Climate change is becoming more severe than ever in human history and the emission of green house gas urgently needs to be reduced while global energy consumption remains booming. Large-scale application of clean fossil fuel combustion shall be considered as a priority for its economical advantages as well as reliability in meeting global energy needs. Oxygen-fired pressurized fluidized bed combustor technology with downstream carbon capture and sequestration is considered a key approach to clean coal combustion. In such technology, the fluidized bed combustor operates at elevated pressures and houses an in-bed heat exchanger tube bundle. It is essential to understand the rate of heat transfer between the immersed heat exchange surface and the fluidized bed as it is a key parameter in heat exchanger design. The goal of this work was to investigate the impact of pressure and presence of fine particles (i.e., surrogate for pulverized fuel) on the overall tube-to-bed heat transfer coefficient. Experiments were conducted in a pilot-scale fluidized bed with an inner diameter of 0.15 m under cold flow conditions. A tube bundle consisting of five horizontal staggered rows was completely submerged in the bed. One of the tubes was replaced by a heating cartridge housed in a hollowed copper rod. Five thermocouples distributed at 45º intervals along the copper rod circumference measured the surface temperature and ensured that local effects were included. The bed material was large glass beads of 1.0 mm in diameter while the fines were glass beads of 60 µm in diameter and thus susceptible to entrainment. The fine particles were continuously fed to the fluidized bed and then captured downstream by a filter system. Fluidization was conducted at 101, 600 and 1200 kPa with excess gas velocities (Ug - Umf) of 0.21, 0.29 and 0.51 m/s. Fine particle feed rates were 0, 9.5 and 14.4 kg/h. Two heating rod positions (2nd row and 4th row) were studies. Overall, the heat transfer coefficient approximately doubled when pressure was increased from 101 to 1200 kPa. At atmospheric conditions, where the slug flow regime occurred, the maximum heat transfer coefficient was at the bottom of the rod, while it moved to the side of the rod at high pressures where the bubbling regime occurred. As the heating rod moving from 2nd row to the 4th row, the averaged heat transfer coefficient increased by respectively 18%, 9% and 6% at 101, 600 and 1200 kPa. The addition of fine particles decreased the average heat transfer coefficient by 10 to 20 W/m2 K where the time – averaged heat transfer coefficient was approximately 220 and 450 W/m2K at 101 kPa and 1200 kPa respectively. There was no effect on the angular profile across the tube surface. The results showed that average heat transfer coefficients matched the correlation developed by Molerus et al. (1995) within a 5% difference across all conditions when fines were not present.

Presented here are the results from the steady state flow boiling of R245FA in a laboratory scale horizontal stainless-steel test tube with an inner diameter of 8.5 mm and a length of 900 mm at a saturation temperature of 35 °C and 40 °C. Experiments were conducted at mass fluxes ranging between 200 and 300 kg/m²s at inlet vapour qualities from 0.2 to 0.7 under uniform, and non-uniform imposed heat flux cases that are expected to exist in horizontal parabolic trough solar collectors. Nine (9) different heat flux distributions were investigated. Local and average heat transfer coefficients (HTC’s) were determined based on wall temperature measurements taken along the length and around the circumference of the test section. Through the choice of the fluid being linked to the possible usage of DSG technology in organic Rankine cycles, the qualitative trends and observed performance variations can be used to predict the same for a working fluid such as water. It was found that the non-uniformity of the heat flux greatly alters the HTC’s of the fluid undergoing boiling but has no effect on the pressure drop characteristics of the fluid undergoing boiling. Heating only on the sides of the tube yielded HTC’s that were 46 % lower than achieved under uniform heating. Heating only from the top proved to be more effective in heat transmission to the fluid than heating only from the bottom (as is the case on PTC solar fields), by only a slight margin, and both these cases yielded HTC’s that were 30 % lower than the uniform heating case. Applying a bell curve heat flux distribution over the tube walls yielded overall HTC’s that differed from the uniform case by a maximum of 5 %, even as the peak heat flux position changes around the circumference of the tube. A further study may be done to quantify the degree to which the non-uniformity of the heat flux influences the local HTC’s, and to develop correlations that may aid in predicting these cases. An integration with flow pattern mapping may also be done to solidify the understanding of the phenomenon governing these observations. / Dissertation (MEng)--University of Pretoria, 2021. / Department for International Development (DFID) through Royal Society-DFID Africa Capacity Building Initiative. / The UK Engineering and Physical Sciences Research Council (EPSRC) [grant numbers EP/T03338X/I and EP/P004709/1]. / Russian Government "Megagrant" project 075-15-2019-1888. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Most heat transfer tubes are designed for either fully uniform wall temperature or fully uniform wall heat flux boundary conditions under forced convection. Several applications, including but not limited to the solar collectors of renewable energy systems, do however operate with non-uniform boundary conditions. Limited research has been conducted on non-uniform wall heat flux heat transfer coefficients in circular tubes, especially for mixed convection conditions. Such works are normally numerical in nature and little experimental work is available. In this experimental investigation the effects of the circumferential heat flux distribution and heat flux intensity on the single phase (liquid) internal heat transfer coefficient were considered for a horizontal circular tube. Focus was placed on the laminar flow regime of water within a stainless steel tube with an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions, including fully uniform and partially uniform heat fluxes were studied for Reynolds numbers ranging from 650 to 2 600 and a Prandtl number range of 4 to 7. The heat flux conditions included 360˚ (uniform) heating, lower 180˚ heating, upper 180˚ heating, 180˚ left and right hemispherical heating, lower 90˚ heating, upper 90˚ heating and slanted 180˚ heating. Depending on the angle span of the heating, local heat fluxes of 6 631 W/m2 , 4 421 W/m2 , 3 316 W/m2 , 2 210 W/m2 and 1 658 W/m2 were applied. Results indicate that the local and average steady state Nusselt numbers are greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for case where the applied heat flux was positioned on the lower half (in terms of gravity) of the tubes circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Variations in the heat transfer coefficient were found to be due to the secondary buoyancy induced flow effect. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for fully uniform heating, lower 180° heating, and upper 180° heating. / Dissertation (MEng)--University of Pretoria, 2018. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

The problem of condensation of steam on a vertical tier of horizontal tubes is investigated by both analytical and experimental methods in this study. A computer program is written to perform the analysis of laminar film condensation on the horizontal tubes. The program is capable to calculate condensate film thickness and velocity distribution, as well as the heat transfer coefficient within the condensate. An experimental setup was also manufactured to observe the condensation phenomenon. Effects of tube diameter and temperature difference between steam and the tube wall on condensation heat transfer have been analytically investigated with the computer program. Experiments were carried out at different inclinations of the tier of horizontal tubes. Effects of the steam velocity and the distance between the horizontal tubes are also experimentally investigated. Results of the experiments are compared to those of the studies of Abdullah et al., Kumar et al. and Nusselt as well as to the analytical results of the present study.

A complete two-phase numerical model of film condensation from a mixture of a vapour and a non-condensing gas that is based on the two-dimensional elliptic governing equations with variable physical properties is presented. The model predicts the full viscous flow and heat and mass transfer for the mixture around the tube and in the entire liquid film from the top of the tube to the falling film below the tube. A finite volume method is used with a segregated solution approach and a dynamically moving computational grid that tracks the phase interface sharply. Fundamental balances of mass, energy, and force are enforced accurately at the phase interface. The model was developed in steps and validated against various experimental and theoretical works in the literature for different two-phase flows. The validation tests included stratified flow of liquid and gas in a horizontal channel, falling liquid film over a vertical wall, and condensation of steam from a steam-air mixture in a vertical channel. The model was used to simulate laminar film condensation from a downward flowing steam-air mixture over an isothermal horizontal tube. The validity of this new model is demonstrated by comparisons with previous theoretical and experimental studies. New results are presented on the effects of free-stream-to-tube temperature difference, upstream Reynolds number, free-stream gas mass fraction, and free-stream pressure on the condensate film development, the local and average heat transfer coefficients, and the total condensate mass flow rate. It was found that the temperature difference had the greatest effect on the condensation rate and film thickness. The presence of non-condensing gas in the vapour has a strong negative impact on the condensation process. For the pure steam case, moderate changes in the upstream Reynolds number showed slight increases in condensate mass flow rate with increased Reynolds number. For the mixture case, however, moderate increase in upstream Reynolds number increases significantly the condensate mass flow rate and film thickness. This trend becomes more noticeable as the free-steam gas mass fraction increases. Changing the free-stream pressure demonstrated that property variation had a relatively smaller effect than temperature difference and gas mass fraction changes. / February 2017

The phenomenon dropwise condensation on a horizontal gold coated tube is investigated by both analytical and experimental methods in this study. A computer program is prepared to calculate the dropwise condensation heat transfer rate on the horizontal gold coated tube. An experimental setup was also manufactured to measure the dropwise condensation heat transfer rate. The effects of flow rate, temperature of cooling water and also steam to wall temperature difference have been analytically investigated by using Mathcad computer program. Experiments were carried out at different inlet temperatures of cooling water. Effects of cooling water at different flow rates are also experimentally investigated. Results of the experiments are compared to those of the literature and the analytical results.

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.

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.

La compréhension des écoulements multiphasiques dans les évaporateurs à mini-canaux est primordiale pour la performance des boucles de climatisation dans le secteur automobile notamment. Cette thèse s’est principalement intéressée à l’écoulement d’entrée de tels évaporateurs ainsi qu’à la répartition des phases dans les mini-canaux. Dans un premier temps, l’écoulement adiabatique diphasique en entrée d’évaporateur a été étudié. Un dispositif expérimental transparent, respectant au mieux la géométrie d’entrée de l’évaporateur, a été réalisé afin de reproduire l’écoulement diphasique d’entrée en eau-air mais en respectant les régimes d’écoulement rencontrés avec du R134a. Plusieurs techniques de caractérisation ont été mises en œuvre (visualisation, conductimétrie, tube de Pitot et prises de pression) afin de quantifier les pertes de pression, les épaisseurs de film et les vitesses du gaz dans un régime principalement annulaire. Suivant le même principe, un autre module en acier-inox a été développé pour caractériser l’écoulement directement en entrée d’évaporateur avec du réfrigérant R134a. Dans un second temps, nous avons étendu l’étude au cas d’un évaporateur compact à mini-canaux. Dans deux situations adiabatiques : monophasique (eau) et diphasique (eau-air), les pertes de pression, la répartition des phases le long de l’évaporateur et le régime d’écoulement dans les mini-canaux ont été étudiés sur un échangeur fabriqué en polycarbonate dont la géométrie s’approche au mieux de celle d’un échangeur réel. Les nombreux résultats ainsi obtenus constituent une base de données conséquente utile à la simulation numérique de ce type d'écoulements diphasiques / Understanding of multiphase flows in mini-channel evaporators is essential for the performance of air-conditioning systems, particularly in automotive sector. This thesis is mainly interested in behavior of inlet flow and phase distribution in the mini-channels. Initially, an adiabatic two-phase flow at the evaporator's inlet was studied. A transparent experimental apparatus with the same geometry as an evaporator's inlet has been designed. This test section helped us to reproduce the same flow regimes with air-water as flow regimes encountered with R134a in an evaporator. Several characterization techniques were used (visualization, conductance probes, Pitot tube and pressure taps) to determine pressure losses, liquid film thickness and gas velocity in a predominantly annular flow regime. Following the same principle, another experimental facility in stainless steel was developed to directly characterize the R134a flow at the evaporator's inlet. Finally, we have extended the study to the case of a compact evaporator in two adiabatic situations: single-phase (water) and two-phase (air-water). Pressure losses, phase distribution along the evaporator and flow regime in mini-channels were studied on an evaporator made of transparent materials (polycarbonate) with a close geometry to that of a real evaporator. Numerous results were obtained to provide a consistent database that would be useful for numerical simulation of this type of two-phase flows

Sprinkled tube bundles with a thin liquid film flowing over them are used in various technology processes where it is necessary to separate the vapour and liquid phases quickly and efficiently. The process occurs predominantly at low temperatures with a corresponding decrease of pressure around the tube bundle. Such a technology is represented for instance by an evaporator at absorption units or an evaporator for sea water desalination. In ideal conditions water boils at the whole surface of an exchanger, but in practice it must be considered that in original spots of contact between water and the exchanger wall the water will not boil at the tubes' surface but the cooling liquid will merely be heated-up. The presented dissertation thesis focuses on this issue. The objective of the thesis was to determine the heat transfer coefficient at the surface of sprinkled tube bundles of various geometries at atmospheric pressure as well as low pressure. For this purpose experiments have been carried out at tube bundles consisting of copper tubes of 12,0 mm diameter placed horizontally one above another that were heated by water. Three types of tubes (smooth, sandblasted and grooved) of four various pitches (15,0 to 30,0 mm by 5,0 mm increments) have been tested. Simultaneously individual bundles' geometries consisted of 4, 6, 8 and 10 tubes with identical surface finish. Based on the conducted experiments the mathematical model of heat transfer that involves mainly analogy criteria has been made more accurate. A temperature field at the sprinkled tube bundle surface has been scanned by a thermographic camera during the performed experiments. Influence of geometry and tube surface finish on flow mode and consequently also on heat transfer has been assessed in accordance with the compiled methodology.