Utilização de superfícies estendidas - espumas metálicas - para intensificação da transferência de calor por ebulição
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Data
2021-12-10
Autores
Manetti, Leonardo Lachi
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
A evolução dos componentes eletrônicos tem resultado em componentes de alto de-sempenho e alta geração de calor. A técnica de ebulição em piscina com contato direto do flu-ido com o componente, resfriamento por imersão, é uma solução para a indústria eletrônica. Os fluidos dielétricos vêm ganhando destaque nas pesquisas, pois permitem a imersão dos dispo-sitivos sem risco de danos e também possuem baixa temperatura de saturação, mantendo o sistema abaixo da temperatura crítica. O uso de técnicas para tratar/modificar a superfície aquecida tem sido proposto para aumentar o desempenho da ebulição. Uma técnica adequada à indústria é o uso de espumas metálicas de estrutura aberta, que têm sido amplamente investi-gadas. Neste trabalho espumas metálicas de diferentes materiais, Cobre e Níquel, com diferen-tes características e espessuras foram primeiramente caracterizadas para determinação da sua porosidade, diâmetros de poro e fibra, densidade de poros, área superficial, permeabilidade e capilaridade. Em seguida, elas foram usadas como superfície aquecida durante a ebulição de dois fluidos dielétricos, HFE-7100 e Etanol, em condições de saturação à pressão atmosférica em dois laboratórios distintos. No geral, as espumas metálicas forneceram um maior coeficien-te de transferência de calor quando comparado com a superfície plana e também preveniram o superaquecimento inicial, tendo um início de nucleação antecipado. Para espessuras idênticas, a espuma de cobre apresentou melhor desempenho, pois há um balanço entre a condutividade térmica do material e a área de troca de calor. Quando houve a variação de espessura, consta-tou-se que a espessura ótima depende do fluxo de calor, sendo a maior espessura melhor para baixos fluxos e a menor espessura melhor para altos fluxos de calor, pois há um balanço entre o aumento de área e a resistência para a saída de vapor e consequente entrada de líquido no inte-rior da espuma. Quando comparado os diferentes fluidos, o Etanol apresentou melhor desem-penho que o HFE-7100 devido às suas melhores propriedades termofísicas, dentre elas o calor latente de vaporização, condutividade térmica e temperatura de saturação. Por fim, um mode-lo foi desenvolvido para predizer o coeficiente de transferência de calor e o fluxo máximo de calor em função das características da espuma metálica e do fluido de trabalho. O modelo prediz satisfatoriamente os nossos dados experimentais assim como os dados da literatura, sendo um bom guia para uso na indústria.
The growth of electronic technology has resulted in components with high perfor-mance and high heat generation. The pool boiling technique with direct contact of the fluid and the component - immersion cooling - is a solution for the industry of electronics. Dielec-tric fluids have been chosen in boiling research because they allow the components to be im-mersed without short-circuiting damage and have a low saturation temperature - which keeps the system below the critical temperature. The use of techniques to treat/modify the heating surface has been proposed to increase boiling performance. An appropriate technique for the industry is using metal foams with an open structure – open-cell metal foam. In this work, metal foams of different materials, Copper and Nickel, with different characteristics and thicknesses were first characterized to determine their porosity, pore and fiber diameters, pore density, surface area, permeability, and wickability. Then, they were used as a heating surface during the boiling of two dielectric fluids, HFE-7100 and Ethanol, under conditions of satura-tion at atmospheric pressure in two different laboratories. In general, the metal foams provid-ed a higher heat transfer coefficient when compared to the flat surface and prevented initial overheating, with an early nucleation onset. For identical thicknesses, the copper foam showed better performance because there is a balance between the thermal conductivity of the material and the metal foam area. For thickness variation, it was noted that the optimum thickness de-pends on the heat flux: the thicker foam has a better performance at low fluxes while the thin-ner one is better at high heat fluxes because there is a balance between the area and the re-sistance to the vapor bubble escape and the consequent liquid flow into the foam. When com-paring the different fluids, Ethanol performed better than HFE-7100 due to its better thermo-physical properties, mainly the latent heat of vaporization, thermal conductivity, and satura-tion temperature. Finally, a predictive model was developed to predict the heat transfer coef-ficient and the maximum heat flux according to the characteristics of the metal foam and the working fluid. The model predicts with a good agreement our experimental data as well as the literature data, being a good guide for use in the industry.
The growth of electronic technology has resulted in components with high perfor-mance and high heat generation. The pool boiling technique with direct contact of the fluid and the component - immersion cooling - is a solution for the industry of electronics. Dielec-tric fluids have been chosen in boiling research because they allow the components to be im-mersed without short-circuiting damage and have a low saturation temperature - which keeps the system below the critical temperature. The use of techniques to treat/modify the heating surface has been proposed to increase boiling performance. An appropriate technique for the industry is using metal foams with an open structure – open-cell metal foam. In this work, metal foams of different materials, Copper and Nickel, with different characteristics and thicknesses were first characterized to determine their porosity, pore and fiber diameters, pore density, surface area, permeability, and wickability. Then, they were used as a heating surface during the boiling of two dielectric fluids, HFE-7100 and Ethanol, under conditions of satura-tion at atmospheric pressure in two different laboratories. In general, the metal foams provid-ed a higher heat transfer coefficient when compared to the flat surface and prevented initial overheating, with an early nucleation onset. For identical thicknesses, the copper foam showed better performance because there is a balance between the thermal conductivity of the material and the metal foam area. For thickness variation, it was noted that the optimum thickness de-pends on the heat flux: the thicker foam has a better performance at low fluxes while the thin-ner one is better at high heat fluxes because there is a balance between the area and the re-sistance to the vapor bubble escape and the consequent liquid flow into the foam. When com-paring the different fluids, Ethanol performed better than HFE-7100 due to its better thermo-physical properties, mainly the latent heat of vaporization, thermal conductivity, and satura-tion temperature. Finally, a predictive model was developed to predict the heat transfer coef-ficient and the maximum heat flux according to the characteristics of the metal foam and the working fluid. The model predicts with a good agreement our experimental data as well as the literature data, being a good guide for use in the industry.
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Palavras-chave
Espuma metálica, Fluido dielétrico, Ebulição, Resfriamento por imersão, Modelo preditivo, Metal foam, Dielectric fluid, Boiling, Immersion cooling, Predictive model