Capes / Neste trabalho, o processo de eletroerosão a frio (wire electrical discharge machining ou wire EDM) foi utilizado como método de fabricação alternativo para a confecção de microranhuras axiais em tubos de calor. Com isso, material foi retirado ao invés de ser adicionado ao invólucro do tubo de calor para a concepção da estrutura capilar, contribuindo para a redução de massa no dispositivo passivo de transferência de calor por mudança de fase. Uma modelagem baseada no projeto térmico e nos limites operacionais (limites capilar, de arrasto, viscoso, sônico e de ebulição) foi proposta para os tubos de calor com microranhuras axiais de geometria semicircular como estrutura capilar considerando diferentes definições disponíveis na literatura. Estes modelos, implementados no software EEStm (Engineering Equation Solver)tm, foram utilizados como ferramenta para o projeto dos tubos de calor ranhurados propostos. Os tubos de calor foram produzidos a partir de um tubo reto de cobre com um diâmetro externo de 9,45 mm, um diâmetro interno de 6,20 mm e um cumprimento total de 200 mm. O fluido de trabalho utilizado foi água deionizada e os tubos e calor foram carregados com uma razão de preenchimento de 60% do volume evaporador. O condensador foi resfriado por convecção forçada de ar, a seção adiabática foi isolada por uma fita de fibra de vidro e o evaporador foi aquecido utilizando um resistor elétrico em fita de liga de níquel-cromo e isolado do ambiente externo por um isolamento térmico aeronáutico. Os tubos de calor foram testados experimentalmente para as inclinações de operações iguais a 0º, 45º, 90º, 225º e 270º com relação ao plano horizontal, sob cargas térmicas compreendidas entre 5 W e 50 W. Os resultados experimentais do desempenho térmico dos tubos de calor mostraram que as microranhuras axiais fabricadas pelo processo de eletroerosão a frio como estrutura capilar funcionaram com sucesso em todos os casos estudados. Além disso, na maioria dos casos estudados, o tubo de calor com micriranhuras apresentou melhor desempenho térmico quando comparado com um tubo de calor contendo tela metálica como estrutura capilar. / In this master's dissertation an alternative fabrication method (wire electrical discharge machining, or wire EDM) was used to manufacture axial microgrooves in heat pipes. With this, material has been removed rather than being added to the heat pipe shell for the design of the capillary structure, contributing to mass reduction in the passive heat transfer device by phase change. A model based on thermal design and operational limits (capillary, trailing, viscous, sonic and boiling limits) was proposed for the heat pipes with axial microstrips of semicircular geometry as capillary structure considering different definitions available in the literature. These models, implemented in the EEStm (Engineering Equation Solver) software, were used as a tool for the design of the proposed grooved heat pipes. The heat pipes were produced from a straight copper tube with an outside diameter of 9.45 mm, an inner diameter of 6.20 mm and a total compliance of 200 mm. The working fluid used was deionized water and the tubes and heat were charged with a fill ratio of 60% of the evaporator volume. The condenser was cooled by forced convection of air, the adiabatic section was insulated by a fiberglass tape and the evaporator was heated using an electric resistor in nickel-chromium alloy tape and isolated from the external environment by an aeronautical thermal insulation. The heat pipes were experimentally tested for slopes of operations equal to 0º, 45º, 90º, 225º and 270º with respect to the horizontal plane, under thermal loads between 5 W and 50 W. The experimental results showed that the axial grooves manufactured by the Wire-EDM process worked satisfactorily in all analyzed cases. In most of the cases, the heat pipe with grooves showed a better performance when compared with the heat pipe with metallic mesh.

A aplicação de nanofluidos em tubos de calor em geral apresenta resultados experimentais satisfatórios em estudos buscando obter uma redução na resistência térmica do tubo de calor. No entanto, os estudos computacionais existentes associando tubos de calor e nanofluidos apresentam resultados conflitantes e carecem de uma discussão mais aprofundada a respeito da validade dos modelos utilizados para a representação computacional do comportamento de um nanofluido em tubo de calor, especialmente utilizando materiais e fluidos não convencionais como nanotubos de carbono ou etilenoglicol. Assim, o presente estudo busca avaliar a exatidão e a precisão obtida em uma série de simulações computacionais utilizando diferentes equações disponíveis na literatura para a modelagem de um nanofluido em um tubo de calor por meio da comparação com dados experimentais da literatura. Esta modelagem utiliza o método dos volumes finitos e permite determinar o efeito da variação dos modelos de propriedades e da concentração volumétrica de um nanofluido nos campos de temperaturas e nas resistências térmicas resultantes das simulações. Os resultados obtidos apresentam concordância com o comportamento esperado do ponto de vista qualitativo, mas falham em representar quantitativamente o comportamento da seção do evaporador dos tubos de calor estudados, apresentando variações máximas entre 1,5% e 23,9% em relação às temperaturas medidas experimentalmente. Isso pode ser justificado pelo fato de que a modelagem do fenômeno de ebulição de um nanofluido é mais complexa do que a modelagem utilizada atualmente em simulações computacionais. Essa consideração possui suporte na literatura e cria possibilidades para pesquisas futuras. / Application of nanofluids in heat pipes usually presents satisfactory experimental results in studies seeking to reduce the thermal resistance of the heat pipe. However, the existing computational studies connecting heat pipes and nanofluids present conflicting results and lack a deeper discussion regarding the validity of the models currently used for the computational representation of the behavior of a nanofluid in a heat pipe, especially using unusual materials and fluids, like carbon nanotubes or ethylene glycol. Thus, the present study seek to analyze the accuracy and the precision obtained in a set of computational simulations using pre-established equations for the modeling of a nanofluid in a heat pipe by using a direct comparison with existing experimental data. This modeling uses the finite volume method and permits to determine the effect of the variation of the properties models and the volume fraction of a nanofluid in the resulting temperature fields and the thermal resistances of the simulations. The obtained results show agreement with the expected behavior qualitatively, but fail to represent the phenomenon quantitatively, presenting maximum variations between 1,5% and 23,9% comparing to the experimentally measured average temperatures. This is justified by the hypothesis that the ebullition phenomenon modeling is more complex than the modeling currently used for computational simulations. This hypothesis is supported by the literature and creates possibilities for future researches.

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<p>  </p> <p>  </p> <p>Concentrator photovoltaics have demonstrated greater solar energy production efficiency than previous solar electric technologies.  However, recent research reveals that heat management is a significant difficulty in CPV systems, and if left unaddressed, it can have a severe influence on system efficiency and lifetime. Traditional CPV cooling relies on active methods such as forced air convection, or liquid cooling, which might lead to an extremely large parasitic power use. In addition, the moving parts of a cooling system result in a shorter lifespan and higher maintenance expenses. </p> <p><br></p> <p>CPV systems can boost their efficiency and lifespan by adopting passive cooling solutions. This work employed radiative cooling and natural convection to construct an efficient and cost-effective cooling system. The excess heat of a solar cell can be dispersed into space via electromagnetic waves via radiative cooling. Due to the fact that the radiative cooling power is related to the difference between the fourth powers of the solar cell and the ambient temperature, much greater cooling powers can be obtained at higher temperatures. Heat pipes were installed to act as a heat pump by transferring excessive heat from solar cells within a system to the exterior, where it can be dissipated via natural air cooling and thermal radiation. Experiments conducted in this study demonstrate that a temperature reduction of 21 ◦C was accomplished through radiative cooling and natural convection, resulting in an increase of 64 mV, or 17% in the open-circuit voltage of a GaSb solar cell.</p>

In the current climate crisis situation, the development and wide operating range of electric mobility is of great importance, with electric rail traction being the main form of electric transport over medium-long distances. In this sense, electric traction converters are undergoing a deep modification, moving towards more powerful, more compact converters with a wider operating range. This project addresses the problem of overheating of the internal air of a railway electric traction converter when it operates in extreme environments with high temperatures, around 50o C or 60o C. In these cases, the existing cooling system, which uses external air at ambient temperature as the coolant fluid, is not enough to ensure safe operation of the converter. This limits the operating range of electric trains with high power density converters, which cannot operate in hot climates. Furthermore, it poses a risk for other operating ranges in the near future, where the development of converters with higher current levels and thus higher power losses will again challenge conventional cooling systems. This project uses the MITRAC/TC1500TM traction converter developed by Alstom as a basis for proposing different additional cooling systems that complement the conventional one, with the use of different cooling technologies, including forced air convection, heat pipes, liquid-cooled cold plates and Peltier cells, also called thermoelectric coolers. The implementation of the different technologies is evaluated based on mathematical models developed in MATLAB® and computational fluid dynamics simulations in StarCCM+® . The results obtained allow to conclude that the use of heat pipes and Peltier cells is the most recommendable for the development of thermal management systems for electric traction converters, provided that they are implemented with a good external heat dissipation medium, preferably ambient temperature air flows already that already exist in the converter. Furthermore, it is shown that the same cooling technology can provide very different results depending on its implementation. / I den rådande klimatkrisen är det mycket viktigt att utveckla elektrisk mobilitet och att ha ett brett användningsområde, där elektrisk järnvägsdrift är den viktigaste formen av elektrisk transport över medellånga avstånd. Omvandlarna för elektrisk traktion genomgår därför en genomgripande förändring, mot kraftfullare och mer kompakta omvandlare med ett bredare driftsområde. Detta projekt behandlar problemet med överhettning av den inre luften i en elektrisk traktionsomvandlare för tåg när den används i extrema miljöer med höga temperaturer, runt 50o C eller 60oC. I dessa fall är det befintliga kylsystemet, som använder extern luft vid omgivningstemperatur som kylmedel, inte tillräckligt för att garantera en säker drift av omvandlaren. Detta begränsar användningsområdet för elektriska tåg med omvandlare med hög effektdensitet, som inte kan användas i varma klimat. Dessutom utgör det en risk för andra driftområden inom den närmaste framtiden, där utvecklingen av omvandlare med högre strömnivåer och därmed högre effektförluster återigen kommer att utmana konventionella kylsystem. I projektet används MITRAC/TC1500TM , en traktionsomvandlare som utvecklats av Alstom, som grund för att föreslå olika ytterligare kylsystem som kompletterar det konventionella, med användning av olika kyltekniker, inklusive luftkonvektion, värmerör, vätskekylda kylplattor och Peltierceller, även kallade termoelektriska kylare. Implementeringen av de olika teknikerna utvärderas baserat på matematiska modeller som utvecklats i MATLAB® och beräkningsflödesdynamiska simuleringar i StarCCM+®. De erhållna resultaten gör det möjligt att dra slutsatsen att användningen av värmerör och Peltier-celler är det mest rekommenderade för utvecklingen av termiska styrsystem för elektriska traktionsomvandlare, förutsatt att de implementeras med ett bra externt värmeavledningsmedium, helst luftflöden vid omgivningstemperatur som redan finns i omvandlaren. Dessutom visas att samma kylteknik kan ge mycket olika resultat beroende på hur den implementeras.

Les recherches que nous présentons dans ce manuscrit s’inscrivent dans le cadre de l’analyse des phénomènes de transport fondamentaux impliqués lors du processus d’évaporation d’un film liquide mince. Outre les mécanismes macroscopiques (résistance thermique du solide, capillarité, thermocapillarité, …) qui influencent le comportement de tels films, des développements fondamentaux et expérimentaux ont mis en évidence le rôle significatif d’effets microscopiques, comme les forces de van der Waals [11,96,117]. L’objectif de cette thèse est double. Il s’agit tout d’abord de caractériser les phénomènes locaux qui influencent le processus d’évaporation et ensuite, d’étendre notre étude à une échelle globale “macroscopique”. Ce manuscrit est divisé en deux parties qui correspondent à ces deux objectifs. <p><p>L’étude décrite dans la première partie propose une contribution originale à la modélisation de l’évaporation des films minces, y compris au voisinage des lignes de contact. De manière générale, nous cherchons à mettre en évidence l’influence de phénomènes qui se déroulent aux petites échelles sur le transfert thermique d’un film mince déposé sur une paroi plane et chauffée. Dans le cadre de l’hypothèse de lubrification, deux modèles sont dès lors développés. Le premier modèle décrit l’évaporation d’un film liquide mince dans sa vapeur pure tandis que le second modèle porte sur l’évaporation d’un film liquide mince dans un gaz inerte. Les diverses recherches menées sont principalement orientées vers la quantification, d’une part, des angles de contact apparents générés par l’évaporation, malgré le caractère parfaitement mouillant du couple liquide-solide utilisé et, d’autre part, des flux de chaleur et de matière interfaciaux. Une particularité du premier modèle est qu’il généralise divers modèles existants [15,25,86,117] en regroupant un ensemble de phénomènes spécifiques et complexes tels que le saut de température à l’interface liquide-vapeur, la résistance thermique de la vapeur et celle du solide ou la variation locale de la température de saturation à l’interface liquide-vapeur suite à la courbure interfaciale et aux forces de van der Waals. En plus de ces effets, d’autres mécanismes plus classiques sont inclus dans le modèle :la tension superficielle, la thermocapillarité, la pression de disjonction, l’évaporation et le recul de vapeur. Des analyses de stabilité linéaires et des études paramétriques ont été réalisées afin de quantifier l’influence de ces phénomènes sur la stabilité d’un film liquide mince, sur son évaporation et sur le transfert de chaleur associé. Au travers des chapitres 3 et 4, nous mettons notamment en évidence <p>• comment les forces de van der Waals compensent l’évaporation du film liquide mince de façon à créer un film stationnaire stable,<p>• pourquoi le recul de la vapeur et la thermocapillarité sont deux phénomènes qui peuvent être négligés dans les conditions étudiées dans ce travail,<p>• des lois analytiques qui décrivent certaines variables du problème, plus particulièrement l’angle de contact et le maximum du flux de chaleur, en fonction de la surchauffe de la paroi solide.<p><p>Faisant suite aux travaux proposés par Haut et Colinet [59], nous avons ensuite développé un second modèle afin de caractériser l’évaporation dans une faible quantité de gaz inerte d’un film liquide mince déposé sur une paroi plate et chauffée. Tout comme dans le cadre de l’étude précédente, notre analyse s’articule autour d’une étude de stabilité linéaire ainsi que d’études paramétriques réalisées sur des nombres caractéristiques du problème. Alors que les conclusions sur la stabilité du film sont indépendantes de la quantité de gaz inerte contenue dans la phase vapeur, il n’en est pas de même pour les transferts de matière et de chaleur interfaciaux comme montré au chapitre 5.<p><p>Dans la seconde partie du travail, nous utilisons les conclusions auxquelles nous sommes arrivés dans la première partie dans le cadre d’une application industrielle. En collaboration avec le Centre d’Excellence en Recherche Aéronautique (CENAERO) et la société Euro Heat Pipes (EHP), une stratégie a été élaborée afin de simuler les transferts thermiques radiaux dans une rainure d’un caloduc au niveau de l’évaporateur. Les résultats numériques, obtenus sur base d’un modèle multi-échelle développé à l’ULB et implémenté numériquement lors d’un stage chez CENAERO, montrent que ces transferts sont influencés par la valeur de l’angle de contact. Celui-ci dépendant des phénomènes microscopiques, il s’avère par conséquent nécessaire de les inclure dans le modèle thermique. En effet, si nous ne considérons que les aspects macroscopiques du problème, qui se résument à la conduction dans le solide et dans le liquide, le coefficient d’échange global au niveau de la rainure est surestimé.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Passive two-phase cooling devices such as capillary pump loops, loop heat pipes, and vapor chambers can utilize capillary-fed boiling in the porous evaporator wick to achieve high heat flux dissipation, while maintaining low thermal resistances. These systems typically rely only on passive capillary pumping through the porous wick to transport fluid. This inevitably leads to limits on the maximum heat flux and power dissipation based on the maximum capillary pressure available. To overcome these capillary pumping limitations in these passive devices, a mechanical pump can be added to the system to create a pump-assisted capillary loop (PACL). The pump can actively transport the fluid to overcome the pressure drop in liquid lines, reserving all of the available capillary action to draw liquid from a compensation chamber into the porous evaporator at the location of the heat input.<br>Previous studies on pump-assisted capillary loops have used a porous pathway to draw liquid to the heated evaporator surface from a liquid supply in the compensation chamber. This pathway typically comprises porous posts distributed over the heated surface area to ensure uniform liquid feeding during boiling and to avoid dryout regions. This thesis presents an evaporator design for a pump-assisted capillary loop system featuring a non-porous manifold connection between the compensation chamber and the evaporator wick base where boiling occurs. By using this approach, microscale liquid-feeding features can be implemented without the manufacturing restrictions associated with the use of porous wick pathways (such as sintered powder copper particles).<br>An analytical model for two-phase pressure drop prediction in the base wick is developed and used to define the evaporator geometry and feeding structure dimensions. A parametric analysis of the evaporator geometry is performed with the target of achieving a maximum heat dissipation of 1 kW/cm2 without a capillary limit. A 24 x 24 microtube array configuration with an outside tube diameter of 0.25 mm was identified as a result of this analysis. This manifold delivers liquid the base wick manufactured from sintered copper particles with a mean particle diameter of 90 microns. <br>The resulting evaporator geometry was translated into a manufacturable copper manifold design. A modular test section design consisting of a cover for attachment of fittings, a support structure for holding the manifold, a sintered copper wick base, and a carrier plate was created and manufactured, to accommodate for future testing scheduled to be performed by an external industry partner. The resulting design provides a testing vehicle to investigate the effect of different tubing arrangements and dimensions, as well as multiple base wick configurations. This knowledge can be used to engineer future evaporator architectures for enhanced performance. The improved understanding providing on the effect of liquid feeding distribution into the base wick, the effects of boiling on the base wick pressure drop, and the manufacturing limitations can each improve the performance prediction of evaporators with top feeding. <br>