Packed bed combustors burn fairly large solid fuel particles within confining walls, with air supplied from below the grate. As combustion occurs and particles are consumed, fresh particles are fed onto the bed so the level is kept roughly constant. Packed bed combustion is used for wood and biomass combustion in small-scale power plants, wood waste combustion in pulp and paper plants, and trash incineration. The structure of a packed bed is very important to the combustion process and can be defined by particle shape and size, sphericity, particle overlap (decreasing area availability) and chiefly by void fraction. Void fraction has already been proven of great influence in packed beds – it is raised to the third power in the pressure loss equation, and it can also affect heat and mass transfer and surface reaction rates. This thesis presents results of several experimental combustion tests that were performed in a packed bed combustor, using commercial spruce lumber particles of parallelepipedal geometry as fuel. At the end of each test the bed contents were removed, taking care to preserve their structure, and fixed with liquefied wax. The solidified bed was then cut into circular cross sections at different heights of the bed, and photographs of the cross sections were taken so the local void fraction could be estimated using image analysis. The bed sampling led to the discovery that, surprisingly, the actual bulk void fraction in the combustor, which is the average of local void fraction measurements, is less than that of the unburnt particles, varying from 19% to 30% in decrease in void fraction depending on the particle type used. Local measurements allowed the development of an empirical linear equation model to represent the variation of void fraction with height above the grate. Each combustion test had measurements of gas volume fractions and temperatures at different heights above the bed grate to be compared with the results of a numerical model simulation. The numerical model used in this work is an existing numerical model of all the relevant processes in packed bed combustion. Previously, the numerical model had assumed the void fraction to be constant and equal to that of the unburnt fuel, since no information on local variation was available, and the packing geometry remained self-similar as particles are consumed. Three models for void fraction were then compared in the combustion model: a constant void equal to that of the unburnt particles, the empirical linear fit of void fraction with height, and a constant void equal to the measured bulk void fraction. Maximum temperatures were higher using the unburnt fuel void fraction because of a thicker oxidation zone, whereas the void fraction model iii based on experiments generated a thicker reduction zone and therefore higher CO concentrations. CO concentrations were experimentally measured and agreed quite well with the CO concentration from the model. Local void fraction differences had the most impact in the diffusion-controlled zone, as shown by comparing the empirical void model and the measured bulk void fraction. How lowering the void fraction can increase gas velocities, heat and mass transfer coefficients, and burning rates is also discussed in this work.

Gas lift is a means of enhancing oil recovery from hydrocarbon reservoirs. Gas injected at the production riser base reduces the gravity component of the pressure drop and thereby, increases the supply of oil from the reservoir. Also, gas injection at the base of a riser helps to mitigate slugging and thus, improving the performance of the topside facility. In order to improve the efficiency of the gas lifting technique, a good understanding of the characteristics of gas-liquid multiphase flow in vertical pipes is very important. In this study, experiments of gas/liquid (air/water) two-phase flows, liquid/liquid of oil/water two-phase flows and gas/liquid/liquid (air/oil/water) three-phase flows were conducted in a 10.5 m high 52 mm ID vertical riser. These experiments were performed at liquid and gas superficial velocities ranging from 0.25 to 2 m/s and ~0.1 to ~6.30 m/s, respectively. Dielectric oil and tap water were used as test fluids. Instruments such as Coriolis mass flow meter, single beam gamma densitometer and wire-mesh sensor (WMS) were employed for investigating the flow characteristics. For the experiments of gas/liquid (air/water) two-phase flow, flow patterns of Bubbly, slug, churn flow regimes and transition regions were identified under the experimental conditions. Also, for flow pattern identification and void fraction measurements, the capacitance WMS results are consistent with those obtained simultaneously by the gamma densitometer. Generally, the total pressure gradient along the vertical riser has shown a significant decrease as the injected gas superficial velocity increased. In addition, the rate of decrease in total pressure gradient at the lower injected gas superficial velocities was found to be higher than that for higher gas superficial velocities. The frictional pressure gradient was also found to increase as the injected gas superficial velocity increased. For oil-water experiments, mixture density and total pressure gradient across the riser were found to increase with increasing water cut (ranging between 0 - 100%) and/or mixture superficial velocity. Phase slip between the oil and water was calculated and found to be significant at lower throughputs of 0.25 and 0.5 m/s. The phase inversion point always takes place at a point of input water cut of 42% when the experiments started from pure oil to water, and at an input water cut of 45% when the experiment’s route started from water to pure oil. The phase inversion point was accompanied by a peak increase of pressure gradient, particularly at higher oil-water mixture superficial velocities of 1, 1.5 and 2 m/s. The effects of air injection rates on the fluid flow characteristics were studied by emphasizing the total pressure gradient behaviour and identifying the flow pattern by analysing the output signals from gamma and WMS in air/oil/water experiments. Generally, riser base gas injection does not affect the water cut at the phase inversion point. However, a slight shift forward for the identified phase inversion point was found at highest flow rates of injected gas where the flow patterns were indicated as churn to annular flow. In terms of pressure gradient, the gas lifting efficiency (lowering pressure gradient) shows greater improvement after the phase inversion point (higher water cuts) than before and also at the inversion point. Also, it was found that the measured mean void fraction reaches its lowest value at the phase inversion point. These void fraction results were found to be consistent with previously published results.

Multiphase flows occurring in pipelines with a serpentine configuration is an important phenomenon, which can be encountered in heat exchangers used in a variety of industrial processes. More specifically, in many industrial units such as a large cracking furnace in a refinery, the tubes are arranged in a serpentine manner and are relatively short. As flow negotiates round the 180o bend at the ends of the tubes, the generated centrifugal force could cause flow maldistribution creating local dry spots, where no steady liquid film is formed on the adjacent straight sections of the pipe. As a result, events including coking, cracking and overheating of heat transfer surfaces may occur and lead to frequent shutdown of the facilities. Consequently, this could increase operating costs and reduce production revenue. Thus, it is desirable to know the effect that the bends exert on the flow in the straight part of the pipe. Apart from this, knowledge of the bend effects on the flows in the pipeline could also be important for the design of other pipelines for gas/liquid transport, e.g. offshore gas and oil pipelines. Quite a large number of studies have been found in the literature. The majority of them were for two-phase flow with small diameter pipes (i.d. ≤ 50 mm). However, studies with large diameter pipes (i.d. ≥ 100 mm), have increasingly been considered in recent years as problems related to large diameter vertical pipes are being encountered more and more often in industrial situations. This thesis studies the effect of 180o bends on the characteristics and development of gas-liquid two-phase flows in large diameter downward and upward pipes. The study particularly focuses on the influence of serpentine configuration on flow structure, cross-sectional void distribution and circumferential liquid film profiles and their development along the downward and upward sections. It was found that both the top and bottom bends have considerable impacts on flow behaviour, although to varying degrees. These impacts were highly dependent on the air and water flow rates. For sufficient flow rates, the bends were observed to create flow maldistribution in the adjacent straight section, due to the effects of centrifugal force. The air moved towards the inner zone of the bend and the water towards the outer zone, while a lesser quantity of water was identified on the other surfaces of the pipe. Investigation of the film thickness development in the downward and upward sections showed that, the liquid film behaviour close to the bends was significantly different from those located further away. This can be attributed to the centrifugal force of the bends. Examination of the power spectral density (PSD) along the downward and upward sections showed that, the shape of PSD located in the adjacent section to the bends, was substantially different from those located further away. Furthermore, several flow regime maps were generated which showed that, in addition to bubbly, intermittent and annular flows, unstable flows existed along the upward section, particularly for low gas and water flow rates. In this study it was found that, the lower bend was periodically blocked by the liquid and then blown through by the accumulated air. The data obtained from this study were compared with different theoretical correlations found in the existing literature. Some discrepancy between the results of the current study and those of previous published materials was noted. Updated correlations were presented which provided well results when they applied for the data obtained from the current study and previous studies.

O presente trabalho envolve um estudo teórico e experimental do escoamento bifásico externo a banco de tubos. Inicialmente, apresenta-se uma ampla revisão da literatura sobre padrões de escoamento, fração de vazio e perda de pressão, durante escoamentos monofásicos e bifásicos externos a banco de tubos. Nesta análise são também descritos os métodos de previsão destes parâmetros. Verificam-se diferenças significativas entre as estimativas proporcionadas por eles, fato que indica a inexistência de métodos generalizados. Posteriormente é apresentada uma descrição detalhada da bancada experimental projetada e construída durante o doutoramento. O aparato completo compõe-se da seção de testes, circuito de água, sistema de compressão e condicionamento de ar, e seções de injeção dos fluxos e condicionamento do escoamento. A seção de testes consiste em um banco de tubos distribuídos segundo configuração triangular normal, com os tubos apresentando diâmetro externo de 19,1 mm, comprimento de 381 mm, e espaçamento transversal de 24 mm. Os experimentos foram realizados para escoamento vertical ascendente de misturas água-ar e velocidades superficiais da fase líquida e gás de 0,020 a 1,500 m/s e de 0,10 a 10,00 m/s, respectivamente. Neste estudo foram desenvolvidas técnicas inéditas para determinação experimental da fração de vazio superficial no interior do banco de tubos baseadas em sistemas óptico e de sensoriamento capacitivo. Os padrões de escoamento foram identificados subjetivamente através de visualização de imagens e vídeos do escoamento, e objetivamente com o auxílio do método de agrupamento de dados k-means utilizando parâmetros baseados no sinal de perda de pressão e do sensoriamento capacitivo. Identificou-se subjetivamente os padrões de escoamento bolhas, bolhas dispersas, bolhas grandes, agitante, intermitente e anular. Constatou-se equivalência entre os padrões de escoamento identificados através dos métodos objetivo e subjetivo. Resultados experimentais para fração de vazio foram obtidos através de técnicas óptica e capacitiva. Constatou-se que o traçador rodamina B utilizado no método óptico altera as condições do escoamento, ainda que em concentrações reduzidas. A partir dos resultados obtidos com o sensoriamento capacitivo estimou-se a fração de vazio para o padrão bolhas. Resultados para a parcela friccional da perda de pressão também foram levantados. Constata-se o incremento da fração de vazio e da parcela friccional da perda de pressão com as velocidades superficiais das fases líquida e gás. Os resultados para fração de vazio foram comparados com métodos de previsão da literatura, e de maneira geral os métodos preveem as tendências dos resultados experimentais apenas para vazões de líquido reduzidas. Analogamente, os resultados para perda de pressão foram comparados com estimativas segundo métodos da literatura, concluindo que os métodos não preveem satisfatoriamente os resultados obtidos. Desta forma, foram propostos novos métodos de previsão para padrões de escoamento, fração de vazio e parcela friccional da perda de pressão, desenvolvidos a partir de análises dos mecanismos dominantes do escoamento, e adotando parâmetros adimensionais para correlacionar os dados. Os métodos propostos preveem satisfatoriamente os resultados experimentais deste estudo e da literatura para escoamentos bifásicos água-ar. / The present thesis concerns a theoretical and experimental study of external two-phase flows across tube bundles. Initially, a comprehensive literature review covering flow patterns, void fraction and pressure drop for single and two-phase flows across tubes bundle is presented. The review also describes predictive methods for these parameters. A comparison of these methods reveals reasonable disagreement among their predictions, indicating the absence of generalized methods. Subsequently, the apparatus and instrumentation designed and built to obtain the experimental data are described. The experimental apparatus comprises the test section, a water loop, air compression and conditioning systems, and sets for fluid flow injections and conditioning. The test section is a normal triangular tube bundle, with 19.1 mm OD tubes, 381 mm long and transversal pitch of 24 mm. The experiments were performed for air-water upward vertical flow, for superficial liquid and gas velocities ranging from 0.020 to 1.500 m/s and 0.10 to 10.00 m/s, respectively. Innovative techniques to evaluate the void fraction within the bundle were developed based on capacitive and optical methods. The flow patterns were identified subjectively and objectively by k-means clustering method, using as clustering parameters the pressure drop and the capacitive signals. Bubbles, dispersed bubbles, large bubbles, churn, intermittent and annular flow patterns were identified subjectively. The data groups identified by the objective method are representative of the flow patterns. Void fraction measurements were obtained for bubbly flow using both techniques (optical and capacitive). The void fraction data based on the optical method had its experimental range limited due to changes in the flow characteristics caused by the addition of the fluorescent dye Rhodamine B. The experimental results indicate that the void fraction increases with increasing the superficial velocities of both phases. In general, the void fraction predictive methods available in the literature capture the trends of the experimental results only for reduced liquid flow rates. According to the experimental results, the frictional pressure drop increases asymptotically with increasing the flow rates of both phases. None of the predictive methods from literature evaluated in the present study predicted satisfactorily the experimental results. Methods for prediction of flow patterns, void fraction and frictional pressure drop parcel were also developed in the present study. These methods provided reasonable predictions of the experimental results obtained in the present study, and also from the literature for air and water flows across tube bundles.

Les écoulements diphasiques avec ou sans changement de phase sont présents dans les applications terrestres et spatiales avec notamment le contrôle thermique des satellites par boucle diphasique, l’alimentation en propergol des moteurs de fusée et le traitement des eaux usées pour les missions d’exploration spatiale. Des expériences d’ébullition convective dans un tube chauffé avec du HFE7000 ont été menées en écoulement vertical ascendant au sol et en microgravité conditions afin de caractériser les régimes d’écoulements et de mesurer les transferts de chaleur, le taux de vide et les pertes de pression. Les mesures de taux de vide ont permis de caractériser la vitesse moyenne de la phase vapeur et l’épaisseur du film liquide en écoulement annulaire. En microgravité, l’épaisseur du film liquide et le frottement interfacial sont inférieurs aux conditions de gravité terrestre. La structure du film liquide a été caractérisée par des visualisation rapides. L’impact de la gravité, des vitesses superficielles du liquide et de la vapeur sur la célérité et la fréquence des ondes perturbatrices a été mis en évidence. Deux techniques de mesure ont été implémentées et comparées pour la mesure du coefficient d’échange de chaleur. En ébullition convective saturée pour des titres massiques supérieurs à 0.2, le transfert de chaleur est peu sensible à la gravité et en bon accord avec des corrélations de la littérature. En ébullition nucléée sous refroidie pour des titres inférieurs à 0.1, le transfert de chaleur est significativement plus faible en microgravité. / Two-phase flows with or without phase change are present in terrestrial and space applications like thermal control of satellites, propellant supply for launchers, and waste water treatment for space exploration missions. Flow boiling experiment with HFE7000 were conducted in a heated tube in vertical upward flow on ground and in microgravity conditions to collect data on flow patterns, pressure drops, heat transfers, void fraction. Void fraction measurements allowed to measure mean gas velocity and the liquid film thickness in annular flow. In microgravity condition, the liquid film thickness and the interfacial shear stress are significantly lower than in normal gravity. A detail analysis of the film structure was performed by image processing. The impact of gravity and liquid and vapour superficial velocities on the disturbance waves velocities and frequencies was investigated. Two different measurement techniques were used and compared to determine the heat transfer coefficient. For quality values greater than 0.2, HTC is not sensitive to gravity and is in good agreement with classical correlations of the literature. For qualities smaller than 0.1, in the subcooled nucleate boiling regime HTC is significantly smaller in microgravityconditions.

Flow mechanisms affect transport processes during condensation. Most studies on two-phase flow regimes are qualitative in nature, typically providing only information to guide the identification of the respective regimes and the transitions between them. These studies have, however, not yielded quantitative information to assist the development of pressure drop and heat transfer models. Such qualitative studies have also yielded results with considerable variability between investigators. A comprehensive investigation of flow mechanisms, void fraction, pressure drop and heat transfer during condensation of R404A in microchannels was conducted. In contrast to all prior investigations, high-speed video recordings and image analyses were used to directly measure void fraction, slug frequencies, vapor bubble velocity, vapor bubble dimensions and liquid film thicknesses in tube diameters ranging from 0.508 to 3.00 mm. Experiments were conducted at reduced pressures and mass fluxes ranging from 0.38 to 0.77 and 200 to 800 kg m-2 s-1, respectively, to document their influences on the condensation process at local vapor qualities ranging from 0 to 1. This information was used to develop a model for the void fraction in condensing flows. A complementing set of heat transfer and pressure drop measurements were conducted on the same geometries at similar conditions, and the void fraction model was used in conjunction with these measurements to develop improved heat transfer and pressure drop models. This comprehensive set of experiments and analyses yields a self-consistent and accurate treatment of high-pressure condensation in small hydraulic diameter geometries. Furthermore, the heat transfer model was found to agree well with condensing ammonia and carbon dioxide data that were obtained at significantly different conditions in different tube diameters. The added physical understanding of the condensation process and the models developed will serve as important building blocks for the design of microscale condensers and thermal systems.

On the efforts to provide a reliable source of experimental information on turbulent subcooled boiling ow, time resolved Particle Tracking Velocimetry (PTV) experiments were carried out using HFE-301 refrigerant ow through a vertical rectangular channel with one heated wall. Measurements were performed at liquid Reynolds numbers of 3309, 9929 and 16549 over a wall heat flux range of 0.0 to 64.0 kW=m2. From the PTV measurements, liquid two dimensional turbulence statistics are available, such as: instantaneous 2-D velocity fields, time-averaged axial and normal velocities, axial and normal turbulence intensities, and Reynolds stresses. The present results agree with previous works and provide new information due to the 2-D nature of the technique, for instance, this work shows that by increasing heat ux, the boiling bubbles influence on the liquid phase is portrayed as a persistent increase of axial velocity on regions close to the heater wall. This persistent increase on the axial velocity reaches a maximum value attributed to the terminal bubble velocity. These new observed phenomena must be considered for the development and improvement of two-phase ow turbulence models. To this end, an extensive error analysis was also performed with emphasis on the applicability of the PTV measurement technique on optically inhomogeneous flows. The error quantification exhibited negligible optically induced errors for the current conditions, making the data acquired in this work a vast and reliable source.

Two-phase liquid-gas flow has a wide variety of applications in space, including active thermal control systems, high-power communications satellites, heat pumps and space nuclear reactors. Two-phase systems have many potential advantages over current single-phase systems due to reductions in system size, weight and power consumption. The mechanisms of pressure drop, heat transfer coefficients, void fractions, and flow regimes must be well understood under microgravity conditions in order to design reliable two-phase systems. The main objective of this present research is to develop a new mathematical model that can accurately predict the annular two-phase friction pressure drop to optimize the design of two-phase systems. The two-phase flow tests were conducted aboard the NASA KC-135 aircraft by the Interphase Transport Phenomena (ITP) group from Texas A&M University. The two-phase flow pressure drops were measured across a single transparent test section 12.7 mm ID and 1.63 m long in annular regimes under microgravity conditions during two flight campaigns. Different from previous work, this was the first time both the void fraction and the film thickness were measured under microgravity conditions. The empirical correlations for the interfacial friction factor and void fraction were developed from 57 experimental data using a linear least squares regression technique. The annular two-phase friction pressure drop can be predicted by the new mathematical model requiring only knowledge of the length and diameter of the tube, liquid and vapor mass flow rates, and properties of the working fluid. In addition, the new mathematical model was validated using Foster-Miller & ITP data collected over twelve flights aboard the KC-135 with working fluid R-12 (77 data points), Sundstrand data collected aboard the KC-135 with working fluid R-114 (43 data points) and Zhao and Rezkallah data aboard the KC-135 with working fluid water and air (43 data points). Compared with the LockhartMartinelli model, Wheeler model, Chen model and homogeneous model, the new mathematical model is the optimal model for predicting the two-phase friction pressure drop in annular regimes. The majority of the data falls within +-20% of the proposed correlation and the average error is 12%.

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.

Ziel der Arbeiten war die Weiterentwicklung und Validierung von Modellen in CFD-Codes. Hierzu wurde am FZD die thermohydraulische Versuchsanlage TOPFLOW aufgebaut und mit räumlich und zeitlich hochauflösenden Gittersensoren ausgestattet. Vertikale Teststrecken mit Nenndurchmessern von DN50 bzw. DN200 für Luft/Wasser- sowie Dampf/Wasser-Strömungen lieferten Ergebnisse zur Entwicklung von Strömungsformen, zum Verhalten der Zwischenphasengrenzfläche sowie zum Wärme- und Impulsaustausch zwischen den Phasen. Die Validierung des CFD-Codes in komplexen Geometrien erfolgte anhand von 3D Gasgehalts- und Geschwindigkeitsfeldern, die bei Umströmung eines asymmetrischen Hindernisses auftreten, das in der Teststrecke DN200 eingebaut war. Im Hinblick auf Strömungen mit freier Oberfläche untersuchte das FZD in zwei horizontalen Acrylglas-Kanälen geschichtete Zweiphasenströmungen im Gleich- bzw. Gegenstrom sowie Schwallströmungen. Bei den Nachrechnungen dieser Versuche gelang die Simulation der Schwallentstehung. Entsprechend des Projektziels wurden die experimentellen Ergebnisse zur Modellentwicklung genutzt. Bei vertikalen Strömungen stand die Wirkung der lateralen Blasenkräfte (z.B. Liftkraft) im Vordergrund. Zum Test unterschiedlicher Modellansätze wurde hierzu ein Mehrblasenklassen-Testsolver entwickelt und genutzt. Darauf aufbauend wurde ein neues Konzept für ein Mehrblasenklassenmodell, das Inhomogene MUSIG Modell erarbeitet und in den kommerziellen CFD Code CFX (ANSYS) implementiert. Bei Validierungsrechnungen zeigte sich, dass vor allem die Blasenkoaleszenz- und -zerfallsmodelle weiter optimiert werden müssen. Untersuchungen zu Einzeleffekten, wie z.B. die Abschätzung von Turbulenzkoeffizienten und die Analyse der Trajektoren von Einzelblasen in unmittelbarer Wandnähe, lieferten weitere wichtige Ergebnisse des Projekts.