Blasenkoaleszenz und -fragmentierung, genutzt werden können. Besonderes interessant ist die Untersuchung der Entwicklung der Zweiphasenströmung über der Rohrhöhe. Aus diesem Grund wurden für jede der 92 betrachteten Kombinationen aus Gas- und Wasser-Volumenstromdichten bis zu 18 Messungen mit variablen Abständen zwischen Gaseinspeisung und Messebene durchgeführt. Dabei wurde der Druck an der Gaseinspeisestelle konstant auf 0,25 MPa(a) gehalten. Diese Randbedingung bietet den Vorteil, dass die so gemessenen Daten die Entwicklung der Strömung über der Rohrhöhe widerspiegeln, d.h. eine Konfiguration beschreiben, bei der das Gas an einer festen Höhenposition eingespeist wird und die Messungen in verschiedenen darüberliegenden Ebenen erfolgen. Wesentliche Ergebnisse dieser Messserie sind radiale zeitgemittelte Profile für den Gasgehalt und die Gasgeschwindigkeit sowie zeit- und querschnittsgemittelte Blasengrößenverteilungen. Außerdem liegen blasengrößen- und ortsaufgelöste Gasgehaltsdaten vor. Wie bereits bei früheren Versuchsserien wurden auch in diesem Fall die Strömungsformen analysiert, wobei die Klassifizierung anhand der Blasengröße erfolgte. Ein wesentlicher Bestandteil dieser neuen Luft/Wasser-Versuche war die Qualitäts- und Plausibilitätsprüfung der Messdaten. Es konnte festgestellt werden, dass die Daten einen eindeutigen, widerspruchsfreien Trend bzgl. ihrer Entwicklung mit zunehmendem Abstand von der Gaseinspeisung aufweisen. Zur Plausibilitätsprüfung wurden Vergleiche des Gasgehaltsverlaufes über der Rohrhöhe mit theoretisch zu erwartenden Kurven durchgeführt. Zusätzlich zu diesen Ergebnissen enthält der Bericht eine Einschätzung des Einflusses des Bohrungsdurchmessers an der Gaseinspeisung auf die sich einstellende Strömung.

This thesis focuses on the lattice-Boltzmann method (LBM) and its ability to simulate immiscible two-phase flow. We introduce the main lattice-Boltzmann-based approaches for analyzing two-phase flow: the color-fluid model by Gunstensen, the interparticle-potential model by Shan and Chen, the free-energy model by Swift and Orlandini, and the mean-field model by He. The first objective is to assess the ability of these methods to maintain continuity at the interface of two fluids, especially when the two fluids have different viscosities or densities. Continuity issues have been mentioned in the literature but have never been quantified. This study presents a critical comparison of the four lattice-Boltzmann-based approaches for analyzing two-phase flow by analyzing the results of the two-phase Poiseuille flow for different viscosity ratios and density ratios. The second objective is to present the capability of the most recent version of the color-fluid model for simulating 3D flows. This model allows direct control over the surface tension at the interface. We demonstrate the ability of this model to simulate surface tension effects at the interface (Laplace bubble test), stratified two-phase flows Poiseuille two-phase flow), and bubble dynamics (the free rise of a bubble in a quiescent viscous fluid).

O presente estudo visa ampliar o conhecimento sobre a dinâmica do escoamento intermitente líquido-gás, caracterizando experimentalmente os principais parâmetros deste tipo de escoamento. Para este fim, é utilizada a bancada experimental que consta de uma linha bifásica horizontal de 9,2 m de comprimento e 25,8 mm de diâmetro. A monitoração das estruturas do escoamento bifásico foi realizada utilizando dois (02) sensores de malha de eletrodos de 12x12 nós separados 3,75 cm entre si. Os testes experimentais foram realizados para 47 condições de vazão de líquido e de gás do escoamento bifásico no padrão intermitente, levando em conta as limitações da bancada experimental. Após processar o sinal medido, é obtida a série temporal dos dados e os parâmetros característicos do escoamento intermitente em golfadas, como a velocidade da bolha alongada, frequência, comprimentos da bolha e do pistão de líquido e a fração de vazio. A distribuição de probabilidades de cada parâmetro foi aproximada com uma função probabilística, do tipo normal ou log-normal. Foram elaboradas relações funcionais para calcular os parâmetros característicos médios do escoamento em golfadas e seus respectivos desvios padrão a partir das condições de operação do escoamento (velocidades superficiais e propriedades dos fluidos). Essas correlações visam predizer o comportamento dos parâmetros do escoamento intermitente líquido-gás e podem ser utilizados como dados de entrada ou de fechamento dos modelos matemáticos existentes. Este estudo também pode servir de referência para futuros estudos e para o desenvolvimento e validação de modelos teóricos do escoamento em golfadas em dutos horizontais. / The goal of the present work is to broaden the knowledge on the dynamics of two-phase intermittent gas-liquid flows through the experimental characterization of their parameters. The experimental work was carried out at the LACIT-UTFPR labs and a two-phase flow apparatus consisting of a 25.8-mm ID, 9-m long transparent line was used. A pair of 12x12-nodes wire mesh sensors 3.75 cm apart was used to identify the existing phase at each node. A test grid comprehending a broad range of gas and liquid flow rates within the intermittent flow pattern region was chosen. The operational limitations of the experimental rig were taken into account on defining the boundaries of the aforementioned test grid. The temporal series of the data, as well as characteristic parameters of the intermittent flows such as frequency, void fraction, elongated bubble velocity and the lengths of the liquid slug and the elongated bubble were obtained after signal processing. The probability distribution of each parameter was found and approximated to a known statistical distribution such as the normal or the log-normal ones. Correlations to fit the averaged parameters and their standard deviations were proposed. The objective of such correlations is to predict the average values of the characteristic parameters of the intermittent flows so that such predicted values might be used as input data for the theoretical models developed. Finally, statistical distributions based on the empirical correlations and their normal and log-normal distributions were found. These distributions validated the aforementioned correlations, which can henceforth be used in future theoretical modelling in this field.

L’opto-pyrotechnie (amorçage de la détonation par système optique) est l’une des innovations les plus prometteuses en termes de fiabilité, de sécurité et de performances pour les futurs lanceurs spatiaux. Le but de la thèse est d’étudier et de modéliser le premier des deux étages d’un Détonateur Opto-Pyrotechnique, constitué d’un explosif confiné dans une chambre de combustion fermée où se déroulent les premières phases d’une Transition Déflagration-Détonation. L’amorçage par laser de l’explosif puis la combustion en chambre isochore sont traités par le code EFAE, lequel est couplé au logiciel LS-DYNA qui simule la déformation et la rupture du disque de fermeture de la chambre, puis la propulsion du projectile résultant vers le second étage. En parallèle, diverses techniques expérimentales (adsorption de gaz, vélocimétrie hétérodyne, microscopie) ont mis en valeur plusieurs procédés physiques, ce qui a permis de tester le couplage entre EFAE et LS-DYNA, puis de déterminer et de hiérarchiser les paramètres affectant les critères industriels. / Opto-pyrotechnics (ignition of detonation by optical systems) is one of the most promising innovations to improve reliability, safety and performances on future space launchers. This thesis aims at studying and modeling the first stage from a two-stage opto-pyrotechnic detonator that consists of a condensed explosive confined in a closed combustion chamber, in which the beginning of a Deflagration-to-Detonation Transition occurs. The laser ignition of the explosive and its isochoric combustion are modeled by the EFAE code. This code is coupled with LS-DYNA software to deal with the deformation and the rupture of the metallic disk that closes the combustion chamber, and then with the subsequent propulsion of the projectile to the second stage. In parallel, various experimental technics (gas adsorption, photonic Doppler velocimetry, microscopy) have underlined several physical processes that allow first to test the coupling between EFAE and LS-DYNA, then to determine and classify influent parameters that affect the industrial specifications.

Orientador: André Luiz Seixlack / Resumo: Neste trabalho apresenta-se um modelo distribuído para análise de condensadores do tipo parede aquecida, hot-wall, usados em refrigeradores domésticos. Nesse modelo considera-se o escoamento do fluido refrigerante no interior do tubo do condensador e a transferência de calor entre a placa de revestimento externo do refrigerador e o ar ambiente. No modelo, inclui-se também a transferência de calor ao longo da fita de alumínio, usada para fixação do tubo na placa de revestimento externo e a transferência de calor através do isolamento térmico para o interior do compartimento refrigerado. O escoamento no interior do tubo é considerado unidimensional e dividido em três regiões: região monofásica de vapor superaquecido, região bifásica líquido-vapor e região monofásica de líquido sub-resfriado. O escoamento bifásico é analisado segundo o modelo homogêneo, ou seja, considerando condições de equilíbrio térmico e hidrodinâmico entre as fases. Considera-se também a queda de pressão do escoamento no interior do tubo. A fita de alumínio e a placa de revestimento externo são consideradas como aletas com extremidades adiabáticas. As equações do escoamento do fluido refrigerante: conservação da massa, quantidade de movimento e conservação de energia, são resolvidas por integração numérica e a equação de conservação de energia ao longo da fita de alumínio é resolvida pelo método de Volumes Finitos. O sistema de equações algébricas, decorrente do processo de discretização das equações difere... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This work presents a distributed model for the analysis of hot-wall condensers used in domestic refrigerators. In this model, the flow of refrigerant inside the condenser tube and the heat transfer between the outer coating plate of the refrigerator and the ambient air are considered. Also included in this model the heat transfer along the aluminum tape used to hold the tube to the outer coating plate and the heat transfer through the thermal insulation into the refrigerated compartment. The flow inside the tube is taken as one-dimensional and divided into three regions: superheated vapor region, two-phase liquid-vapor region and sub-cooled liquid region. The two-phase flow is analyzed according to the homogeneous model, that is, considering conditions of thermal and hydrodynamic equilibrium between the phases. It is also considered the pressure drop of the flow inside the tube. The aluminum tape and the outer coating plate are considered as fins with adiabatic ends. The equations of the refrigerant flow: mass conservation, momentum and energy conservation, are solved by numerical integration and the energy conservation equation along the aluminum tape is solved by the Finite Volume method. The set of algebraic equations, resulting from the discretization process of the differential equations, is solved iteratively by successive iterations, point-to-point along the condenser tube and the aluminum tape. The results obtained are compared with experimental data available in the ... (Complete abstract click electronic access below) / Mestre

The key to success in separation of liquid mixtures is the efficient creation and utilization of vapour-liquid contact area. By packing the column with gas-liquid contact devices such as structured packing, the vapour-liquid contact area can be increased. However, the efficiency of these packed columns depends strongly on the local flow behaviour of the liquid and vapour phase inside the packing. The aim of this work was to develop three-dimensional CFD models to study the hydrodynamic behaviour on the corrugated sheets of packing. Different approaches are possible to simplify the problem and to extend it for more complex flow scenarios. In this work, three-dimensional CFD simulations were performed to study the complete fluid-dynamic behaviour. This was performed in two steps. As a first step, the developed model was validated with experimental studies using a simplified geometry i.e., an inclined plate. The three-dimensional Volume-of-Fluid (VOF) model was utilized to study the flow behaviour of the gas-liquid countercurrent flow. The influence of the liquid surface tension was taken into consideration using the Continuum Surface Force (CSF) model. The wetting characteristics of liquids with different viscosity (1 and 5 mPas) and contact angle (70° and 7°) were studied for different flow rates. Three different mixtures (water, water-glycerol (45 wt. %) and silicon-oil (DC5)) were considered. Initially, the rivulet width of experiments and simulations were compared and an error of 5 % maximum was determined. The results were also in good agreement with earlier studies. The percentage of wetting due to changes in flow rate, viscosity and contact angle was compared and discussed. For all tested systems, excellent agreement between the experiments and simulation studies was found. In addition, profiles of the velocity in the film at film flow conditions over a smooth inclined plate obtained from simulations were compared with experimental profiles obtained using a μPIV technique. A detailed sensitivity study was also performed in order to understand the changes in the velocity profiles due to small change in liquid flow rate, temperature and inclination angle. As a next step, the developed model was extended to geometries resembling real corrugated sheets of packing used in industrial applications. In earlier numerical studies of structured packing, geometries were simplified to enable easy meshing and faster computation. In this work, the geometries of corrugated sheets of packing were developed without any simplification and the flow behaviour was studied using the model validated in the first step. The flow behaviour on sheets with different geometrical modifications such as smooth and triangular crimp surfaces as well as perforations on the sheets were numerically studied and quantitatively compared with experimental studies for the three different fluid test systems. The agreement between the simulations and experiments was within an acceptable range for all system. The difference in the interfacial area between the corrugated sheets of a packing with and without perforation was analyzed and the prediction ability of different empirical correlations for the interfacial area available in literature was also compared and discussed. Furthermore, the numerical study was extended to understand the influence of the second corrugated sheet. Studying the flow behaviour between two sheets experimentally is very challenging, especially inside opaque packing. The model proved to be a very suitable tool to study the hold-up of the liquid between two sheets, the change in wetting behaviour due to small change in liquid inlet position. The results are also in good agreement with the earlier experimental studies, where researchers measured the liquid hold-up mainly in the region where two corrugated sheets touch each other. The three-dimensional CFD model was validated to study the flow behaviour on corrugated sheets of packing. The results from the simulations agree very well with findings from the experimental studies in terms of wetting and hold-up.