In modern heat transfer, shape and geometric optimisation are new considerations in the evaluation of thermal performance. In this research, we employed constructal theory and design to present three-dimensional theoretical and numerical solutions of conjugate forced convection heat transfer in heat generating devices with cooling channels of different cross-sectional shapes. In recent times, geometric configurations of cooling channel have been found to play an important role in thermal performance. Therefore, an efficient ways of optimally designing these cooling channels shapes is required. Experimentation has been extensively used in the past to understand the behaviour of heat removals from devices. In this research, the shapes of the cooling channels and the configurations of heat-generating devices were analytically and numerically studied to minimise thermal resistance and thus illustrate cooling performance under various design conditions. The cooling channels of five different cross-sectional shapes were studied: Circular, square, rectangular, isosceles right triangular and equilateral triangular. They were uniformly packed and arranged to form larger constructs. The theoretical analysis is presented and developed using the intersection of asymptotes method. This proves the existence of an optimal geometry of parallel channels of different cross-sectional shapes that penetrate and cool a volume with uniformly distributed internal heat generation and heat flux, thus minimising the global thermal resistance. A three-dimensional finite volume-based numerical model was used to analyse the heat transfer characteristics of the cross-sectional shapes of various cooling channels. The numerical computational fluid dynamics (CFD) package recently provided a more cost-effective and less time-consuming means of achieving the same objective. However, in order to achieve optimal design solutions using CFD, the thermal designers have to be well experienced and carry out a number of trial-and-error simulations. Unfortunately, this can not always guarantee an accurate optimal design solution. In this thesis a mathematical optimisation algorithm (a leapfrog optimisation program and DYNAMIC-Q algorithm) coupled with numerical CFD was employed and incorporated into the finite volume solver, –FLUENT, and grid (geometry and mesh) generation package, – GAMBIT to search and identify the optimal design variables at which the system would perform optimally for greater efficiency and better accuracy. The algorithm was also specifically designed to handle constraint problems where the objective and constraint functions were expensive to evaluate. The automated process was applied to different design cases of cooling channels shapes. These cooling channels were embedded in a highly conductive solid and the peak temperature was minimised. The trend and performance of all the cooling channel shapes cases studied were compared analytically and numerically. It was concluded that an optimal design can be achieved with a combination of CFD and mathematical optimisation. Furthermore, a geometric optimisation of cooling channels in the forced convection of a vascularised material (with a localised self-cooling property subjected to a heat flux) was also considered. A square configuration was studied with different porosities. Analytical and numerical solutions were provided. This gradient-based optimisation algorithm coupled with CFD was used to determine numerically the optimal geometry that gave the lowest thermal resistance. This optimiser adequately handled the numerical objective function obtained from numerical simulations of the fluid flow and heat transfer. The numerical results obtained were in good agreement with results obtained in the approximate solutions based on scale analyses at optimal geometry dimensions. The approximate dimensionless global thermal resistance predicted the trend obtained in the numerical results. This shows that there were unique optimal design variables (geometries) for a given applied dimensionless pressure number for fixed porosity. The results also showed that the material property had a significant influence on the performance of the cooling channel. Therefore, when designing the cooling structure of vascularised material, the internal and external geometries of the structure, material properties and pump power requirements would be very important parameters to be considered in achieving efficient and optimal designs for the best performance. Finally, this research investigated a three-dimensional geometric optimisation of conjugate cooling channels in forced convection with an internal heat generation within the solid for an array of cooling channels. Three different flow orientations based on constructal theory were studied numerically- firstly, an array of channels with parallel flow; secondly, an array of channels in which flow of every second row was in a counter direction and finally, an array of channels in which the flow direction in every channel was opposite to that of previous channel. The geometric configurations and flow orientations were optimised in such a way that the peak temperature was minimised subject to the constraint of fixed global volume of solid material. The optimisation algorithm coupled with CFD was also used to determine numerically the optimal geometry that gave the lowest thermal resistance. The use of the optimisation algorithm coupled with the computational fluid dynamics package; render the numerical results more robust with respect to the selection of optimal structure geometries, internal configurations of the flow channels and dimensionless pressure difference. / Thesis (PhD(Eng))--University of Pretoria, 2012. / Mechanical and Aeronautical Engineering / unrestricted

Dropwise condensation, which collects the condensate liquid in the form of droplets, has attracted a growing interest due to much higher heat transfer coefficient. One important and challenging issue in dropwise condensation is the presence of non-condensable gas (NCG) which drastically reduces its heat transfer performance. Concerning the mechanism understanding, this thesis is aiming to investigate dropwise condensation in case of NCG by combing different methods. Firstly, convective dropwise condensation out of moist air is experimentally investigated under controllable conditions. In modeling, some crucial aspects are reasonably captured: the coupled heat and mass transfer during droplet growth by a multi-scale droplet growth model; the inter-droplet interaction defined by a distributed point sink method; the enhancement of the convective mass transfer using the droplet Sherwood number. Furthermore, a multi-component multi-phase thermal pseudopotential-based LB model is developed to advance the directly numerical simulation of dropwise condensation.

E-bikes, often known as electric bicycles, are becoming more and more well-liked as green modes of mobility. High-capacity lithium-ion (Li-ion) batteries are utilised to power these e-bikes because of their extended cycle life, high energy density, and low self-discharge rate. The performance and longevity of these batteries may be impacted by temperature fluctuations, however. To guarantee the safe and dependable functioning of Li-ion batteries used in e-bikes, it is crucial to do temperature analysis on the batteries. In this dissertation, the thermal behaviour of a 48V 60AH Li-ion battery used in an e-bike will be studied under various cooling scenarios. The research specifically contrasts forced convection cooling using fans with broad and limited outlet ports to natural air convection cooling with large and reduced outlet ports. The study sheds light on the ideal cooling setups that might increase battery longevity and performance. The results of this study have important ramifications for e-bike producers and designers, battery producers, and energy storage system researchers. Simulations based on computational fluid dynamics (CFD) are used to simulate the thermal behaviour of the Li-ion battery under various cooling settings for the investigation. 25°C has been selected as the ambient temperature. For forced convection, the airflow rate is set at 3.5 m/s, whereas the airflow rate for natural convection is set at 0.1 m/s. The study's findings demonstrate that both natural and forced convection cooling methods may successfully lower the temperature of a Li-ion battery. However, forced air convection cooling using fans is more efficient than natural air convection at dispersing heat. These findings suggest that, owing to the higher air velocity, shrinking the outlet ports in both cooling approaches improves thermal performance.

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The purpose of this study is to investigate forced convection of air through open cell foams. It can be numerically investigated either by implementing the time efficient macroscopic models or computationally expensive microscopic models. However, during the course of this study, it was observed that the macroscopic models are not sufficient for determining the desired key parameters. Nevertheless, it is still possible that these macroscopic models can be used to design an application accurately with minimum time efforts if the concerned key parameters are already known through other means. Accordingly, in this work, a methodology is developed to determine the desired key parameters by implementing the microscopic models, which are further used into the macroscopic models for designing different applications. To validate the proposed methodology, a set of steady state and transient forced convection experiments were performed for a set of ceramic foams having different pore diameter (10−30 PPI) and porosity (0.79−0.87) for a superficial velocity in the range of 0.5−10 m/s.

EDF exploite sur son parc de production de nombreuses machines électriques tournantes. Les contraintes thermiques subies par celles-ci engendrent des échauffements locaux qui nuisent à leur intégrité. Le présent travail contribue à fournir des méthodes de calcul adaptées à la détection et à la localisation des points chauds. Il participe à améliorer la compréhension des écoulements en rotation et leurs effets sur les transferts thermiques. Plusieurs dispositifs expérimentaux, de complexité ascendante, ont été utilisés pour comprendre et valider les simulations numériques. Une première étude sur une marche descendante (demi-pôle) parcourue par un jet de paroi non-confiné a mis en avant des différences par rapport à un jet confiné ; ces deux cas existent dans un alternateur. Une seconde étude menée sur une cavité tournante confinée a analysé l’impact d’un écoulement de Taylor-Couette-Poiseuille sur la température et la position des points chauds créés, en balayant l’ensemble des régimes d’écoulement. Ces études ont mis en exergue une première méthode de calcul fiable, fondée sur l’étude numérique CHT. Une autre méthode, basée sur la FEM couplée à une méthode inverse, a été testée sur une maquette d’alternateur hydraulique afin de pallier aux temps de calcul longs de la première. Cette méthodologie remonte aux coefficients d’échanges convectifs numériques à partir des mesures du champ thermique du rotor, mais n’est envisageable que lorsque l’on dispose de données expérimentales suffisantes. Ces travaux ont aussi mis en évidence de nouvelles techniques de mesures sans contact, comme l’utilisation d’un pyromètre à haute fréquence pour la mesure de température sur des machines tournantes. / EDF operates a large number of electrical rotating machines in its electricity generation capacity. Thermal stresses which affect them can cause local heating, sufficient to damage their integrity. The present work contributes to provide methodologies for detecting hot spots in these machines, better understanding the topology of rotating flows and identifying their effects on heat transfer. Several experimental scale model were used by increasing their complexity to understand and validate the numerical simulations. A first study on a turbulent wall jet over a non-confined backward-facing step (half-pole hydrogenerator) notes significant differences compared to results from confined case : both of them are present in an hydrogenerator. A second study was done on a small confined rotating scale model to determinate the effects of a Taylor-Couette-Poiseuille on temperature distribution and position of hot spots on the heated rotor, by studying the overall flow regimes flow. These studies have helped to obtain a reliable method based on conjugate heat transfer (CHT) simulations. Another method, based on FEM coupled with the use of an inverse method, has been studied on a large model of hydraulic generator so as to solve the computation time issue of the first methodology. It numerically calculates the convective heat transfer from temperature measurements, but depends on the availability of experimental data. This work has also developped new no-contact measurement techniques as the use of a high-frequency pyrometer which can be applied on rotating machines for monitoring temperature.

The problem of laminar and turbulent conjugate mixed convection flow and heat transfer in shallow enclosures with a series of block-like heat generating components is studied numerically for a Reynolds number range of zero (pure natural convection) to typically 106, Grashof number range of zero (pure forced convection) to 1015 and various block-to-fluid thermal conductivity ratios, with air as the working medium. The shallow enclosure has modules consisting of heat generating elements, air admission and exhaust slots. Two problems are considered. In the first problem, the enclosure has free boundaries between the modules and in the second problem, there are partitioning walls between the different modules. The flow and temperature distributions are taken to be two-dimensional. Regions with the same velocity and temperature distributions can be identified assuming repeated placement of the blocks and fluid entry and exit openings at regular distances, neglecting end wall effects. One half of such rectangular region is chosen as the computational domain taking into account the symmetry about the vertical centreline. On the basis of the assumption that mixed convection flow is a superposition of forced convection flow with finite pressure drop and a natural convection flow with negligible pressure drop, the individual flow components are delineated. The Reynolds number is based on forced convection velocity, which can be determined in practice from the fan characteristics. This is believed to be more meaningful unlike the frequently used total velocity based Reynolds number, which does not vanish even in pure natural convection and which makes the fan selection difficult. Present analysis uses three models of turbulence, namely, standard k-ε (referred to as Model-1), low Reynolds number k-ε (referred to as Model-2) and an SGS kinetic energy based one equation model (referred to as Model-3). Results are obtained for aiding and opposing mixed convection, considering also the pure natural and pure forced convection limiting cases. The results show that higher Reynolds numbers tend to create a recirculation region of increasing strength at the core region and that the ranges of Reynolds number beyond which the effect of buoyancy becomes insignificant are identified. For instance, in laminar aiding mixed convection, the buoyancy effects become insignificant beyond a Reynolds number of 500. Results are presented for a number of quantities of interest such as the flow and temperature distributions, local and average Nusselt numbers and the maximum dimensionless temperature in the block. Correlations are constructed from the computed results for the maximum dimensionless temperature, pressure drop across the enclosure and the Nusselt numbers.

Cette thèse a pour objectif d’étudier les performances thermiques et rhéologiques des ferrofluides sous champ magnétique pour des applications de refroidissement. L’approche adoptée dans cette thèse est de nature macroscopique, et est basée sur plusieurs études expérimentales. Cette caractérisation des performances des ferrofluides est focalisée sur trois aspects : i) étude de la rhéologie ii) étude de la convection forcée iii) étude la conductivité thermique. Différents outils de caractérisation correspondant à chaque domaine d’étude ont été développés. Dans le domaine de la rhéologie, une cellule magnétique a été construite et adaptée à un rhéomètre afin d’étudier le comportement rhéologique du ferrofluide sous un champ magnétique allant jusqu’à 0,8 T. Cette démarche met en évidence l’influence du champ magnétique et de son intensité sur les forces de cisaillement. Dans le domaine des transferts de chaleur, une boucle thermohydraulique pour l’étude de l’échange de chaleur en convection forcée avec une paroi à flux imposée sous champ magnétique a été mise au point. Ce type de dispositif permet l’étude de plusieurs paramètres liés à la configuration spatiale du champ magnétique appliqué, à l’effet de l’uniformité du champ sur l’intensification des échanges de chaleur. La compréhension et l’analyse de ces résultats sont consolidées par l’étude de la conductivité thermique du ferrofluide sous champ magnétique. Un banc a été mis en place et a permis de mettre en évidence l’influence de la température ainsi que de l’intensité du champ magnétique sur cette grandeur. Á l’issue de ces caractérisations, l’application des ferrofluides pour le refroidissement de composants électroniques de puissance est discutée par une mise en œuvre expérimentale. Ces essais ouvrent la voie pour de nouvelles recherches et permettent de mener des réflexions relatives aux domaines d’application des ferrofluides. / This thesis aims to study the thermal and rheological performances of ferrofluids under magnetic field for an application in cooling systems. The approach consists on macroscopic analysis based on experimental studies. Our approach is focused on three aspects: i) rheology ii) internal forced convection iii) thermal conductivity. We developed different characterization benches. For rheological studies, a magnetic circuit is developed and integrated into rheometer to create magnetic fields reaching 0,8 T. This approach demonstrates the influence of magnetic flux density on the shear forces. Concerning heat transfers, we carried out experimental tests based on the use of a closed loop flow system to study forced convection of ferrofluids with imposed wall flux under magnetic field. This test bench allows us to understand the impact of several parameters related to the configuration of the applied magnetic field on the enhancement of convective heat transfers. To analyze why convective heat transfers are better under magnetic field, we carried out a system for measuring the thermal conductivity. This bench tests allows us to show the effect of temperature and magnetic flux density on this physical property. All these tests are paving the way for new research activities and to the ferrofluids applications in cooling systems.

L’exploitation des énergies renouvelables est une voie nécessaire afin de lutter contre le réchauffement climatique, et afin d’anticiper la raréfaction des matières premières. Le mur capteur/stockeur solaire appliqué aux bâtiments à très basses consommations d’énergie s’inscrit dans cette volonté d’une transition vers les énergies renouvelables. Dans le cadre de ces travaux de thèse, l’énergie solaire est stockée dans des matériaux à changement de phase qui permettent un stockage de chaleur latente plus dense que le stockage sensible des matériaux de construction traditionnels. Cette énergie est restituée à l’ambiance intérieure par la circulation d’air neuf à travers l’élément de stockage. Un mur capteur/stockeur solaire a été développé en s’appuyant sur une revue bibliographique préalable des différents travaux scientifiques menés pour des problématiques similaires. Le dispositif a été expérimenté en environnement réel dans un premier temps, intégré à l’enveloppe d’un petit bâtiment en bois fortement isolé. La quantité de chaleur captée par le mur peut atteindre 2 kWh.m-2.jour-1, pour une quantité de chaleur restituée à l’air de 1,5 kWh.jour-1. Le dispositif a été testé en conditions maîtrisées de laboratoire. Une attention particulière a été portée à la mesure de température au sein même du MCP, afin d’analyser le comportement thermique de ce dernier. Deux phénomènes ont été observés : le recouvrement de la phase liquide sur la phase solide et l’homogénéisation des températures en phase liquide. Le comportement thermique du MCP dépend des interactions entre trois flux : le flux de charge (apport solaire), le flux de décharge (énergie restituée à l’air) et un flux vertical induit par le recouvrement de la phase liquide sur la phase solide. Par ailleurs, un modèle numérique dynamique du mur capteur a été développé en volumes finis. Ce modèle permet de simuler l’effet de serre du mur capteur, le stockage de chaleur et les phases de solidification et de fusion du MCP, et la restitution de chaleur à l’air entrant dans le bâtiment. Les résultats numériques alors obtenus ont été confrontés aux données expérimentales. Le modèle a été validé pour la température d’air soufflée (en sortie du mur capteur). L’écart entre valeurs expérimentales, sur des périodes journalières, est en moyenne de 0,6°C pour la température d’air soufflé et est inférieur à 10 % pour l’énergie fournie à l’air préchauffé. Ces différences sont inférieures aux incertitudes de mesures et à l’incertitude du calcul énergétique. Le modèle ainsi validé peut être couplé au code de simulation thermique dynamique du bâtiment TRNSYS. / Use of renewable energy is a necessary way to fight global warming and to anticipate scarcity of raw materials. The solar/storage wall used in buildings with lower energy consumption meets this evolution to renewable energy sources. In this thesis, solar energy is stored in a phase charge material (PCM), which provides latent storage. The latent storage is higher than sensible storage in usual building materials. This energy is restored to indoor air, by circulation and heating of inlet air through the wall storage element. In this thesis work, the solar storage wall was developed, based on previous published works dealing with similar systems. An experiment has been carried out with the solar storage integrated in a small wood building with a high insulation. The solar energy recovered by the wall reaches 2 kWh.m-2.day-1 and 1,5 kWh.day-1 was restored to air. In a second experiment, a prototype was developed to be used in controlled laboratory conditions. Special attention was given to PCM temperature measures to analyze the PCM thermal behavior. Two phenomena were observed: (i) liquid phase recovering solid phase, (ii) temperature homogenization in liquid phase. The PCM thermal behavior depends on interactions between three energetic flows: the charge flow (solar energy recovered), the restored flow (energy restored to the inlet air) and a vertical flow created by the liquid phase recovering. Furthermore, a numerical dynamic model for the solar storage wall was developed. It is based on a finite volume approach. This model simulates: (i) the ground effect in a solar wall, (ii) the thermal energy storage and phase changes, and (iii) heat recovery energy to air inlet. Numerical results were compared to experimental values. The model was validated for air temperature for daily cycle defined with a charge period (during sunning) and a continue air heating. The difference between numerical values and experimental values are lower than 0.6°C in mean temperature, and 10% in energy. This difference is lower than measurement uncertainties and energy calculation error margins. So the model is valeted and can be coupled with the dynamic thermal simulation code: TRNSYS.