The effect of nano-structured surfaces on pool boiling heat transfer is explored in this study. Experiments are conducted in a cubical test chamber containing fluoroinert coolant (PF5060, Manufacturer: 3M Co.) as the working fluid. Pool boiling experiments are conducted for saturation and subcooled conditions. Three different types of ordered nano-structured surfaces are fabricated using Step and flash imprint lithography on silicon substrates followed by Reactive Ion Etching (RIE) or Deep Reactive Ion Etching (DRIE). These nano-structures consist of a square array of cylindrical nanofins with a longitudinal pitch of 1 mm, transverse pitch of 0.9 mm and fixed (uniform) heights ranging from 15 nm - 650 nm for each substrate. The contact angle of de-ionized water on the substrates is measured before and after the boiling experiments. The contact-angle is observed to increase with the height of the nano-fins. Contact angle variation is also observed before and after the pool boiling experiments. The pool boiling curves for the nano-structured silicon surfaces are compared with that of atomically smooth single-crystal silicon (bare) surfaces. Data processing is performed to estimate the heat flux through the projected area (plan area) for the nano-patterned zone as well as the heat flux through the total nano-patterned area, which includes the surface area of the fins. Maximum heat flux (MHF) is enhanced by ~120 % for the nanofin surfaces compared to bare (smooth) surfaces, under saturation condition. The pool boiling heat flux data for the three nano-structured surfaces progressively overlap with each other in the vicinity of the MHF condition. Based on the experimental data several micro/nano-scale transport mechanisms responsible for heat flux enhancements are identified, which include: "microlayer" disruption or enhancement, enhancement of active nucleation site density, enlargement of cold spots and enhancement of contact angle which affects the vapor bubble departure frequency.

This dissertation focuses on the effects of superhydrophobic (SHPo) surfaces on thermal transport. The work is divided into two main categories: thermal transport without phase change and thermal transport with phase change. Thermal transport without phase change is the topic of four stand-alone chapters. Three address jet impingement at SHPo surfaces and the fourth considers natural convection at a vertical, SHPo wall. Thermal transport with phase change is the topic of a single stand-alone chapter exploring pool boiling at SHPo surfaces.Two chapters examining jet impingement present analytical models for thermal transport; one considered an isothermal wall and the other considered an isoflux wall. The chapter considering the isothermal scenario has been archivally published. Conclusions are presented for both models. The models indicated that the Nusselt number decreased dramatically as the temperature jump length increased. Further, the influence of radial position, jet Reynolds number, Prandtl number and isoflux versus isothermal heating become negligible as temperature jump length increased. The final chapter concerning jet impingement reports an experimental exploration of jet impingement at post patterned SHPo surfaces with varying microfeature pitch and cavity fraction. The empirical results show a decrease in Nusselt number relative to smooth hydrophobic surfaces for small pitch and cavity fraction and the isoflux model agrees well with this data when the ratio of temperature jump length to slip length is 3.1. At larger pitch and cavity fractions, the empirical results have higher Nusselt numbers than the SHPo surfaces with small pitch and cavity fraction but remain smaller than the smooth hydrophobic surface. We attribute this to the influence of small wetting regions. The chapter addressing natural convection presents an analytical model for buoyant flow at a vertical SHPo surface. The Nusselt number decreased dramatically as temperature jump length increased, with greater decrease occurring near the lower edge and at higher Rayleigh number. Thermal transport with phase change is the topic of the final stand-alone chapter concerning pool boiling, which has been archivally published. Surface heat flux as a function of surface superheat was reported for SHPo surfaces with rib and post patterning at varying microfeature pitch, cavity fraction, and microfeature height. Nucleate boiling is more suppressed on post patterned surfaces than rib patterned surfaces. At rib patterned surfaces, transition superheat decreases as cavity fraction increases. Increasing microfeature height modestly increases the transition superheat. Once stable film boiling is achieved, changes in surface microstructure negligibly influence thermal transport.

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Orientador: Elaine Maria Cardoso / Resumo: O avanço de novas tecnologias, associado à minimização dos custos de fabricação e instala-ção, constitui um grande desafio para a área de refrigeração, uma vez que a geração de calor tem aumentado gradativamente nos últimos anos. Assim, a busca de novos fluidos com pro-priedades térmicas superiores aos comumente usados tornou-se indispensável para melhorar a eficiência energética. Nas últimas décadas os nanofluidos - dispersões de partículas de escala nanométrica (1 a 100nm) em um fluido-base - têm atraído especial interesse não somente da comunidade acadêmica, mas também da indústria em áreas como: a microeletrônica, microflu-ídica, transporte, manufatura, assistência médica, entre outras. O melhor desempenho térmico e a vasta gama de aplicações fazem dos nanofluidos potenciais substitutos dos refrigerantes utilizados em diversos segmentos da engenharia. Dentro desse contexto, o presente trabalho teve como objetivos: o estudo teórico e experimental da influência das propriedades termofísi-cas e concentração de nanofluidos, bem como, das características geométricas da superfície aquecedora sobre o ângulo de contato e a molhabilidade. Também, atenção foi dada à prepa-ração e caracterização dos nanofluidos (Al2O3-água e Fe2O3-água), por meio da análise expe-rimental da condutividade térmica e da viscosidade dinâmica para diferentes concentrações; uma bancada experimental, para aquisição de imagens de gota séssil, foi construída a fim de viabilizar as análises de ângulo de conta... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The advance of new technologies, associated to the minimization of manufacturing and installation costs, presents a great challenge for the refrigeration area, since the heat generation has increased in recent years. Thus, the search for new fluids with thermal properties higher than those commonly used has become indispensable to improve energy efficiency. In recent decades, nanofluids-dispersions of nanometer-scale particles (1 to 100 nm) in a base fluid - have attracted special interest not only from the academic community but also from industry in areas such as microelectronics, microfluidics, transport, manufacturing, medical assistance, among others. In this context, the present work had the following goals: the theoretical and experimental study of the influence of thermophysical properties and nanofluid concentration, as well as the geometric characteristics of the heating surface on the contact angle and wetta-bility. Attention was also given to the preparation and characterization of nanofluids (Al2O3-water and Fe2O3-water) by the experimental analysis of thermal conductivity and dynamic viscosity for different concentrations; an experimental apparatus for the acquisition of sessile droplet images was designed in order to analyze the contact angle and wettability; and a computational routine was developed to obtain the drop profile and the surface-fluid interaction for the different nanofluids and surfaces used. Based on database, it was possible to evaluate the pre... (Complete abstract click electronic access below) / Doutor

O presente trabalho envolve a análise experimental do efeito da deposição de nanopartículas por meio da ebulição em piscina na molhabilidade e na textura da superfície. Inicialmente, este estudo apresenta uma análise da literatura sobre métodos de avaliação do ângulo de contato, preparo de nanofluidos, procedimentos de avaliação da rugosidade e possíveis efeitos que a deposição de nanopartículas tem sobre a textura da superfície. Verificou-se para as superfícies recobertas com nanopartículas ângulos de contato próximos a zero e comportamento dinâmico para gotas de água depositadas sobre elas. Desta forma, optou-se por avaliar a molhabilidade qualitativamente através da análise da velocidade de espalhamento de uma gota depositada sobre a superfície recoberta. Caracterizou-se também a massa de nanopartículas depositadas, a morfologia e a rugosidade das superfícies. Efetuou-se o recobrimento das superfícies por meio da ebulição em piscina de nanofluidos a base de água deionizada contendo nanopartículas de Al2O3 (10, 20-30 e 40-80 nm), Cu (25 nm) e SiO2 (15 e 80 nm) para concentrações volumétricas de 0,001, 0,01, 0,1 e 0,5%, submetidos a tempos de ebulição de 15, 30, 45 e 180 minutos em superfícies de alumínio e aço inoxidável. Como resultado final deste estudo concluiu-se que a rugosidade superficial e a molhabilidade se elevam com a deposição das nanopartículas. Além disso, a molhabilidade aumenta com o incremento da área da superfície recoberta com aglomerados. / The present study concerns an investigation on the wettability and the surface texture behavior of flat aluminum and stainless steel plates covered with porous thin-films of nanoparticles obtained through pool boiling of nanofluids. Since the contact angle of the obtained surfaces is small and in many cases the deposited droplet exhibits a dynamic behavior, dynamic top-down analyses of spreading droplets were performed. Evaluations were performed of nanoparticles mass deposition on the sample, surface roughness and micro-structural with an SEM (Scanning Electron Microscopy). Experiments were performed for nanofluids containing nanoparticles of Al2O3 (10, 20-30 and 40-80 nm), Cu (25nm) and SiO2 (15 and 80 nm) for volumetric concentrations of 0.001, 0.01, 0.1 and 0.5% for pool boiling time set to 15, 30, 45 and 180 minutes over aluminum and stainless steel plates. As a final result of this study it was found that surface roughness and wettability increase with the deposition of the nanoparticles. In addition, the wettability increases with increasing of the surface area covered with clusters.

O estudo consistiu em uma análise teórica experimental, com o desenvolvimento de uma correlação, dos parâmetros que afetam a ebulição nucleada de refrigerantes halogenados em superfícies cilíndricas lisas. Uma ampla análise da literatura permitiu levantar os distintos tópicos envolvendo este tema destacando entre eles o presente estudo. A análise da literatura envolveu, ainda, um estudo detalhado dos fundamentos da ebulição e das correlações para a previsão do coeficiente de transferência de calor. Foi projetado e construído um aparato experimental no qual foram realizados ensaios envolvendo superfícies de aço inoxidável, cobre e latão, os refrigerantes R-11, R-123, R-12, R-22 e R-134a, fluxos específicos de calor entre 0,8 e 120 kW/m&sup2, pressões reduzidas de 0,008 a 0,26 e rugosidade aritmética média variando entre 0,02 e 3,3 &#956m. A análise destes resultados possibilitou a verificação de comportamentos físicos inéditos segundo a literatura consultada. Destaca-se a análise de efeitos da condução longitudinal e da resistência térmica de contato entre a superfície de transferência de calor e os termopares. Esta análise foi incorporada ao procedimento de determinação da temperatura superficial, e permitiu estabelecer a distribuição de temperaturas ao longo do perímetro da superfície. Finalmente, com base no banco de dados levantado, foi desenvolvida uma correlação para o coeficiente de transferência de calor em ebulição nucleada plenamente desenvolvida de refrigerantes halogenados. / The present research has been focused in a theoretical and experimental analysis of the parameters that affect the pool boiling of halocarbon refrigerants in smooth cylindrical surfaces with the development of a correlation. A wide survey of the literature allowed to list the distinct topics related to this subject and to highlight among them the theme of this study. The literature analysis still involved a detailed study of the pool boiling fundamentals and of the correlations for the pool boiling heat transfer coefficient. An experimental set up has been developed and constructed in which were raise data involving surfaces of stainless steel, copper and brass, the refrigerants R-11, R-123, R-12, R-22 and R-134a, specific heat flux range of 0.8 to 120 kW/m&sup2, reduced pressures from 0.008 to 0.26 and arithmetic mean roughness varying between 0.02 and 3.3 &#956m. The analysis of these results revealed, according to the consulted literature, unpublished physical behaviors. In this work the effects of the longitudinal conduction and of the thermal contact resistance between the heat transfer surface and the thermocouple were analyzed carefully. This analysis was incorporated to the superficial temperature determination procedure and allowed to establish the temperature distribution along the surface perimeter. Finally, using the data base raised in this research, a simple and accurate correlation for the fully developed pool boiling heat transfer coefficient for halocarbon refrigerants applications has been developed.

Durante um acidente em uma usina nuclear, a integridade do vaso pressurizado deve ser assegurada. Em resposta a um possível derretimento do combustível nuclear, a atual geração de usinas possui um sistema para a injeção de água potável na cavidade do vaso pressurizado com intuito de resfriar sua parede, prevenindo danos a sua estrutura e evitando o vazamento de material radioativo. Esse estudo considerou o uso de água marinha como refrigerante para inundar a cavidade do vaso pressurizado combinado com a fixação de um estrutura porosa em forma de grade em sua parede externa como meio de aprimorar a margem de segurança durante a contenção de acidentes. Experimentos de longa duração para a ebulição em piscina de água marinha artificial foram conduzidos em uma superfície circular de cobre plana com 30 mm de diâmetro. Foi encontrado um fluxo de calor crítico de 1; 6 MW/m2 sob pressão atmosférica. Esse valor é significantemente maior que aquele obtido (1; 0 MW/m2) nas mesmas condições experimentais. Foi verificado que os depósitos de sais marinhos podem aumentar a molhabilidade e a capilaridade da superfície de teste, aprimorando assim o fluxo crítico. Combinando a água marinha e a fixação da estrutura porosa sobre a superfície de teste, verificou-se um melhora no coeficiente de transmissão de calor e no fluxo de calor crítico de até 110 % (2; 1 MW/m2), quando comparado a água destilada na superfície limpa, sem a instalação da estrutura. Após os experimentos, foi identificado que muitos dos poros presentes nas superfícies da estrutura porosa encontravam-se bloqueados devido ao aglutinamento de sais marinhos. Isso levou a conclusão que o aumento no valor do fluxo crítico observado para a água marinha artificial ocorreu devido, principalmente, a separação das fases líquida e gasosa do fluido na região próxima a superfície de teste, efeito proporcionado pela forma de grade da estrutura porosa, e ao aumento da molhabilidade e capilaridade da superfície devido a formação dos depósitos marinhos. / During a severe nuclear power plant accident, the integrity of the reactor pressure vessel must be assured. In response to a possible fuel meltdown, operators of the current generation of nuclear power plants are likely to inject water into the reactor pressure vessel to cool down the reactor vessel wall, preserving its integrity and avoiding leakage of radioactive material. This study considers the use of seawater to flood a reactor pressure vessel combined with the attachment of a honeycomb porous plate (HPP) on the vessel outer wall as a way to improve the safety margins for in-vessel retention of fuel. In long-duration experiments, saturated pool boiling of artificial seawater was performed with an upward-facing plain copper heated surface 30 mm in diameter. The resulting value for critical heat flux (CHF) was 1; 6 MW/m2 at atmospheric pressure, a value significantly higher than the CHF obtained when the working fluid was distilled water (1; 0 MW/m2). It was verified that sea-salt deposits could greatly improve surface wettability and capillarity, enhancing the CHF. The combination of artificial seawater and an HPP attached to the heated surface improved the boiling heat transfer coefficient and increased the CHF up to 110% (2; 1 MW/m2) as compared to distilled water on a bare surface. After the artificial seawater experiments, most of the wall micropores of the HPP were clogged because of sea-salt aggregation on the HPP top and bottom surfaces. Thus, the CHF enhancement observed in this case was attributed mainly to the separation of liquid and vapor phases provided by the HPP channel structure and improvement of surface wettability and capillarity by sea-salt deposition.

"Electrohydrodynamic (EHD) pumping relies on the interaction between electrical and flow fields in a dielectric fluid medium. Advantages such as simple and robust design as well as negligible vibration and noise during operation make EHD conduction pumping suitable for various applications. This work investigates meso-scale EHD conduction pumping used as an active flow distribution control mechanism for thermal management systems. Two different scenarios are considered for this purpose: alteration of uniform flow distribution and flow maldistribution correction. Its capability of actively controlling the flow distribution is examined in terms of the value of applied potential for initiation of flow divergence or flow equalization and the flow rate difference between each branch. Experimental results confirm that the reverse pumping direction configuration of EHD pumping is more effective than the same pumping direction configuration. A fundamental explanation of the heterocharge layer development is provided for the effect of flow direction on EHD conduction pumping performance. This study also involves a macro-scale EHD conduction pump used as an alternative mechanism of mixing liquid within a storage tank, for example under low-g condition. A numerical analysis of a simplified model of the experimental setup is provided to illustrate the liquid mixing and thermal homogenization process. The experimental and numerical study provide fundamental understanding of liquid mixing and thermal homogenization via EHD conduction pumping. Liquid-vapor phase change phenomena are used as effective mechanisms for heat transfer enhancement and have many applications such as HVAC&R systems. With this in mind, two detailed studies in pool boiling and in-tube flow condensation are carried out. Specifically, nucleate pool boiling on nano-textured surfaces, made of alumina ceramic substrate covered by electrospun nanofiber, is experimentally investigated. Also, the role of surface roughness and orientation in pool boiling is experimentally characterized. The in-tube convective condensation of pure water in mini-channels under sub-atmospheric pressure is also experimentally explored. This study provides valuable information for the design of condensers in a vapor compression cycle of HVAC&R systems using water as the refrigerant, this process has zero global warming potential. "

O presente trabalho envolve a análise experimental do efeito da deposição de nanopartículas por meio da ebulição em piscina na molhabilidade e na textura da superfície. Inicialmente, este estudo apresenta uma análise da literatura sobre métodos de avaliação do ângulo de contato, preparo de nanofluidos, procedimentos de avaliação da rugosidade e possíveis efeitos que a deposição de nanopartículas tem sobre a textura da superfície. Verificou-se para as superfícies recobertas com nanopartículas ângulos de contato próximos a zero e comportamento dinâmico para gotas de água depositadas sobre elas. Desta forma, optou-se por avaliar a molhabilidade qualitativamente através da análise da velocidade de espalhamento de uma gota depositada sobre a superfície recoberta. Caracterizou-se também a massa de nanopartículas depositadas, a morfologia e a rugosidade das superfícies. Efetuou-se o recobrimento das superfícies por meio da ebulição em piscina de nanofluidos a base de água deionizada contendo nanopartículas de Al2O3 (10, 20-30 e 40-80 nm), Cu (25 nm) e SiO2 (15 e 80 nm) para concentrações volumétricas de 0,001, 0,01, 0,1 e 0,5%, submetidos a tempos de ebulição de 15, 30, 45 e 180 minutos em superfícies de alumínio e aço inoxidável. Como resultado final deste estudo concluiu-se que a rugosidade superficial e a molhabilidade se elevam com a deposição das nanopartículas. Além disso, a molhabilidade aumenta com o incremento da área da superfície recoberta com aglomerados. / The present study concerns an investigation on the wettability and the surface texture behavior of flat aluminum and stainless steel plates covered with porous thin-films of nanoparticles obtained through pool boiling of nanofluids. Since the contact angle of the obtained surfaces is small and in many cases the deposited droplet exhibits a dynamic behavior, dynamic top-down analyses of spreading droplets were performed. Evaluations were performed of nanoparticles mass deposition on the sample, surface roughness and micro-structural with an SEM (Scanning Electron Microscopy). Experiments were performed for nanofluids containing nanoparticles of Al2O3 (10, 20-30 and 40-80 nm), Cu (25nm) and SiO2 (15 and 80 nm) for volumetric concentrations of 0.001, 0.01, 0.1 and 0.5% for pool boiling time set to 15, 30, 45 and 180 minutes over aluminum and stainless steel plates. As a final result of this study it was found that surface roughness and wettability increase with the deposition of the nanoparticles. In addition, the wettability increases with increasing of the surface area covered with clusters.

This dissertation focuses on the effects of superhydrophobic (SHPo) surfaces on thermal transport. The work is divided into two main categories: thermal transport without phase change and thermal transport with phase change. Thermal transport without phase change is the topic of four stand-alone chapters. Three address jet impingement at SHPo surfaces and the fourth considers natural convection at a vertical, SHPo wall. Thermal transport with phase change is the topic of a single stand-alone chapter exploring pool boiling at SHPo surfaces. Two chapters examining jet impingement present analytical models for thermal transport; one considered an isothermal wall and the other considered an isoflux wall. The chapter considering the isothermal scenario has been archivally published. Conclusions are presented for both models. The models indicated that the Nusselt number decreased dramatically as the temperature jump length increased. Further, the influence of radial position, jet Reynolds number, Prandtl number and isoflux versus isothermal heating become negligible as temperature jump length increased. The final chapter concerning jet impingement reports an experimental exploration of jet impingement at post patterned SHPo surfaces with varying microfeature pitch and cavity fraction. The empirical results show a decrease in Nusselt number relative to smooth hydrophobic surfaces for small pitch and cavity fraction and the isoflux model agrees well with this data when the ratio of temperature jump length to slip length is 3.1. At larger pitch and cavity fractions, the empirical results have higher Nusselt numbers than the SHPo surfaces with small pitch and cavity fraction but remain smaller than the smooth hydrophobic surface. We attribute this to the influence of small wetting regions. The chapter addressing natural convection presents an analytical model for buoyant flow at a vertical SHPo surface. The Nusselt number decreased dramatically as temperature jump length increased, with greater decrease occurring near the lower edge and at higher Rayleigh number. Thermal transport with phase change is the topic of the final stand-alone chapter concerning pool boiling, which has been archivally published. Surface heat flux as a function of surface superheat was reported for SHPo surfaces with rib and post patterning at varying microfeature pitch, cavity fraction, and microfeature height. Nucleate boiling is more suppressed on post patterned surfaces than rib patterned surfaces. At rib patterned surfaces, transition superheat decreases as cavity fraction increases. Increasing microfeature height modestly increases the transition superheat. Once stable film boiling is achieved, changes in surface microstructure negligibly influence thermal transport.