This thesis is based on eight articles all related to the characteristics of the cooling system and plate evaporator of a household refrigerator. Through these articles, knowledge is provided that can be used to increase the operational efficiency in household refrigeration. Papers A, B and C focus on heat transfer and pressure drop in a commonly used free convection evaporator – the plate evaporator. Applicable correlations are suggested on how to estimate the air side heat transfer, the refrigerant side pressure drop and the refrigerant side heat transfer. Papers D, E and F hold a unique experimental study of the refrigerant charge distribution in the cooling system at transient and steady state conditions. From this cyclic losses are identified and estimated and ways to overcome them are suggested. In paper G the topic “charging and throttling” is investigated in an unparalleled experimental study based on more than 600 data points at different quantities of charge and expansions device capacities. It results in recommendations on how to optimize the capillary tube length and the quantity of refrigerant charge. Finally, Paper H holds a thermographic study of the overall cooling system operating at transient conditions. Overall, a potential to lower the energy use by as much as 25 % was identified in the refrigerator studied. About 10 % was found on the evaporator’s air side. 1-2 % was identified as losses related to the edge effect of the evaporator plate. About 8 % was estimated to be cyclic losses. About 5 % was found in cycle length optimization.  It is believed that most of these findings are of general interest for the whole field of household refrigeration even though the results come from one type of refrigerator. Suggestions of simple means to reduce the losses without increasing the unit price are provided within the thesis / <p>QC 20120411</p>

Cette thèse est motivée par le besoin de compléter les connaissances actuelles des phénomènes ayant lieu lors d’un renvoi d’eau dans un lit de débris radioactifs, opération appelée « renoyage » et qui intervient dans une séquence d’accident grave où un cœur de réacteur nucléaire est dégradé suite à une perte prolongée de refroidissement primaire. Notre étude, de nature expérimentale, vise à mieux caractériser la crise d’ébullition en convection forcée, dans un milieu poreux chauffant. Le cœur du dispositif expérimental est un milieu poreux modèle quasibidimensionnel, composé de 276 cylindres disposés entre deux plaques de céramique distantes de 3 mm, dont l’une, transparente, permet de visualiser les écoulements. Les cylindres, de 2 mm de diamètre, sont des sondes thermo-résistives qui ont une double fonction : elles sont utilisées comme éléments chauffants et comme capteurs de température. Une boucle fluide permet de contrôler le débit d’injection de liquide dans la section test, la température d’injection ainsi que la pression. La section test est placée verticalement, le liquide est injecté par le bas à une température proche de la saturation. Dans une première série d’expériences, la puissance thermique dissipée globalement par un ensemble de cylindres chauffants est augmentée de façon progressive jusqu’à atteindre l’assèchement d’une zone du milieu poreux. Les résultats montrent deux types de phénoménologies dans le déclenchement de la crise d’ébullition. Pour des débits d’injection faibles (densités de flux massique de l’ordre de 4 kg.m^-2.s^-1 maximum), l’atteinte de la puissance d’assèchement se traduit par un lent recul du front diphasique jusqu’à sa stabilisation en haut de la zone chauffée ; en aval de la zone chauffée, l’écoulement est monophasique vapeur. Pour des débits d’injection plus élevés, la crise d’ébullition apparaît autour d’un des éléments chauffants, conduisant à une ébullition en film localisée, tandis qu’un écoulement diphasique liquide-vapeur continue de parcourir l’aval de la section test. Les visualisations de ces expériences permettent d’identifier qualitativement la structure des écoulements. D’autres expériences consistent à mesurer le flux critique local autour d’un cylindre choisi, pour différentes configurations d’écoulements. Le débit d’injection est fixé. Une puissance de chauffe est imposée à une ligne horizontale de cylindres en amont du cylindre choisi. Les résultats montrent que le flux critique sur ce cylindre diminue en fonction de la puissance délivrée à la ligne chauffée. La distance du cylindre étudié à la ligne chauffée semble avoir peu d’influence sur le flux critique. Des visualisations expérimentales sont utilisées pour caractériser l’écoulement diphasique en aval de la ligne chauffée, dans le but de mettre en relation le flux critique local avec des paramètres hydrodynamiques (saturations, vitesses des phases). Les images obtenues sont difficiles à exploiter. Afin de calibrer les paramètres des algorithmes de traitement d’images, nous avons reproduit une cellule d’essai de géométrie identique à l’originale, mais où l’on injecte du gaz par une ligne de cylindres en amont de la section test dans une configuration d’écoulement diphasique isotherme. Dans ce dispositif, le débit d’injection de gaz est contrôlé et mesuré. Les visualisations obtenues servent alors de références auxquelles sont comparées les visualisations d’ébullition convective. / This work is motivated by the need to better understand the phenomena occurring while some water is injected into a heated porous debris bed. This reflooding operation is a part of the planned mitigation procedure used during a Loss Of Coolant Accident (LOCA) that may occur into a nuclear power plant and results into a severe core damage. Our experimental study aims to characterize the boiling crisis that can happen in a boiling flow taking place within a heatgenerating model porous medium. The test section is a two-dimensional model porous medium, composed of an array of 276 cylinders placed between two ceramic plates spaced from one another by 3 mm, one of which is transparent and allows visualizations of the flow. The 2 mm diameter cylinders are Pt100 resistance temperature detectors that perform a dual function: they act as heating elements (heated by Joule effect) and are also used as temperature probes. A fluid loop allows controlling the liquid injection flow rate, its inlet temperature as well as its pressure. The test section is held vertically, the liquid injected from bottom at a temperature close to the saturation temperature. In a first series of experiments, the thermal power applied to a bundle of heating cylinders is progressively increased until a dry zone is detected in the porous medium. Two kinds of phenomenology are observed during these “dryout experiments”. First, at low liquid injection rate (4 kg.m^-2.s^-1 maximum mass flux), reaching the dryout power results into a liquid front receding down to the upper limit of the heated zone, while downstream the heated zone, the porous medium is vapour-saturated. Second, at higher flow rate, the boiling crisis happens at the surface of a single heating element, resulting in a local film boiling, whereas a two-phase flow still go through the whole test section. High-speed visualizations allow characterizing the flow regimes. Other experiments focus on determining the local critical heat flux on a given cylinder, for different upstream flow configurations. The inlet liquid flow rate is fixed. A thermal power is uniformly applied to a line of heating cylinders, upstream the cylinder under study. Results show that the local critical heat flux decreases as the power applied to the heated line increases. The distance from the cylinder under study to the heated line seems not to have a significant effect on the critical heat flux. Visualizations are used to characterize the two-phase flow upstream the heated line, aiming at expressing the critical heat flux as a function of the hydrodynamic parameters (saturations, phase velocities). The image analysis is particularly challenging. In order to calibrate the image processing parameters, we use a second model porous medium with the same geometry as the heat generating one, but where an isothermal two-phase flow is obtained by injecting gas into the liquid flow rather than generated by boiling. The gas injection flow rate is controlled and measured. Isothermal two-phase flow visualizations provide a reference case and are compared to flow boiling visualizations.

A pesquisa realizada envolveu a avaliação experimental dos efeitos do fluido e da orientação do escoamento no desempenho de um dissipador de calor baseado na ebulição convectiva em microcanais. Estes dissipadores de calor são usados como uma nova aplicação para a refrigeração dos novos dispositivos eletrônicos que geram altas taxas de calor. Efetuou-se inicialmente uma extensa pesquisa bibliográfica sobre o escoamento monofásico e a ebulição convectiva em microcanais e em multi-microcanais através da qual levantou-se os principais métodos de previsão do coeficiente de transferência de calor e da perda de pressão. Então, utilizando o aparato experimental desenvolvido durante o mestrado de Do Nascimento (2012) avaliou-se a transferência de calor e perda de pressão de um dissipador de calor baseado em multi-microcanais paralelos. O dissipador de calor avaliado possui 50 microcanais retangulares dispostos paralelamente com 15 mm de comprimento, 100 µm de largura, 500 µm de altura e espaçados de 200 µm. Ensaios experimentais foram executados para o R245fa, fluido de baixa pressão utilizado em ciclos frigoríficos de baixa pressão, e R407C, fluido de alta pressão usado para conforto térmico, temperatura de saturação de 25 e 31°C, velocidades mássicas de 400 a 1500 kg/m²s, graus de subresfriamento do líquido de 5, 10 e 15°C, título de vapor máximo de até 0,38, fluxos de calor de até 350 kW/m², e para 3 orientações diferentes do dissipador de calor, horizontal, vertical com os canais alinhados horizontalmente e vertical com escoamento ascendente. Os resultados obtidos foram parametricamente analisados e comparados com métodos da literatura. Coeficientes de transferência de calor médios de até 35 kW/m² °C foram obtidos. Resultados adquiridos para o R245fa e R407C foram inferiores aos levantados por Do Nascimento (2012) para o R134a utilizando o mesmo dissipador. O fluido R407C apresentou frequências e amplitudes de oscilações inferiores aos fluidos R134a e R245fa. Nenhum método para o coeficiente de transferência de calor e perda de pressão proporcionou previsões satisfatórias dos dados experimentais. O modelo Homogêneo com viscosidade da mistura bifásica dada por Cicchitti et al. (1960) apresentou as melhores previsões da perda de pressão, já para o coeficiente de transferência de calor, os métodos de Bertsch et al. (2009) e Liu e Winterton (1991) apresentaram as melhores previsões. O dissipador com sua base posicionada horizontalmente fornece coeficientes de transferência de calor superiores enquanto sua base na vertical e escoamento ascendente verificam-se perdas de pressão inferiores. Imagens do escoamento bifásico foram obtidas com uma câmera de alta velocidade e analisadas. / This study presents an experimental investigation on the effect of the fluid and the footprint orientation on the performance of a heat spreader based on flow boiling inside micro-scale channels. This heat spreader is used in an electronics cooling application with high-power density. Initially an extensive investigation of the literature concerning single-phase and two-phase flow inside a single microchannels and multi-microchannels was performed. In this literature review the leading predictive methods for heat transfer coefficient and pressure drop are described. The experimental study was carried out in the apparatus developed by Do Nascimento (2012). The heat sink evaluated in the present study is comprised of fifty parallel rectangular microchannels with cross-sectional dimensions of 100 µm width and of 500 µm depth, and total length of 15 mm. The fins between consecutive microchannels are 200 µm thick. Experimental tests were performed for R245fa, low-pressure fluid used in low pressure refrigeration cycles, and R407C, high-pressure fluid used for heat comfort, saturation temperature of 25 and 31°C, mass velocities from 400 to 1500 kg/m² s, degrees of subcooling of the liquid of 5, 10 and 15°C, outlet vapor quality up to 0.38, heat fluxes up to 350 kW/m², and for the following footprint heat sink orientations: horizontal, vertical with the microchannels aligned horizontally and vertical with upward flow. The results were parametrically analyzed and compared again the predictive methods from literature. Average heat transfer coefficients up to 35 kW/m² °C were obtained. The results for R134a from Do Nascimento (2012) for the same heat sink presented heat transfer coefficients higher than R245fa and R407C. The fluid R407C presented oscillation of the temperature due to thermal instability effects with lower frequency and amplitude lower than R134a, and R245fa. None predictive method provided satisfactory heat transfer coefficient and pressure drop predictions of the experimental data. The Homogeneous model with the viscosity given by Cicchitti et al. (1960) provided the best pressure drop prediction while the heat transfer coefficient was best predicted by Bertsch et al. (2009) and Liu and Winterton (1991). The horizontal orientation of the footprint provided the highest heat transfer coefficients while the vertical footprint orientation with upward flow the lowest pressure drops. Images of the two-phase flow were obtained with a high-speed camera and analyzed.

A pesquisa realizada tratou do estudo da transferência de calor e do fluxo crítico durante a ebulição convectiva no interior de canais de diâmetro reduzidos a partir de dados levantados em bancadas experimentais construídas para esta finalidade. Extensa pesquisa bibliográfica foi efetuada e os principais métodos disponíveis para previsão de coeficiente de transferência de calor, fluxo crítico e mapas de escoamento foram levantados. Os resultados obtidos foram parametricamente analisados e comparados com os métodos da literatura. Pela primeira vez para microcanais, resultados experimentais foram levantados por um mesmo autor em laboratórios distintos buscando verificar a tendência e comportamentos. Tal comparação tem sua importância destacada em face das elevadas discrepâncias observadas na literatura quando resultados de autores distintos, obtidos em condições similares, são comparados. Os resultados levantados foram utilizados na elaboração de modelos que consideram os padrões de escoamento observados em microcanais. A incorporação dos padrões permitiu o desenvolvimento de modelos mecanísticos para coeficiente de transferência de calor, fluxo crítico e critérios para a caracterização da transição entre macro e microcanais baseados na formação do padrão de escoamento estratificado e na simetria do filme líquido no escoamento anular. / This research comprises an experimental and theoretical study on flow boiling heat transfer and critical heat flux inside small diameter tubes based on data obtained in experimental facilities specially designed for this purpose. A broad literature review was carried out and the main methods to predict the heat transfer coefficient, critical heat flux and flow patterns were pointed out. The experimental results were parametrically analyzed and compared against the predictive methods from literature. For the first time, microchannels experimental results obtained by an unique researcher in distinct laboratories were compared and a reasonable agreement was observed. The importance of such a comparison is high-lighted for flow boiling inside microchannels due to the high discrepancies ob-served when results from independent laboratories obtained under similar experimental conditions are compared. Moreover, the experimental results obtained in the present study were used to develop correlations and models for the heat transfer coefficient and heat flux that takes into account the flow patterns observed in microchannels. The heat transfer coefficient and critical heat flux models were developed based on mechanistic approach. In addition, criteria to characterize macro to microchannel transition were proposed based in the occurrence of the stratified flow pattern and the liquid film symmetry under annular flow conditions.

A presente pesquisa trata de um estudo experimental da transferência de calor e da perda de carga de fluidos refrigerantes puros e suas misturas em mudança de fase convectiva no interior de tubos lisos e aqueles dotados de ranhuras internas. Para tanto, foi desenvolvido um equipamento experimental cujo componente básico é composto por um tubo horizontal, aquecido por intermédio de uma resistência elétrica do tipo fita, aderida à superfície externa do tubo. As condições de ensaio variaram numa ampla faixa, permitindo cobrir as condições verificadas na maioria das instalações frigoríficas. Os resultados experimentais foram agrupados em duas faixas de velocidades mássicas: elevadas (G > ou = 200 kg/s.m2), onde prepondera o padrão anular de escoamento, e reduzidas (G < 200 kg/s.m2), predominando o padrão estratificado. Os principais parâmetros que afetam o coeficiente de transferência de calor, tais como, velocidade mássica, fluxo de calor, tipo de refrigerante, temperatura de evaporação e diâmetro do tubo foram analisados. O desempenho termo-hidráulico, relativo ao efeito combinado da transferência de calor e da perda de carga dos tubos ranhurados, foi sensivelmente superior quando comparados aos tubos lisos. A análise dos resultados experimentais permitiu a proposição de correlações para a perda da carga, avaliada através do multiplicador bifásico, &#966L, e para coeficiente de transferência de calor, em tubos lisos e ranhurados. As correlações propostas se mostraram adequadas para aplicações práticas, proporcionando desvios reduzidos em relação aos resultados experimentais. Destacam-se as correlações obtidas para o multiplicador bifásico para tubos microaletados e para o coeficiente de transferência de calor para vazões reduzidas em tubos lisos. Diversos registros fotográficos dos principais padrões de escoamento foram levantados, tendo sido importante na análise e entendimento da mudança de fase. / Present research deals with an experimental study of the heat transfer and pressure drop of pure and mixtures of refrigerants undergoing convective boiling inside horizontal smooth and microfin tubes. An experimental apparatus has been developed and constructed whose main component is a horizontal tube electrically heated. Experimental results have been grouped into two mass velocity ranges: the one corresponding to mass velocities lower than 200 kg/s.m2, where the stratified flow pattern is dominant, and that for mass velocities higher than 200 kg/s.m2, where typically the annular flow pattern can be found. Effects over the heat transfer coefficient of physical parameters such as mass velocity, heat flux, diameter, saturation temperature, and refrigerant have been investigated and analyzed. It has been found out that the thermo-hydraulic performance of microfin tubes is better than that of the smooth ones. Empirical correlations have been proposed for both the two-phase flow multiplier and the heat transfer coefficient for different ranges of operating conditions as well as for smooth and microfin tubes. Results from the proposed correlations can be deemed adequate for practical applications given the limited dispersion obtained with respect to their experimental counterpart. Noteworthy are the results obtained from correlations for both the two phase flow multiplier for microfin tubes and the heat transfer coefficient for the lower range of mass velocities in smooth tubes. Finally, worth mentioning is the photographic essay developed in present research involving the flow patterns that occur under convective boiling of refrigerants in horizontal tubes.

This thesis deals with the problem of critical heat flux (CHF) on technically smooth and treated surfaces at low pressures. The theoretical part presents the basic concepts of two-phase flow and an analysis of existing work on the influence of the surface on CHF. The main part of the work describes the built experimental apparatus for CHF research at low pressures of 100 -1500 kPa (1-15 bar) with a vertical internally heated annular test section. The internal annuli consists of an outer glass tube with an inner diameter of 14.8 mm and an inner tube made of Inconel ™ 625 / Optimized ZIRLO ™ with an outer diameter of 9.14 mm and a heated length of 380/365 mm. CHF experiments on technically smooth surface were performed at outlet pressures 120 kPa, 200 kPa and 300 kPa, at an inlet temperature of 64, 78 and 91 °C and at mass flux of 400, 500, 600 and 800 kg / m2s. The Inconel tubes were tested in two different surface modifications - abraded and bead blasted. Experiments were performed at mass flows of 400, 500 and 600 kg / m2s. The total number of 122 experimental runs were conducted and the results were compared with other literature experimental data. The maximum increase of CHF on abraded / bead blasted tube was 18.12% / 16.17%. The surface structure was analysed by laser microscopy. The wetting behaviour of the surface structures was measured by the sessile drop method. The elemental analysis of the surface was evaluated using the EDS method.

<p>Advancements in future technologies such as artificial intelligence, electric vehicles, and renewable energy create a consistent need for more powerful and smaller electronic devices and systems. As a result, thermal management components such as heat sinks need to remove higher heat loads from more compact spaces to keep electronics within their operational temperature limits. Constraints imposed by conventional manufacturing processes restrict the design of heat sinks to simple geometries with limited cooling performance. Recent widespread commercialization of metal additive manufacturing (AM) tools offers new potential for leveraging the design freedom of these manufacturing technologies to design and fabricate heat sinks with improved performance. </p> <p>In AM, three dimensional parts are created through layer-by-layer depositing of materials, which allows fabrication of complex geometries that would be impossible or too costly using conventional subtractive methods. Many novel heat sink geometries have been proposed in literature which incorporate features such as manifolds, flow mixers, and curved channels using engineering intuition to reduce pressure drop or enhance heat transfer. Although such designs have been shown to offer improved performance, mathematical design algorithms such as topology optimization (TO) have been shown to outperform engineering intuition. Topology optimization optimizes the material distribution within a given design space, guided by physics-based simulations, to achieve a user-defined objective such as minimization of thermal resistance. Previous TO approaches have used penalization methods to ensure the final designs are composed of macroscopic and non-porous features due to the past precedent of fabrication capabilities. This traditional penalization approach is well-suited to the constraints of conventional manufacturing methods; however, microstructures and porous features are easily fabricable with additive manufacturing. There is a need to develop TO approaches that are better suited for leveraging AM for the design of heat sinks. In this thesis, a homogenization approach to topology optimization is proposed wherein the material distribution is represented as parametrized microstructures. This formulation allows design of thermal management components that have sub-grid features and leverages AM for fabrication. The focus of this thesis is the development of the homogenization approach for TO of heat sinks, as well as the exploration of the design problems it can address, the performance benefits made available, and the two-phase flow physics that it uniquely allows to be incorporated into the topology optimization process.</p> <p>A topology optimization algorithm using the homogenization approach is developed by representing the material distribution as arrays of pin fins with varying gap sizes. To this end, the pin fins are modeled as a porous medium with volume-averaged effective properties. Height-averaged two-dimensional flow and non-equilibrium thermal models for porous media are developed for transport in the pin fin array. Through multi-objective optimization, TO designs are generated for an example case involving a hotspot over a uniform background heat input. The resulting topologies have porous-membrane-like designs where the liquid is transported through a fractal network of open, low-hydraulic-resistance manifold pathways and then forced across tightly spaced arrays of pin fins for effective heat transfer. The TO designs are revealed to offer significant performance improvements relative to the benchmark straight microchannel (SMC) heat sink with features optimized under the same multi-objective cost function. A series of microchannel heat sinks are fabricated using direct metal laser sintering to investigate the printing capabilities and to experimentally demonstrate the performance of topology optimized designs. Advantages of the homogenization approach over the penalization approach can be summarized as follows: (1) reduced computational costs due to its ability to create sub-resolution features, (2) intrinsically fabricable parts using available metal AM tools, and (3) easier to use due to significantly reduced number of hyperparameters (e.g., penalization factors) that are controlled by the user. </p> <p>Topology optimization has been applied to thermal management methods involving single-phase flows such as natural convection, forced air cooling, and pumped liquid cooling. Compared to these conventional heat sink technologies, flow boiling offers very high heat transfer coefficients and effective heat capacities, making it a promising candidate for future cooling electronics applications. The final goal of this thesis is to enable topology optimization of flow boiling heat sinks. However, TO of flow boiling heat sinks has been avoided due to difficulties in modeling the boiling phenomena; of note, there are no examples of TO being applied to the design of heat sink under flow boiling throughout the literature. Multi-dimensional two-phase flow models require prior knowledge of friction factor and heat transfer coefficients. Correlations are available in literature but are not universal and depend significantly on channel/fin geometries, surface roughness, and operating conditions. Given that traditional penalization-based TO approach results in fin and channel geometries with unknown shapes, dimensions, and alignment before the optimization is completed, this prohibits their use for optimization of flow boiling heat sinks. However, the homogenization approach to topology optimization developed in this thesis enables the optimization of flow boiling heat sinks. As it relies on user-defined microstructures with known shapes, alignments, and ranges of geometric dimensions, a universal correlation for flow boiling in microchannels is not needed. Instead, correlations for the user-defined microstructures are sufficient to simulate flow boiling in TO designs generated using the homogenization approach. To this end, a predefined microstructure geometry is chosen for which two-phase flow correlations exist and therefore topology optimization can be performed. Topology optimized heat sink designs under flow-boiling are generated and investigated at various heat inputs, topology optimization grid sizes, and maximum vapor quality constraints. Topology optimized heat sinks designed for single-phase versus two-phase flow are compared.  There are significant differences in hydraulic and thermal responses of the single-phase and two-phase designs due to high effective heat capacity rates and high heat transfer coefficients of flow boiling. The algorithm demonstrated in this work extends the capabilities of topology optimization to two-phase flow physics, and thereby enables the design of various two-phase flow components such as evaporators, condensers, heat sinks, and cold plates.</p> <p>The flow and heat transfer of the TO algorithm for microchannel heat sinks under flow boiling use a two-phase mixture model featuring an effective porous medium formulation. However, closure of the governing equations requires empirical correlations for pressure drop and heat transfer that are specific to the operating conditions, microstructure geometry, and surface finish. Therefore, it must be demonstrated these available correlations can be successfully calibrated over a range of microstructural variations present within the homogenization framework, so as to attain the required prediction generality and accuracy needed to ensure the resulting designs achieve Pareto-optimality. To this end, a set of uniform pin fin calibration samples are additively manufactured and experimentally tested under flow boiling at various flow rates and heat inputs for model calibration. All of the unknown/free coefficients in the adopted correlations are determined by minimizing the error between the model predictions and the experimental measurements using gradient-based optimization. The calibrated topology optimization algorithm is then used to generate a Pareto-optimal set of heat sinks optimized for minimum pressure drop and thermal resistance during flow boiling. Experimental characterization of these additively manufactured heat sinks, unseen during the model coefficient calibration process, reveals that the measured Pareto optimality curve matches that predicted by the topology optimization algorithm. Lastly, a heat sink design is generated for a design space involving multiple hot spots and background heating to showcase the capability of the experimentally calibrated two-phase topology optimization algorithm at handling complex boundary conditions. The optimized heat sink intelligently distributes an adequate amount of coolant flow to each of the heated regions to avoid local dry-out. This work demonstrates a complete framework for two-phase topology optimization of heat sinks through experimental calibration of flow boiling correlations to the porous medium used by the homogenization approach. </p> <p>The major contribution of this thesis is the development of a homogenization approach for TO of additively manufactured microchannel heat sinks under single- and two-phase flows. Not only does the homogenization approach provide several advantages over the traditional penalization approaches such as reduced computational costs, intrinsic fabricability using AM, and ease of use, but it also enables TO of heat sinks under flow boiling and potentially TO of other two-phase thermal management components. The work discussed in this thesis serves a comprehensive end-to-end guide on TO of microchannel heat sinks using the homogenization approach with experimental demonstrations for validation.</p>

A novel surface structure comprising dendritically ordered nano-particles of copper was developed during the duration of this thesis research project. A high current density electrodeposition process, where hydrogen bubbles functioned as a dynamic mask for the materials deposition, was used as a basic fabrication method. A post processing annealing treatment was further developed to stabilize and enhance the mechanical stability of the structure. The structure was studied quite extensively in various pool boiling experiments in refrigerants; R134a and FC-72. Different parameters were investigated, such as; thickness of the porous layer, presence of vapor escape channels, annealed or non-annealed structure. Some of the tests were filmed with a high speed camera, from which visual observation were made as well as quantitative bubble data extracted. The overall heat transfer coefficient in R134a was enhanced by about an order of magnitude compared to a plain reference surface and bubble image data suggests that both single- and two-phase heat transfer mechanisms were important to the enhancement. A quantitative and semi-empirical boiling model was presented where the main two-phase heat transfer mechanism inside the porous structure was assumed to be; micro-layer evaporation formed by an oscillating vapor-liquid meniscus front with low resistance vapor transport through escape channels. Laminar liquid motion induced by the oscillating vapor front was suggested as the primary single-phase heat transfer mechanism. The structure was applied to a standard plate heat exchanger evaporator with varying hydraulic diameter in the refrigerant channel. Again, a 10 times improved heat transfer coefficient in the refrigerant channel was recorded, resulting in an improvement of the overall heat transfer coefficient with over 100%. A superposition model was used to evaluate the results and it was found that for the enhanced boiling structure, variations of the hydraulic diameter caused a change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance. For the standard heat exchanger, it was mostly the convective boiling mechanism that was affected by the change in hydraulic diameter. The structure was also applied to the evaporator surface in a two-phase thermosyphon with R134a as working fluid. The nucleate boiling mechanism was found to be enhanced with about 4 times and high speed videos of the enhanced evaporator reveal an isolated bubble flow regime, similar to that of smooth channels with larger hydraulic diameters. The number and frequency of the produced bubbles were significantly higher for the enhanced surface compared to that of the plain evaporator. This enhanced turbulence and continuous boiling on the porous structure resulted in decreased oscillations in the thermosyphon for the entire range of heat fluxes. / QC 20111111

This thesis is divided into four distinct chapters all linked by the topic of spray cooling. Chapter one gives a detailed categorization of future and current spray cooling applications, and reviews the major advantages and disadvantages that spray cooling has over other high heat flux cooling techniques. Chapter two outlines the developmental goals of spray cooling, which are to increase the output of a current system and to enable new technologies to be technically feasible. Furthermore, this chapter outlines in detail the impact that land, air, sea, and space environments have on the cooling system and what technologies could be enabled in each environment with the aid of spray cooling. In particular, the heat exchanger, condenser and radiator are analyzed in their corresponding environments. Chapter three presents an experimental investigation of a fluid management system for a large area multiple nozzle spray cooler. A fluid management or suction system was used to control the liquid film layer thickness needed for effective heat transfer. An array of sixteen pressure atomized spray nozzles along with an imbedded fluid suction system was constructed. Two surfaces were spray tested one being a clear grooved Plexiglas plate used for visualization and the other being a bottom heated grooved 4.5 x 4.5 cm2 copper plate used to determine the heat flux. The suction system utilized an array of thin copper tubes to extract excess liquid from the cooled surface. Pure water was ejected from two spray nozzle configurations at flow rates of 0.7 L/min to 1 L/min per nozzle. It was found that the fluid management system provided fluid removal efficiencies of 98% with a 4-nozzle array, and 90% with the full 16-nozzle array for the downward spraying orientation. The corresponding heat fluxes for the 16 nozzle configuration were found with and without the aid of the fluid management system. It was found that the fluid management system increased heat fluxes on the average of 30 W/cm2 at similar values of superheat. Unfortunately, the effectiveness of this array at removing heat at full levels of suction is approximately 50% & 40% of a single nozzle at respective 10[degrees]C & 15[degrees]C values of superheat. The heat transfer data more closely resembled convective pooling boiling. Thus, it was concluded that the poor heat transfer was due to flooding occurring which made the heat transfer mechanism mainly forced convective boiling and not spray cooling. Finally, Chapter four gives a detailed guide for the design and construction of a high heat flux heater for experimental uses where accurate measurements of surface temperatures and heat fluxes are extremely important. The heater designs presented allow for different testing applications; however, an emphasis is placed on heaters designed for use with spray cooling.

In-situ vapor extraction is performed by applying a pressure differential across a hydrophobic porous membrane that forms a wall of the channel as a means of reducing the local quality of flow boiling within the channel. As the local quality is reduced, the heat transfer capability can be improve while large pressure drops and flow instability can be mitigated. The present study investigates the potential of vapor extraction, by examining the characteristics and mechanisms of extraction. The physics based models for transition among extraction regimes are developed which can be used as a basis for a regime-based vapor extraction rate model. The effects of vapor extraction on flow boiling in a microscale fractal-like branching network and diverging channels are studied by using a one-dimensional numerical model based on conservation of mass and energy, along with heat transfer and pressure drop correlations. The results show the improvement in reduced pressure drop and enhanced flow stability, and show the potential of heat transfer enhancement. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from March 25, 2013 - March 25, 2014