<p>Heat sinks have the capability of increasing operating heat flux limits for improved thermal management in the immersion cooling of electronics using dielectric fluids. However, even for arrays of simple, straight fins, the generation of vapor between and along fins during pool boiling lead to performance effects that are not well understood. Further investigation of the heat-flux-dependent variation of boiling modes that can manifest along the fin height is required. Although methods for the prediction of fin boiling heat transfer exist that incorporate a variable heat transfer coefficient determined from a flat surface, they have been developed and assessed for single, isolated fins under the assumption that the sides of the fin at any location behave like that of a flat surface. As a result, when applied to fin arrays, these methods may not always be accurate for the full range of heat flux operation along boiling curve up to the critical heat flux, due to the fins interfering with each other when arranged in arrays of differing spacing and height. To establish when the fins in an array can be described as isolated and having the flat surface boiling behavior, pool boiling experiments are performed using copper heat sinks in two fluids with vastly different properties: HFE-7100 and water. The spacing and height of the longitudinal fins are varied across a range from much larger to less than half of the scale of the capillary length scale of both fluids, <em>L</em>_<em>b</em>. High-speed visualizations enable the identification of different boiling regimes to identify correspondence between flow observations and the boiling performance, such as when there is bubble confinement from fin interference. Trends in the pool boiling data are also compared, noting changes in superheat at various heat fluxes to establish when fin height or spacing affects boiling behavior. The experimental boiling performance is compared to predictions developed assuming isolated fins so as to identify the spacings and heights for which the fin arrays follow this behavior. Overall, the data from both fluids strongly support a hypothesis that <em>L</em>_<em>b</em> is the key length scale. Heat transfer from fin array heat sinks with heights and spacings above <em>L</em>_<em>b</em> are shown to be accurately predicted in both fluids. However, spacings smaller than <em>L</em>_<em>b</em> lead to bubble confinement which affects the superheat, particularly at low heat fluxes, while heights shorter than <em>L</em>_<em>b</em> are unable to support multiple boiling regimes along the fin sidewall. This work identifies the capillary length as the key length scale at which confinement and height effects need to be considered for accurate predictions of immersion cooling applications.</p>

Analytical, closed-form solutions governing thermoelectric behavior are derived. An analytical model utilizing a thermal circuit is presented involving heat transfer into, through, out of, and around a thermoelectric device. A nondimensionalization of the model is presented. Linear heat transfer theory is applied to the model to obtain a series of closed form equations predicting net power output for the thermoelectric device. Fluid streams flowing through shrouded heat sinks with square pin fins are considered for the thermal pathways to and from the device. Heat transfer and pressure drop are characterized in a manner conducive to an analytical model using previously published experimental results. Experimental data is presented which validates and demonstrates the usefulness of the model in predicting power output for commercially available thermoelectric generators. A specific design for a thermoelectric power harvester is suggested consisting of a pattern of thermoelectric generators. An economic model for calculating payback time is developed. An optimization process is demonstrated that allows for the payback time of such a system to be minimized through optimization of the physical design of the system. It is shown that optimization of the thermal pathways dramatically reduces payback time. Optimized design of a system is discussed in light of theoretical cases with feasible payback times.

This thesis work researched the temperature effects the insert tool life. This was examined through physical laboratory testing with designed milling tools and temperature simulations in ANSYS. The designed milling tools altered the temperature in the insert through external parameters like design and material. Three milling tools were used in total: A milling tool created in an copper alloy that cools the inserts, a milling tool created in the material idun that increases the temperature in the inserts, and the milling tool Coro Mill 245 that was used as a reference. These tools were tested with two inserts, insert 1130 with a coating of PVD, and insert 4330 which has a coating of CVD.  The idun milling tool and the reference milling tool was already designed, but the copper milling tool was designed during the thesis work. The designed copper milling tool implemented a heat sink to cool the milling tool with convection and was manufactured in a copper alloy with high strength. The copper milling tool was 20 $\%$ colder than the reference milling tool when comparing the maximum temperature of the insert. Through testing in dry milling of the three milling tools with different inserts and a simulation in ANSYS of the temperature development, the conclusion was that insert 4330 had crater wear which increased with increased temperature. This crater wear can be the main factor in the insert tool life.

This thesis presents a simulation study on the thermal performance of the fuel systemin a fighter aircraft. The main objective is to investigate the potential utilization of thefuel system as a heat sink and explore various operational aspects that can enhance itsperformance. To achieve this, the study focuses on analyzing the impact of tank emptyingsequences, fuel types, and the implementation of bypass recirculation under low thrustconditions.The fuel system is modeled using the Modelica language-based tool, Modelon Impact,which enables a thorough and detailed examination of its thermal behavior. A functionalmodel of the fuel system has been successfully built in this modeling tool. The investiga-tion of the three operational aspects has been divided into two heat sink studies. Heat sinkstudy 1 examines the tank emptying sequences, with three different sequences being inves-tigated. The simulation results did not demonstrate any clear and significant benefits fromusing different emptying sequences. However, emptying the fuselage tank first showedslightly better behavior compared to the sequence that involved emptying the wing tanksfirst. Heat sink study 2 focuses on the implementation of bypass recirculation of the fuelback to the tanks. The primary objective of heat sink study 2 is to maintain the feed linetemperature close to the threshold determined by the fuel type in order to . The simulationresults revealed significant improvements in the heat sink capacity of the system, reachingup to 168%.

No description available.

A presente dissertação trata de um estudo teórico-experimental sobre escoamento monofásico e bifásico em um dissipador de calor baseado em microcanais. Este tipo de dissipador de calor tem sido usado para a intensificação da troca de calor em sistemas compactos e de alto desempenho. A intensificação da troca de calor promovida pelo escoamento em microcanais é acompanhada de um incremento na perda de pressão, portanto o estudo destes dois parâmetros é essencial para o entendimento dos fenômenos relacionados e fundamental para o desenvolvimento de ferramentas de projeto para dissipadores de calor baseados em microcanais. Inicialmente, um levantamento bibliográfico extenso sobre a ebulição convectiva em microcanais de reduzido diâmetro foi realizado. Este estudo da literatura trata de critérios de transição entre micro- e macro-escala, padrões de escoamento, métodos de previsão do coeficiente de transferência de calor e perda de pressão. Atenção específica foi dada a estudos de dissipadores de calor baseados em microcanais. Com base nesta análise da literatura, uma bancada experimental foi confeccionada para que dados experimentais de transferência de calor e perda de pressão pudessem ser levantados a partir de um dissipador de calor de microcanais. O dissipador de calor fabricado para este estudo é constituído de 50 microcanais retangulares dispostos paralelamente com 15 mm de comprimento, 100 µm de largura, 500 µm de profundidade e espaçados entre si de 200 µm. Experimentos foram executados para o R134a, velocidades mássicas de 400 a 1500 kg/m²s, título de vapor máximo de 0,35 e fluxos de calor de até 310 kW/m². Como conclusão deste trabalho observa-se perda de pressão elevada em relação aos valores fornecidos pelos métodos de previsão da literatura e um coeficiente de transferência de calor próximo ao estimado pelo modelo de três zonas proposto por Thome et al. (2004). / This study presents a theoretical and experimental investigation on single and two-phase flows in a microchannel based heat sink. Multi-microchannel heat sinks are able of dissipating extremely high heat fluxes under confined conditions. Such characteristics have attracted the attention of academia and industry and actually several studies are being carried out in order to evaluate and optimize such devices. Initially, an extensive investigation of the literature concerning convective boiling in micro-scale channels was performed. This literature review covers transitional criteria between micro- and macro-scale flow boiling, two phase flow patterns, heat transfer coefficient and pressure drop during convective boiling. Special attention was given to studies concerning microchannels based heat sinks. Based on this investigation, an experimental facility was built for performing heat transfer and pressure drop measurements during single-phase flow and flow boiling in microchannel based heat sinks. For this study, a microchannel based heat sink was also manufactured. The heat sink contains 50 rectangular parallel microchannels, 15 mm long, 100 µm wide by 500 µm deep and separated by 200 µm walls. Experiments were performed for R134a, mass velocity of 400-1500 kg/m²s, maximum vapor quality of 0,35 and heat fluxes up to 310 kW/m². The database obtained in the present study was compared against pressure drop and heat transfer coefficient prediction methods from the literature. It was found that no one method is accurate in predicting heat sink pressure drop while heat transfer coefficient results were accurately predicted by the 3-zone model proposed by Thome et al. (2004).

La convection turbulente dans des géométries régulières et complexes trouve son importance dans de nombreuses applications industrielles, notamment les échangeurs de chaleurs (dissipateurs à picots). L’objectif de ce travail est de faire une étude diagnostic sur la qualité d’échange thermique dans des géométries représentatives d’échangeurs de chaleur, en utilisant la simulation numérique comme outil d’investigation. L’approche qui sera utilisée est basée sur la résolution des équations de Navier-Stockes (moyennées) RANS avec les modèles statistiques.La finalité des résultats est de proposer des solutions pour promouvoir l’échange de chaleur dans ce type de configuration à petites échelles (micro systèmes de dissipateurs de chaleur à picots). Pouvoir déterminer une nouvelle approche pour le calcul du coefficient d’échange moyen par convection (coefficient d’échange convectif) dans des géométries complexes et compactes, et adopter une nouvelle approche pour calculer les températures de référence. Proposition et adaptation d’une nouvelle forme de géométrie pour une éventuelle optimisation du modèle existant (picots en forme losange). / The turbulent convection in regular and complex geometries is important in many industrial applications including mini/micro heat exchangers (pin-fins heat sinks). The objective of this work is to conduct a parametric study of the quality of heat exchange in representative forms of geometries of mini-channels, by using numerical simulation as an investigative tool. The approach taken is based on solving Averaged Navier-Stokes equations, RANS approach with the statistical models.The purpose of the results is to propose solutions to promote the exchange of heat in this type of configuration (micro systems of pin-fins heat sinks). Get a new approach for the calculation of averaged heat transfer coefficient in complex and compact geometries, and adopt a new approach to calculate reference temperatures. Suggestion and adaptation of a new form of geometry for an eventual optimization of the existing model (diamond shaped pin-fins).

Dans cette thèse, on présente une étude numérique de la convection forcée turbulente dans un dissipateur thermique muni d'une chicane transversale dans le by-pass. Le premier modèle est composé d’ailettes planes et le second consiste à ajouter des ailettes broches entre les ailettes planes. Les équations gouvernantes basées sur le modèle de turbulences k- SSt sont discrétisées et résolues par la méthode des volumes finis et l'algorithme SIMPLE. Les résultats dynamiques sont présentés en en termes de champs de vitesse, des profils de vitesse axiales dans des sections choisies ainsi que la perte de charge. L'étude thermique est présentée en terme de champs de température et de distribution du nombre de Nusselt. Un rapport entre les performances thermique et dynamique est présenté pour évaluer les différents dissipateurs thermiques. / In this thesis, we present an numerical study of turbulent forced convection in a heat sink provided with a transverse baffle in the bypass. The first model is composed of plates fins and the second consists of adding pin fins between the plates fins. The governing equations, based on the k- SSt turbulence model, are disscredized and solved by the finite volume method and the SIMPLE algorithm. Dynamic results are presented in terms of velocity fields, profiles of the axial velocities in selected sections and pressure drop. The thermal study is presented in terms of temperature fields and the distribution of Nusselt number. A ratio between the thermal and dynamic performances is presented to evaluate the different heat sinks.

Since most of the energy input is converted to thermal energy in laser applications, the proper thermal management of laser systems is an important issue. Maintaining the laser diode and crystal temperature distributions in a narrow range during the operation is the most crucial requirement for the cooling of a laser system. In the present study, thermoelectric cooling (TEC) of a 1064 nm wavelength diode pumped laser source is investigated both experimentally and numerically. During the heat removal process, the thermal resistance through and between the materials, the proper integration of the TEC assembly, and the heat sink efficiency become important. For the aim of evaluating and further improving the system performance, various assembly configurations, highly conductive components, efficient interface materials and heat sink alternatives are considered. Several experiments are conducted during the system development stage, and parallel numerical simulations are performed both for comparison and also for providing valuable input for the system design. Results of the experiments and the simulations agree well with each other. As the laser device works in the transient regime, the experiments and the simulations are also implemented in this regime. In the final part of the study, the experiments are performed under the actual device working conditions. It is proved that with the designed TEC module and the copper heat sink system, the laser device can operate longer than the required operational time successfully.

In this study, forced cooling of heat sinks mounted on CPU&rsquo / s was investigated. Heat sink effectiveness, effect of turbulence models, effect of radiation heat transfer and different heat sink geometries were numerically analyzed by commercially available computational fluid dynamics softwares Icepak and Fluent. The numerical results were compared with the experimental data and they were in good agreement. Conjugate heat transfer is simulated for all the electronic cards and packages by solving Navier-Stokes equations. Grid independent, well converged and well posed models were run and the results were compared. The best heat sink geometry is selected and it is modified in order to have lower maximum temperature distribution in the heat sink.