The thermophysical properties of nanofluids have attracted the attention of researchers to a far greater extent than the heat transfer characteristics of nanofluids have. Contradictory results on the thermal-fluid behaviour of nanofluids have been numerically and experimentally reported on in the open literature. Natural convection has not been investigated experimentally as much as the other properties of nanofluids. In this study, the characteristics and stability of Al2O3-water nanofluids (d = 20 30 nm) were analysed using a Malvern zetasizer, zeta potential and UV-visible spectroscopy. The natural convection of Al2O3- water nanofluids (formulated with a single-step method) was experimentally studied in detail for the volume fractions 0, 0.05, 0.1, 0.2, 0.4 and 0.6% in a rectangular cavity with an aspect ratio of 1, heated differentially on two opposite vertical walls for the Rayleigh number (Ra) range 3.49 x 10⁸ to 1.05 x 10⁹. The viscosity of Al2O3-water nanofluids measured between 15 and 50 °C. The effect of temperature and volume fraction on viscosity was also investigated. A detailed study of the nanoparticle concentration effect on the natural convection heat transfer coefficient was performed. It was found that increasing the concentration of nanoparticles improves the heat transfer coefficient by up to 15% at a 0.1% volume fraction. Further increasing the concentration of nanoparticles causes the natural convection heat transfer coefficient to deteriorate. This research also supports the idea that "for nanofluids with thermal conductivity more than the base fluids an optimum concentration may exist that maximises heat transfer in an exact condition as natural convection, laminar force convection or turbulence force convection". / Dissertation (MEng)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

This Master of Engineering investigation focuses on the natural convection of nanofluids in rectangular cavities. The governing equations applied to analyse the heat transfer and fluid flow occurring within the cavity are given and discussed. Special attention is given to the models that were developed to predict the thermal conductivity and dynamic viscosity of such nanofluids. A review concerning past investigations into the field of natural convection of nanofluids in cavities is made. The investigation is divided into experimental works and computational fluid dynamics (CFD) numerical investigations. Through the literature review, it was discovered that many numerical models exist for the prediction of the thermophysical properties of nanofluids, specifically thermal conductivity and viscosity. Depending on the nanofluid and the application, different models can be used. The literature study also revealed that most previous works were done in the CFD field. Very few experimental studies have been performed. Numerical CFD investigations, however, need experimental results for validation purposes, leading to the conclusion that more experimental work is needed. The heat transfer capability and thermophysical properties of the nanofluid are investigated based on models found in literature. The investigation incudes measuring the heat transfer inside a cavity filled with a nanofluid and subjected to a temperature gradient. The experiment is performed for several volume fractions of particles. An optimum volume fraction of 0.005 is obtained. At this volume fraction the heat transfer enhancement reaches a maximum for the present investigation. The investigation is repeated as a numerical investigation using the commercially available CFD software ANSYS-FLUENT. The same case as used in the experimental investigation is modelled as a two-dimensional case and the results are compared. The same optimum volume fraction and maximum heat transfer is obtained with an insignificantly small difference between the two methods of investigation. This error can be attributed to the minor heat losses experienced from the experimental setup as in the CFD adiabatic walls considered. It is concluded that, through the inclusion of TiO2 particles in the base fluid (deionised water), the thermophysical properties and the heat transfer capability of the fluid are altered. For a volume fraction of 0.005 and heat transfer at a temperature difference of 50 °C, the heat transferred through the fluid in the cavity is increased by more than 8%. From the results, it is recommended that the investigation is repeated with TiO2 particles of a different size to determine the dependency of the heat transfer increase on the particle size. Various materials should also be tested to determine the effect that material type has on the heat transfer increase. / Dissertation (MEng)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Les phénomènes de transport de matière ainsi que la cinétique des réactions chimiques sont des processus importants en électrochimie car ceux-ci contrôlent le courant mesuré. Dans ce contexte, nous nous intéressons à de simples réactions électrochimiques et à la classe des réactions de transfert couplés électrons-protons (PCET), jouant un rôle important dans les phénomènes biologiques et la conversion d’énergie. Ces réactions impliquent le transfert d’électron(s) et de proton(s) et sont représentées par un schéma carré. Alors que la cinétique de transfert d’électrons est largement étudiée, la cinétique de transfert de protons l’est plus rarement. Ces réactions sont en effet supposées être très rapides alors qu’il existe des situations où les réactions de protonation constituent l’étape limitante. La première partie de la thèse consiste à étudier la cinétique des réactions de protonation en tenant compte de la catalyse de Brönsted. Par le biais de simulations numériques, nous montrons que la catalyse augmente la réversibilité des voltampérogrammes cycliques, à des pH où le transfert couplé s’opère. Les prédictions numériques ont été comparées aux données expérimentales et les résultats sont encourageants car une même tendance est observée. L’accord quantitatif n’est cependant pas satisfaisant à ce stade. Les phénomènes de transport étant connus pour affecter les processus à l’électrode, la seconde partie de la thèse est consacrée à l’étude de l’influence de la convection. Nous commençons par présenter les différentes raisons qui peuvent expliquer les déviations expérimentales par rapport à la diffusion seule, comme la convection naturelle induite par des gradients de densité ou de tension superficielle. Nous présentons le concept de convection spontanée associé aux mouvements microscopiques de la solution. Bien que les fondements théoriques de la convection spontanée soient discutables, la théorie permet de reproduire les résultats d’un certain nombre d’expériences, souvent pratiquées en conditions non contrôlées. Ensuite, nous évaluons l’influence de la convection naturelle sur de simples réactions électrochimiques, avant de passer à l’étude des réactions PCET. Les simulations numériques nous ont permis de prévoir la déviation des chronoampérogrammes par rapport à une situation diffusive en fonction de la durée de l’expérience et de la contribution de chaque espèce à la densité de la solution. Pour une électrode située au bord supérieur, la production d’espèces plus denses amène une déviation du courant plus importante, dû au développement d’instabilités hydrodynamiques. La convection due aux gradients de densité est supposée être accentuée lorsque que les réactions électrochimiques sont couplées avec des réactions chimiques, ce qui est la définition même des PCET. Cependant, nous avons conclu à un impact négligeable de celles-ci, sauf pour de faibles valeurs de constantes cinétiques. Pour conclure, nous avons évalué d’une part l’impact de la convection due aux effets Marangoni et d’autre part son couplage à la convection induite par des gradients de densité. L’influence de ces mouvements convectifs sur le courant résultant dépend des propriétés des réactifs et des produits de la réaction, mais également de la présence d’une surface libre. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The influence of electromagnetic stirring in an electric arc furnace (EAF) has been studied. Using numerical modeling the effect of electromagnetic stirring on the thermal stratification and fluid flow has been investigated. The finite element method (FEM) software was used to compute the electromagnetic forces, and the fluid flow and heat and mass transfer equations were solved using a finite volume method (FVM) software. The results show that electromagnetic stirring has a significant effect on temperature homogenization and mixing efficiency in the bath. The important part of this study was calculation of heat transfer coefficient. The results show, electromagnetic stirring improves the heat transfer from the melt to scrap which is dependent on the stirring direction and force magnitudes.

No description available.

This work can be viewed in three separate sections, each of which build off of the prior. The first part of this study examined the flow in a 1/16th scale calandria test section based on a typical CANDU moderator layout. The experiments utilized forced flow supplied to the vessel and electrical heated rods to mimic the heat flow from calandria tubes. The size of the vessel, flow rates, and power levels were used to scale the experiments such that the provided representative temperature fields. The temperature field inside the vessel was measured and shown to compare well with CFD predictions over a wide range of inlet conditions and power levels. Additionally, this work addressed the scaling distortions in the experiment which occurred due to physical limitations when performing experiments at 1/16 scale (e.g., a smaller number of heater rods with a larger diameter were used in the experiment because at 1/16-scale direct fabrication of 390 fuel channel simulators is not feasible). The work proposed the H factor addition to the Ar. This additional scaling criteria was shown to better maintain the flow regimes expected CANDU moderators by taking into account distortions introduced by surface heating instead of volumetric heating in addition to the reduction in total number of tubes. While this work involved forced convective flows at the inlet of the vessel, in some regions of the calandria buoyancy induced forces were sufficiently high such that these phenomena altered the direction and magnitude of the flows as compared to purely forced convective behavior. Hence further work, discussed below, was initiated to better understand and measure these local phenomena where buoyancy forces are of similar magnitude as those of forced convection. Such local conditions we have terms mixed convection regime for the purposes of this thesis. The second part of this work further examined the mixed convection between a subset of the CANDU calandria tubes, namely how does a lower tube effect the mixed convection heat transfer of the upper tube in an inline arrangement. To isolate and measure the phenomena with sufficient detail, a small number of tubes was studied and advanced diagnostics such as Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) were employed. This study combined fluid velocity, temperature and wall temperature measurements with CFD simulations to develop a mechanistic model and understanding of the effect of natural convection plumes from lower elevations on the natural circulation phenomena on an upper cylinder. Superposition of the natural convection phenomena combined with pseudo forced convection effects from the lower elevation cylinder’s plume was used to model the mixed convection phenomena. This model was shown to perform well, with nearly all data being predicted to with +-20% for experiments performed in this work, and experiments in literature. A major finding from the preceding discussion is the importance of the lower elevation plume velocity on the local phenomena on the upper cylinder. The third section further expanded upon the prior two by replacing the lower cylinder with a diffuser nozzle which could provide a forced convective component with accurately defined velocities. Such measurements allow for accurate definition of the local Ri number and allowed full access for instrumentation to observe the velocity fields. The major contribution of this work was a flow regime map that defined the phenomena around a heated cylinder under mixed convection conditions. Additionally, the establishment of a database of fluid temperature and velocity measurements for a wide range of Ri was also developed and used to further validate CFD predictions. / Thesis / Doctor of Philosophy (PhD)

Designing buildings to use energy more efficiently can lead to lower energy costs, while maintaining comfort for occupants. Computational fluid dynamics (CFD) can be utilized to visualize and simulate expected flows in buildings and structures. CFD gives architects and designers the ability to calculate the velocity, pressure, and heat transfer within a building. Previous research has not modeled natural ventilation situations that challenge common design rules of thumb used for cross-ventilation and single-sided ventilation. The current study uses a commercial code (FLUENT) to simulate cross-ventilation in simple structures and analyzes the flow patterns and heat transfer in the rooms. In the Casa Giuliana apartment and the Affleck house, this study simulates passive cooling in spaces well-designed for natural ventilation. Heat loads, human models, and electronics are included in the apartment to expand on prior research into natural ventilation in a full-scale building. Two different cases were simulated. The first had a volume flow rate similar to the ambient conditions, while the second had a much lower flow rate that had an ACH of 5, near the minimum recommended value Passive cooling in the Affleck house is simulated and has an unorthodox ventilation method; a window in the floor that opens to an exterior basement is opened along with windows and doors of the main floor to create a pressure difference. In the Affleck house, two different combinations of window and door openings are simulated to model different scenarios. Temperature contours, flow patterns, and the air changes per hour (ACH) are explored to analyze the ventilation of these structures. / Master of Science

Solar chimney power plants use the buoyancy-nature of heated air to harness the Sun's energy without using solar panels. The flow is driven by a pressure difference in the chimney system, so traditional chimneys are extremely tall to increase the pressure differential and the air's velocity. Computational fluid dynamics (CFD) was used to model the airflow through a solar chimney. Different boundary conditions were tested to find the best model that simulated the night-time operation of a solar chimney assumed to be in sub-Saharan Africa. At night, the air is heated by the energy that was stored in the ground during the day dispersing into the cooler air. It is necessary to model a solar chimney with layer of thermal storage as a porous material for FLUENT to correctly calculate the heat transfer between the ground and the air. The solar collector needs to have radiative and convective boundary conditions to accurately simulate the night-time heat transfer on the collector. To correctly calculate the heat transfer in the system, it is necessary to employ the Discrete Ordinates radiation model. Different chimney configurations were studied with the hopes of designing a shorter solar chimney without decreases the amount of airflow through the system. Clusters of four and five shorter chimneys decreased the air's maximum velocity through the system, but increased the total flow rate. Passive advections wells were added to the thermal storage and were analyzed as a way to increase the heat transfer from the ground to the air. / Master of Science

No description available.

The Division for Electricity of Uppsala University is developing a wave power concept. The energy of the ocean waves is harvested with wave energy converters, consisting of one buoy and one linear generator. The units are connected in a submerged substation. The mechanical design is kept as simple as possible to ensure reliability. The submerged substation includes power electronics and different types of electrical power components. Due to the high cost of maintenance operations at sea, the reliability of electrical systems for offshore renewable energy is a major issue in the pursuit of making the electricity production economically viable. Therefore, proper thermal management is essential to avoid the components being damaged by excessive temperature increases. The chosen cooling strategy is fully passive, and includes no fans. It has been applied in the second substation prototype with curved heatsinks mounted on the inner wall of the pressurized vessel. This strategy has been evaluated with a thermal model for the completed substation. First of all, 3D-CFD models were implemented for selected components of the electrical conversion system. The results from these submodels were used to build a lumped parameter model at the system level. The comprehensive thermal study of the substation indicates that the rated power in the present configuration is around 170 kW. The critical components were identified. The transformers and the inverters are the limiting components for high DC-voltage and low DC-voltage respectively. The DC-voltage—an important parameter in the control strategy for the WEC—was shown to have the most significant effect on the temperature limitation. As power diodes are the first step of conversion, they are subject to large power fluctuations. Therefore, we studied thermal cycling for these components. The results indicated that the junction undergoes repeated temperature cycles, where the amplitude increased with the square root of the absorbed power. Finally, an array of generic heat sources was optimized. We designed an experimental setup to investigate conjugate natural convection on a vertical plate with flush-mounted heat sources. The influence of the heaters distribution was evaluated for different dissipated powers. Measurements were used for validation of a CFD model. We proposed optimal distributions for up to 36 heat sources. The cooling capacity was maximized while the used area was minimized.