Les fréquences de fonctionnement élevées des puces semi-conductrices génèrent des flux de chaleurs importants qu'il est nécessaire d'évacuer pour éviter la destruction de la puce. Un module standard dans le domaine de l'électronique de puissance est composé d'une puce en silicium, d'un isolant électrique (substrat) et d'un dissipateur thermique (drain) permettant l'évacuation de la chaleur. Cette chaleur induit des contraintes thermomécaniques dues à la dilatation différentielle des matériaux.Deux concepts nouveaux proposés permettent de palier ces problèmes et d'augmenter la fiabilité générale des systèmes électroniques. Le premier est la conception et l'élaboration d'un drain composite à propriétés thermiques adaptatives (coefficient de dilatation thermique et conductivité thermique). Dans le deuxième, une nouvelle méthode d'assemblage est présentée. Elle permet, au moyen d'un film métallique Sn ou Au, de créer des composés intermétalliques stables dans le temps. / The high operating frequencies of semiconductor chips generate heat fluxes it is important to be evacuated in order to avoid the destruction of the chip. A standard module in the field of power electronics is composed of a silicon chip, an electrical insulator (substrate) and a heat sink (drain) for the evacuation of heat. This heat induces thermomechanical stresses due to differential expansion of materials.Two new concepts proposed can overcome these problems and increase the overall reliability of electronic systems. The first is the design and development of a drain composite adaptive thermal properties (thermal expansion coefficient and thermal conductivity). In the second, a new assembly method is presented. It allows, by means of a metal film Sn or Au, intermetallic compounds to create stable over time.

Submitted by JORGE ANDRES ARROYAVE ORTEGON (jaarroyaveo@unal.edu.co) on 2018-08-20T15:11:59Z No. of bitstreams: 1 Dissertação_Jorge_Arroyave_Versão_Final_FC(1).pdf: 6142009 bytes, checksum: 185f73f2530ec7bc30607d8a9e004919 (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-08-20T20:10:47Z (GMT) No. of bitstreams: 1 arroyaveortegon_ja_me_ilha.pdf: 6142009 bytes, checksum: 185f73f2530ec7bc30607d8a9e004919 (MD5) / Made available in DSpace on 2018-08-20T20:10:47Z (GMT). No. of bitstreams: 1 arroyaveortegon_ja_me_ilha.pdf: 6142009 bytes, checksum: 185f73f2530ec7bc30607d8a9e004919 (MD5) Previous issue date: 2018-04-27 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / A energia solar pode ser aproveitada como fonte de energia térmica para aquecimento de água, por exemplo, em coletores solares ou como fonte de energia elétrica usando sistemas de células fotovoltaicas. Entretanto, as células fotovoltaicas, geralmente, de custos relativamente altos, têm algumas restrições relacionadas a altas temperaturas de operação e distribuições de temperatura não homogêneas levando a redução da vida útil e eficiência elétrica de tais sistemas. Essas limitações têm sido o foco de pesquisas, a fim de melhorar as eficiências elétricas, regular as temperaturas de operação e reduzir os materiais necessários para fabricação das células. Assim, este projeto de pesquisa tem como objetivo avaliar o desempenho de um dissipador de calor, baseado em microcanais retangulares paralelos, no resfriamento de uma célula fotovoltaica de alta concentração (HCPV-High Concentration Photovoltaic Cell), utilizando-se de análise teórica (modelo térmico), simulação numérica (usando o software comercial CFD ANSYS® Fluent v15) e de uma bancada experimental. Neste trabalho, foram consideradas as condições de máxima radiação (denominado de pior cenário, quando a célula não gera eletricidade e todo o calor deve ser dissipado) e de radiação média ao longo do período considerado. Os dados climatológicos foram obtidos do site Canal Clima - UNESP, com dados historicos do clima na região noroeste paulista. Foi realizada uma revisão do estado da arte a fim de compreender como os sistemas de geração elétrica fotovoltaica podem ser otimizados pelo uso de concentradores solares e materiais mais eficientes (células de junção-múltipla). A influência da temperatura nestes sistemas e como sistemas de resfriamento podem melhorar seu desempenho também foram analisados. Uma bancada experimental permitiu validar os resultados teóricos e numéricos obtidos. Comprovou-se que o uso de dissipador de calor baseado em microcanais pode permitir um controle efetivo da temperatura da célula HCPV, melhorando sua eficiência de conversão de energia solar em energia elétrica. O dissipador de calor foi avaliado sob condições de fluxo de calor constante, variando-se a velocidade mássica, G, no intervalo de 300 kg/m2s a 1500 kg/m2s. Assim, foi possível manter a superfície da célula a uma temperatura de 40°C, aproximadamente, para uma queda de pressão de, em média, 6 kPa. Os resultados das três análises apresentaram comportamentos similares e a concordância entre eles foi razoável, considerando as limitações de cada abordagem. / Solar energy can be used as a source of thermal energy in solar collectors, for example, or as a source of electricity using photovoltaic cell systems. However, photovoltaic cells requires high investments having some restrictions related to high operating temperatures and nonhomogeneous temperature distributions, leading to a reduction in the useful life and electrical efficiency. These limitations have been the focus of researches in order to improve electrical efficiencies, to regulate operating temperatures, and to reduce required materials in the cells. Thus, this research project aims to evaluate the performance of a heat sink based on parallel rectangular microchannels for cooling of a high concentration photovoltaic cell (HCPV), using theoretical analysis (thermal model), numerical simulation (using commercial software CFD ANSYS® Fluent v15) and an experimental bench. In this work, it was considered the conditions of maximum radiation (named worst scenario, when the cell does not generate electricity and all the heat must be dissipated) and the average radiation over the period considered. These climatological data were obtained from the Canal Clima – UNESP site, in the northwestern region of São Paulo state. A review on the subject was carried out in order to understand how solar photovoltaic systems can be optimized using solar concentrators and more efficient materials (multiple-junction cells). The influence of temperature and cooling systems were analyzed. An experimental bench was built, which allowed the validation of the theoretical and numerical results. The use of microchannel heat sinks can allow an effective temperature control of the HCPV cell, improving its efficiency of converting thermal energy into electrical energy. The heat sink was evaluated for different heat flux values and for mass velocity, G, in a range of 300 kg/m2s to 1500 kg/m2s. It was possible to maintain the high concentration cell at 40 °C with a pressure drop of 6 kPa, for the worst scenario. The three analyzes presented similar behavior and the agreement between them was reasonable, considering the approaches limitations. / FAPESP 2013/15431-7 / CNPq 458702/2014-5

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).

Orientador: Elaine Maria Cardoso / Resumo: A energia solar pode ser aproveitada como fonte de energia térmica para aquecimento de água, por exemplo, em coletores solares ou como fonte de energia elétrica usando sistemas de células fotovoltaicas. Entretanto, as células fotovoltaicas, geralmente, de custos relativamente altos, têm algumas restrições relacionadas a altas temperaturas de operação e distribuições de temperatura não homogêneas levando a redução da vida útil e eficiência elétrica de tais sistemas. Essas limitações têm sido o foco de pesquisas, a fim de melhorar as eficiências elétricas, regular as temperaturas de operação e reduzir os materiais necessários para fabricação das células. Assim, este projeto de pesquisa tem como objetivo avaliar o desempenho de um dissipador de calor, baseado em microcanais retangulares paralelos, no resfriamento de uma célula fotovoltaica de alta concentração (HCPV-High Concentration Photovoltaic Cell), utilizando-se de análise teórica (modelo térmico), simulação numérica (usando o software comercial CFD ANSYS® Fluent v15) e de uma bancada experimental. Neste trabalho, foram consideradas as condições de máxima radiação (denominado de pior cenário, quando a célula não gera eletricidade e todo o calor deve ser dissipado) e de radiação média ao longo do período considerado. Os dados climatológicos foram obtidos do site Canal Clima - UNESP, com dados historicos do clima na região noroeste paulista. Foi realizada uma revisão do estado da arte a fim de compreender como os sistemas de... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Solar energy can be used as a source of thermal energy in solar collectors, for example, or as a source of electricity using photovoltaic cell systems. However, photovoltaic cells requires high investments having some restrictions related to high operating temperatures and nonhomogeneous temperature distributions, leading to a reduction in the useful life and electrical efficiency. These limitations have been the focus of researches in order to improve electrical efficiencies, to regulate operating temperatures, and to reduce required materials in the cells. Thus, this research project aims to evaluate the performance of a heat sink based on parallel rectangular microchannels for cooling of a high concentration photovoltaic cell (HCPV), using theoretical analysis (thermal model), numerical simulation (using commercial software CFD ANSYS® Fluent v15) and an experimental bench. In this work, it was considered the conditions of maximum radiation (named worst scenario, when the cell does not generate electricity and all the heat must be dissipated) and the average radiation over the period considered. These climatological data were obtained from the Canal Clima – UNESP site, in the northwestern region of São Paulo state. A review on the subject was carried out in order to understand how solar photovoltaic systems can be optimized using solar concentrators and more efficient materials (multiple-junction cells). The influence of temperature and cooling systems were analyzed. An exp... (Complete abstract click electronic access below) / Mestre

Future defense, aerospace and automotive technologies involve electronic systems that release high pulsed waste heat like during high power microwave and laser diode applications in tactical and combat aircraft, and electrical and electronic systems in hybrid electric vehicles, which will require the development of an efficient thermal management system. A key design issue is the need for fast charging so as not to overheat the key components. The goal of this work is to study the fabrication and technology implementation feasibility of a novel high energy storage, high heat flux passive heat sink. Key focus is to verify by theory and experiments, the practicability of using phase change materials as a temporary storage of waste heat for heat sink applications. The reason for storing the high heat fluxes temporarily is to be able to reject the heat at the average level when the heat source is off. Accordingly, a concept of a dual latent heat sink intended for moderate to low thermal duty cycle electronic heat sink applications is presented. This heat sink design combines the features of a vapor chamber with rapid thermal energy storage employing graphite foam inside the heat storage facility along with phase change materials and is attractive owing to its passive operation unlike some of the current thermal management techniques for cooling of electronics employing forced air circulation or external heat exchangers. In addition to the concept, end-application dependent criteria to select an optimized design for this dual latent heat sink are presented. A thermal resistance concept based design tool/model has been developed to analyze and optimize the design for experiments. The model showed that it is possible to have a dual latent heat sink design capable of handling 7 MJ of thermal load at a heat flux of 500 W/cm2 (over an area of 100 cm2) with a volume of 0.072 m3 and weighing about 57.5 kg. It was also found that with such high heat flux absorption capability, the proposed conceptual design could have a vapor-to-condenser temperature difference of less than 10 0C with a volume storage density of 97 MJ/m3 and a mass storage density of 0.122 MJ/kg. The effectiveness of this heat sink depends on the rapidness of the heat storage facility in the design during the pulse heat generation period of the duty cycle. Heat storage in this heat sink involves transient simultaneous laminar film condensation of vapor and melting of an encapsulated phase change material in graphite foam. Therefore, this conjugate heat transfer problem including the wall inertia effect is numerically analyzed and the effectiveness of the heat storage mechanism of the heat sink is verified. An effective heat capacity formulation is employed for modeling the phase change problem and is solved using finite element method. The results of the developed model showed that the concept is effective in preventing undue temperature rise of the heat source. Experiments are performed to investigate the fabrication and implementation feasibility and heat transfer performance for validating the objectives of the design i.e., to show that the VCTES heat sink is practicable and using PCM helps in arresting the vapor temperature rise in the heat sink. For this purpose, a prototype version of the VCTES heat sink is fabricated and tested for thermal performance. The volume foot-print of the vapor chamber is about 6"X5"X2.5". A custom fabricated thermal energy storage setup is incorporated inside this vapor chamber. A heat flux of 40 W/cm2 is applied at the source as a pulse and convection cooling is used on the condenser surface. Experiments are done with and without using PCM in the thermal energy storage setup. It is found that using PCM as a second latent system in the setup helps in lowering the undue temperature rise of the heat sink system. It is also found that the thermal resistance between the vapor chamber and the thermal energy storage setup, the pool boiling resistance at the heat source in the vapor chamber, the condenser resistance during heat discharging were key parameters that affect the thermal performance. Some suggestions for future improvements in the design to ease its implementation and enhance the heat transfer of this novel heat sink are also presented.

No description available.

<p>  </p> <p>As the power density of computing devices increases, advanced liquid cooling thermal solutions offer an attractive thermal management approach. In particular, the low thermal resistance offered by microchannel heat sinks used in liquid cooling systems may enable increased total heat dissipation within fixed component temperature limits. There has been extensive work on the design of microchannel heat sinks, with many recent efforts to explore novel geometries and emerging manufacturing techniques. Of particular interest is additive manufacturing to allow for designs having complex, non-traditional internal geometries and package structures that cannot be made through conventional means. Despite the potential benefits for design and construction, additive manufacturing introduces new geometric uncertainties that could affect device performance. Direct metal laser sintering methods suitable for printing metal heat sinks typically produce a high internal roughness and other shape deviations in the flow paths of the final part. This extreme relative roughness and potential tortuosity in fluid flow through additively manufactured microchannels could lead to significant deviations in pressure drop and heat transfer predicted with traditional correlations and models. This work seeks to characterize the effects of high relative roughness on the friction factor and Nusselt number in additively manufactured microchannels having a rectangular cross section. Straight microchannel samples of 500 µm, 750 µm, and 1000 µm channel heights, and aspect ratios from 1 to 10 were manufactured to identify the design dimensions that resulted in visibly open channels, albeit with deviations in cross-sectional shape for submillimeter channel sizes and high internal roughness. Heat sink test samples were then printed with an array of these microchannels connected in parallel by inlet and outlet headers. Using water as the working fluid, the pressure drop and heat transfer performance of these sample heat sinks were characterized to explore how their behavior deviated from conventional predictions assuming smooth-walled channels. Flow through these additively manufactured microchannels displayed higher pressure drops than predicted, as well as a flow rate dependence of the hydrodynamic and thermal performance. These observed deviations are explored as effects of the physical conditions inside the channel as a result of additive manufacturing. Severe constriction of the channel would account for the difference in magnitude between the experimental and predicted results, while the introduction of flow redevelopment could lead to a flow rate dependence.  By further understanding the impact of these artifacts and deviations, these factors can be accounted for in the design and modelling of more complex additively manufactured heat sinks. </p>

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.

Avec le remplacement des réseaux hydrauliques et pneumatiques à bord des aéronefs par des réseaux électriques, le nombre d'équipements embarqués pour assurer un bon fonctionnement augmentera. Le passage à un avion entièrement électrique permettrait de réduire les couts de production et fonctionnement, assurerait une meilleure fiabilité des systèmes, et réduirait l'impact écologique de la circulation d'un tel appareil. En effet, un tel avion serait plus léger qu'un avion actuel. Pour s'assurer de cela, il est nécessaire de réduire la masse des équipements embarqués servant à la gestion, la mise en forme, la distribution d'énergie électrique. Le dimensionnement et l'optimisation de la masse des équipements embarqués est donc une problématique fondamentale dans le développement de l'avion plus électrique. Cette masse est majoritairement fixée par les systèmes de refroidissement lorsque l'on considère des systèmes de conversion d'énergie. Parmi l'ensemble des systèmes de refroidissement disponibles et dédiés au refroidissement des convertisseurs statiques, deux grandes technologies ont été sélectionnées, dans l'objectif d'en produire des modèles dédiés à des routines d'optimisation. Les dissipateurs à ailettes droites en convection forcée, ainsi que les systèmes associant dissipateurs à ailettes et caloducs seront modélisés au cours de ces travaux de thèse. Des modèles analytiques de ces systèmes de refroidissement seront proposés, dans l'optique de pouvoir optimiser au mieux leur masse tout en assurant un bon fonctionnement thermique. Même si de nombreuses méthodes de dimensionnement et d'optimisation dédiées aux systèmes de refroidissement existent, notre choix de modélisation s'est porté sur une représentation analytique. En effet, ce type de modélisation est déduit d'une résolution exacte de l'équation de la chaleur pour représenter des configurations géométriques et thermiques simples. Les configurations sélectionnées correspondent à des configurations simples à modéliser analytiquement. L'avantage de tels modèles réside dans le fait que le comportement thermique de systèmes de refroidissement, i.e de la température de la source de chaleur à l'ambiant, est une fonction des paramètres géométriques, des matériaux et des conditions environnementales des systèmes de refroidissement. Ce sont donc des modèles très rapides d'exécution qui donnent une solution exacte du comportement thermique des dissipateurs modélisés. Ils présentent donc un réel intérêt dans l'optique d'optimiser la masse de ces systèmes. / The replacement of hydraulic and pneumatic network embedded in aircraft by electrical network will increase the number of embedded systems to ensure the effective functioning of the aircraft. The development of an electrical aircraft will allow the reduction of production and functioning costs. It will also help ensure a better reliability of systems and will reduce the ecological impact of the aircraft circulation. This kind of plane would be lighter than a usual one. To be sure of this, it is necessary to reduce weight of embedded equipment's dedicated to management, conversion and distribution of electrical energy. The sizing and the optimization of embedded equipment's weight is a critical issue in the development of more electrical aircraft. This weight is mostly defined by heat transfer systems, when we consider the sizing of energy conversion system A lot of heat transfer system exists and are dedicated to the cooling of power converters. We selected two of these heat transfer system, in order to produce models of them. These models will be used in optimization routines. Plate fin heat sink in forced convection, and system assembly, combining heat pipe and plate fin heat sink, will be modelled during this thesis. Analytical models of these heat transfer systems will be developed, to optimize their weight and ensure a good cooling of electrical systems. Even if lots of dimensioning and optimization methods exists, dedicated to heat transfer systems, we choose to use analytical modelling. This kind of models gives an exact solution to the heat equation, to describe simple geometric and thermic configurations. Selected heat transfer systems can be simply described. The main advantage of these models is that it represents the thermal behavior of the system as a function of its geometrical parameters, materials and environmental conditions. Execution of these models is very fast and gives a precise solution of the thermal parameters of the described configuration. There is then a real interest to use this type of models to optimize weight of heat transfer systems, and then power converter.

A stacked microchannel heat sink was developed to provide efficient cooling for microelectronics devices at a relatively low pressure drop while maintaining chip temperature uniformity. Microfabrication techniques were employed to fabricate the stacked microchannel structure, and experiments were conducted to study its thermal performance. A total thermal resistance of less than 0.1 K/W was demonstrated for both counter flow and parallel flow configurations. The effects of flow direction and interlayer flow rate ratio were investigated. It was found that for the low flow rate range the parallel flow arrangement results in a better overall thermal performance than the counter flow arrangement; whereas, for the large flow rate range, the total thermal resistances for both the counter flow and parallel flow configurations are indistinguishable. On the other hand, the counter flow arrangement provides better temperature uniformity for the entire flow rate range tested. The effects of localized heating on the overall thermal performance were examined by selectively applying electrical power to the heaters. Numerical simulations were conducted to study the conjugate heat transfer inside the stacked microchannels. Negative heat flux conditions were found near the outlets of the microchannels for the counter flow arrangement. This is particularly evident for small flow rates. The numerical results clearly explain why the total thermal resistance for counter flow arrangement is larger than that for the parallel flow at low flow rates. In addition, laminar flow inside the microchannels were characterized using Micro-PIV techniques. Microchannels of different width were fabricated in silicon, the smallest channel measuring 34 mm in width. Measurements were conducted at various channel depths. Measured velocity profiles at these depths were found to be in reasonable agreement with laminar flow theory. Micro-PIV measurement found that the maximum velocity is shifted significantly towards the top of the microchannels due to the sidewall slope, a common issue faced with DRIE etching. Numerical simulations were conducted to investigate the effects of the sidewall slope on the flow and heat transfer. The results show that the effects of large sidewall slope on heat transfer are significant; whereas, the effects on pressure drop are not as pronounced.