[pt] O uso de nanofluidos na área de Termociências tem sido de grande interesse tendo em vista possibilitar uma melhora significativa na capacidade de transferência de calor em máquinas térmicas. Isto se deve em função da presença de partículas em suspensão com diâmetros menores que 100 nm que são dispersas em fluidos convencionais. Parâmetros como viscosidade e condutividade térmica são fatores determinantes no desempenho desses materiais como fluidos térmicos em diferentes aplicações. O presente trabalho teve como objetivo a realização de ensaios experimentais para determinação da viscosidade dinâmica e da tensão cisalhante de três nanofluidos com diferentes composições, sob condições de temperatura controlada (faixa de -10 graus Celsius a 30 graus Celsius). Dois parâmetros adicionais - taxa de cisalhamento e taxa de rotação - foram condicionantes nos ensaios. A montagem da bancada experimental foi realizada a partir do acoplamento de um viscosímetro rotativo com um banho termostático. As curvas obtidas mostraram incremento da viscosidade dinâmica com a temperatura para os nanofluidos em relação aos seus fluidos base. Houve também incremento da viscosidade relativa, sobretudo para temperaturas mais elevadas. Os nanofluidos apresentaram comportamento não newtoniano. Para elevadas taxas de cisalhamento, as medições de viscosidade tendem a valores constantes. Correlações entre viscosidade com temperatura e com taxa de cisalhamento foram construídas seguindo funções exponenciais decrescentes e de potência, respectivamente. Os resultados obtidos neste trabalho acrescentam conhecimento à literatura científica sobre a viscosidade e o comportamento reólógico de nanofluidos a baixas temperaturas. / [en] The use of nanofluids in the Thermosciences has been of great interest since it enables a substantial improvement in the heat transfer capacity in thermal machines. This is possible due to the presence particles in suspension, with diameters less than 100 nm, dispersed in conventional fluids. Parameters such as viscosity and thermal conductivity are determining factors in the performance of these materials as thermal fluids in different applications. The objective of the present work was to determine the dynamic viscosity and the shear stress of three distinct nanofluids under controlled temperature conditions (range -10 Celsius degrees to 30 Celsius degrees) at laboratory. In addition, shear rate and rotation rate were also conditioning parameters. The experimental apparatus was assembled coupling a viscometer with a thermal bath. The obtained results showed an increase of the dynamic viscosity with temperature for all nanofluids with respect to their base fluids. An increase of the relative viscosity was also observed especially at higher temperatures. The three nanofluids have shown a non-newtonian behavior. With high shear rates, the measured viscosities tended towards constant values. Curve adjustments were performed between viscosity vs temperature and viscosity vs shear rate using exponential decay function and the Power Law, respectively. The achievements of this research added scientific understanding about the viscosity and rheological behavior of nanofluids at low temperatures.

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.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-08T19:46:26Z No. of bitstreams: 1 lyanemarisemoreirarochacosta.pdf: 4564563 bytes, checksum: e7d42b4e4fd043b15223cacd5d982eac (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-06-26T20:27:37Z (GMT) No. of bitstreams: 1 lyanemarisemoreirarochacosta.pdf: 4564563 bytes, checksum: e7d42b4e4fd043b15223cacd5d982eac (MD5) / Made available in DSpace on 2017-06-26T20:27:37Z (GMT). No. of bitstreams: 1 lyanemarisemoreirarochacosta.pdf: 4564563 bytes, checksum: e7d42b4e4fd043b15223cacd5d982eac (MD5) Previous issue date: 2012-03-09 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho teve por objetivo estudar a influencia de materiais nanoestruturados no transporte térmico em meio líquido e sua aplicabilidade como fluidos de dissipação de calor. Foram estudados nanofluidos de base aquosa contendo nanopartículas esféricas de ouro e prata, bem como nanofolhas de grafeno, nanotubos de carbono, fulerenos e nanofios de fulerenos. Para a caracterização estrutural dos nanofluidos foram utilizadas a espectroscopia de extinção/absorção UV-visível, microscopia de força atômica (AFM) e espectroscopia via espalhamento Raman. Com base na teoria de espalhamento de Mie foi possível estimar a distribuição de tamanhos para as nanopartículas metálicas. Com as imagens de AFM foram obtidos tamanhos de partículas, principalmente para as nanoestruturas de carbono. A qualidade e tipo de nanoestrutura de carbono foram definidos com auxílio da espectroscopia Raman. Os nanofluidos foram termicamente caracterizados através da espectroscopia via efeito de lente térmica. Obteve-se aumento na eficiência térmica para os nanofluidos com nanopartículas metálicas. No entanto, o mesmo não ocorreu com as dispersões de carbono nanoestruturado. Assim, mostraram-se como alternativas promissoras de fluidos de resfriamento os nanofluidos com nanopartículas metálicas. / This work was intended to study the influences of nanostructured materials in thermal transport in liquid medium and the applicability of these new materials as heat dissipation media. We studied aqueous nanofluids containing spherical nanoparticles of gold and silver, as well as Graphene nanosheets, carbon nanotubes, Fullerenes and Fullerene nanowires. For structural characterization of these nanofluids, extinction/ absorption UV-visible spectroscopy, atomic force microscopy (AFM) and Raman scattering spectroscopy have been used. Based on Mie scattering theory it was possible to estimate the distribution of sizes for metal nanoparticles. With AFM images were obtained particle sizes, mainly for the nanocarbon structures. The quality and type of nano carbon structure were defined with help of Raman spectroscopy. The heat transfer characteristics were obtained through thermal lens spectroscopy. We obtained thermal transport enhancement for metallic nanofluids. However, the same did not occur with nanocarbon nanofluids. Thus, the metallic nanofluids proved to be promising alternatives as cooling fluids.

L'évaporation d'une goutte reposant sur un support plat semble être un système relativement simple à étudier et a fait l'objet d'études scientifiques depuis plus d'un siècle. Cependant, l'étude de l'évaporation de gouttes sessiles est toujours d'actualité aujourd'hui avec l'essor de nouvelles techniques de visualisation ou de l'apparition de nouveaux types de fluides complexes.Cette étude expérimentale sera focalisée sur deux aspects distincts :- L'étude sur l'évaporation de fluides purs permettra d'étudier la dynamique d'évaporation et les ondes hydrothermales qui apparaissent dans les gouttes de fluides volatils lors du changement de phase. L'influence du type de fluide (différents alcools et alcanes) et du niveau de gravité (terrestre, lunaire et martienne) seront étudiés. De plus, l'utilisation de différents niveaux de gravité permet de développer un modèle empirique afin de prendre en compte dans le modèle quasi-stationnaire limité par diffusion de la vapeur la convection naturelle qui augmente fortement le débit d'évaporation.- Si les fluides complexes présentent une dynamique de séchage similaire à celle des fluides purs, d'autres mécanismes entrent en jeux, comme la gélification, l'organisation des particules et l'apparition de craquelures (voir Figure 2). Le mouillage et les différents groupes fonctionnels graphés sur les particules seront étudiés en regard du motif final de craquelures. / Sessile droplets are widely found in day to day life: it might be a coffee spilt, rain onto a waterproof raincoat or again, water falling onto a cooking plate. However, despite the vast number of studies devoted to droplets for almost half a century, the fundamental phenomenon of the evaporation of sessile droplets is still a field that attracts a high level of interest due to its wide applicability and the development of new visualisation techniques or new types of complex fluids. This experimental study is focused two distinct aspects:- The evaporation of pure fluids has allow to study hydrothermal waves that appear in the droplets of volatile fluids during phase change. The influence of the type of fluid ---different alcohols and alkanes--- and the gravity levels ---Terrestrial, Lunar and Martian--- is investigated to have a better understanding of the flow motion inside droplet. Moreover, the use of different gravity levels allows to experimentally evidence the contribution of the atmospheric convective transport to sessile droplet evaporation. This investigation has allowed to develop an empirical model to take account of natural convection which greatly increases the evaporation rate in the quasi-steady diffusion-controlled evaporation model.- If complex fluids exhibit an evaporation dynamic similar to pure fluid, other mechanisms come into play, such as gelation, particles organisation and cracks formation. Wetting and different functional groups on the particles graphs will be studied in relation to the final pattern of cracks.

L’évaporation de gouttes de liquides contenant des particules non volatiles représente un phénomène largement présent dans la vie quotidienne, à l’image des traces laissées par le marc de café après séchage. L’étude de la morphologie des dépôts de particules présente un grand intérêt dans les domaines de la biologie et trouve de nombreuses applications dans l’industrie. De ce fait, elle a fait l’objet de nombreuses recherches durant les dernières décennies. Malgré les nombreuses récentes recherches sur les morphologies des dépôts de particules, les mécanismes les contrôlant restent encore non complétement expliqués. Certains facteurs influençant les morphologies des dépôts sont nombreux (température de substrats…) mais restent encore peu documentés dans la littérature. Cette étude expérimentale s’intéresse à l’influence de la température du substrat sur la morphologie des dépôts de nanoparticules après séchage de gouttes sessiles de liquides. L’augmentation de la température du substrat accélère le processus d’évaporation et entraine des morphologies de dépôts très différentes de celles obtenues sur des substrats à température ambiante. Dans cette étude, la microscopie combinée à la thermographie infrarouge et à l’interférométrie ont permis d’expliquer la dynamique de formation de dépôts. De plus, l’étude a permis d’analyser les effets d’autres paramètres sur la morphologie des dépôts, tel que la composition chimique du liquide composant les gouttes. / Evaporation of liquid droplets containing non-volatile solutes is an omnipresent phenomenon in daily life, e.g., coffee stains on solid surfaces. The study of pattern formation of the particles left after the evaporation of a sessile droplet has attracted the attention of many researchers during the past two decades due to the wide range of biological and industrial applications. Despite the significance of controlling the deposition morphology of droplets, the underlying mechanisms involved in pattern formation are not yet fully understood. There is a varied range of factors that affect the final deposition patterns and some, e.g., substrate temperature, are poorly studied in the literature. This experimental study investigates the effect of a wide range of substrate temperatures on the deposition patterns of nanoparticles from drying sessile droplets. Increasing substrate temperature and accelerating the drying process lead to the formation of the patterns not observed on non-heated substrates. This research elucidates the formation mechanisms of these patterns by optical microscopy, infrared thermography, and white light interferometry techniques. Furthermore, the combined effects of substrate temperature and other factors such as chemical composition of base fluid and particle size on the dried patterns are studied. The underlying mechanisms involved in the formation of the patterns influenced by the combined factors are also discussed and presented.

No description available.

The scarcity of freshwater is becoming a global challenge worldwide due to limited resources availability and increasing demand both for manufacturing and household use. For this reason, there is an important need to develop efficient, economic and sustainable desalination technologies able to take advantage of unconventional sources of water (seawater, brackish groundwater and wastewater) in order to produce freshwater. Sun is considered as the most promising abundant renewable (and free) energy source that can be employed in steam and vapor generation processes, which has a great importance in many applications such as: water desalination, domestic water heating, and power generation. This doctoral dissertation presents a study on the efficiency of different carbon based systems - nanofluids and hybrid natural composites - for the improvement of direct-solar evaporation systems, for the production of freshwater. The two main goals of this work consist of: (i) the synthesis and characterization of stable carbon-based nanofluids in water and of re-usable, economical and ecological hybrid composite materials, and (ii) the comparison of such carbon-based systems applied to water evaporation, understanding mechanisms, advantages and limitations. Carbon based materials (carbon black, graphene and multi-walled carbon nanotubes) were chosen because of their high sunlight absorption ability, unique thermal properties, as well as low cost and abundant availability. However, the hydrophobic character of such materials makes necessary to find efficient strategies to overcome this problem when dealing with water. In this work, the suspension stability of graphene-based nanofluids in water - a key parameter for the application of nanofluids in any field - was effectively improved by combining physical (by RF Sputtering coating) or chemical (by NaClO-NaBr solution) graphene surface modification treatments, and the use of common additives (Triton X-114, SDBS and gum arabic) showing different stabilization mechanisms. The best strategy to obtain long-time graphene suspension stability in water (both deionized water and saline solution with 3.5 wt% NaCl) turned out to be the combination of the easy chemical treatment with the electro-steric stabilization effect of gum arabic. In addition to nanofluids, a re-usable devices based on gum arabic cross-linked gelatin hydrogel were synthesized and characterized. Hydrophobic carbon-based materials were easily and uniformly embedded into the porous hydrogel matrix, thanks to the amphiphilic character of both gelatin and gum arabic. The effect of carbon-nanoparticles nature, morphology and concentration on the measured effective thermal conductivity of the composite material was studied and the thermal conductivity of the nanoparticles was evaluated applying several models based on the effective medium approach. The values obtained for the nanoparticles were far from the tabulated thermal conductivity values because of the combination of the composite features (such as nanoparticles concentration, Kapitza resistance) and the particles characteristics (such as aspect ratio, crystalline structure). The performance of carbon-based nanofluids and hybrid hydrogels on direct-solar evaporation of water was tested and compared to that of carbon-wood bilayer composite (which presents both hydrophilic character and natural channels for water transportation) under solar simulator. The effect of surface temperature, light-to-heat conversion efficiency of carbon-based materials, heat losses, water transport through a porous medium and suspension stability (in the case of nanofluids) were investigated in order to understand the advantages and limitations of such systems. All the tested systems were able to improve water evaporation rate and evaporation efficiency up to 70% and 82% under 1 sun and 2 suns respectively using a small amount of nanoparticles: the same amount of particles dispersed in nanofluid (0.01 wt%) was embedded into hydrogels or deposited onto wood. The high sunlight absorption ability of carbon-based nanoparticles appeared as a dominant parameter for the improvement of water evaporation rate. In fact, enhanced light absorption was directly related to a high photothermal conversion efficiency, which caused an improvement in the surface temperature, leading to a consequent enhancement in evaporation rate. It has been found that an adequate supply of water to the evaporation surface represents a fundamental parameter as well considering floating systems.

Les caméras infrarouges modernes permettent d’accéder à la mesure de champs thermiques et de leur évolution temporelle. Le traitement d’images obtenues permet d’analyser la signature thermique d’objets mobiles ou de fluides en écoulement. Dans ce contexte nous avons fait l’étude de trois expériences différentes. La première consiste à suivre des billes mobiles et à évaluer leurs coefficients d’échanges thermiques avec l’environnement par l’estimation de temps caractéristiques. Dans le deuxième cas, nous faisons une comparaison entre deux fluides qui s’écoulent dans un microcanal, afin de déterminer les variations relatives des propriétés thermiques. La dernière application consiste à réaliser une cartographie de diffusivité thermique avec une source de chaleur mobile. / The modern infrared cameras allow the measurement of thermal fields and their temporal evolution. Infrared images processing is suitable to analyze the thermal signature of moving objects or fluid flows. In this context, we made the study of three different experiments. The first one is relative to infrared tracking of randomly moving balls and then estimate their thermal exchanges with the environment by the estimation of some characteristic time. In the second case we made a comparison between two fluids which flow inside a microchannel in order to determine the relative changes of thermal properties. The last application was to estimate a thermal diffusivity field with a mobile heat source.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-08T12:29:33Z No. of bitstreams: 1 elaineaparecidacarvalho.pdf: 7755814 bytes, checksum: b31278b6963ad259e85e174cf59c5391 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-06-26T18:10:31Z (GMT) No. of bitstreams: 1 elaineaparecidacarvalho.pdf: 7755814 bytes, checksum: b31278b6963ad259e85e174cf59c5391 (MD5) / Made available in DSpace on 2017-06-26T18:10:31Z (GMT). No. of bitstreams: 1 elaineaparecidacarvalho.pdf: 7755814 bytes, checksum: b31278b6963ad259e85e174cf59c5391 (MD5) Previous issue date: 2013-03-18 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho, três diferentes materiais (vidros, filmes finos e nanofluidos) foram analisados com o objetivo de verificar mudanças nas propriedades térmicas e ópticas devido a presença de nanopartículas de metais nobres. Nos vidros e filmes, o crescimento de nanopartículas foi induzido através do tratamento térmico das amostras enquanto que nos nanofluidos, a nucleação das nanopartículas foi obtida através de um procedimento químico de uma única etapa. A formação das nanopartículas metálicas nestes materiais foi confirmada pelas imagens de microscopia eletrônica e pelo pico de plasmon superficial presente nos espectros de absorção. Espectros Raman foram obtidos com a finalidade de verificar se ocorreu alguma mudança estrutural. Medidas de lente térmica foram realizadas em todas as amostras, fornecendo os valores da difusividade térmica. As matrizes vítreas estudadas foram germanato (Ge02 - PbO) e telurito (Te02 - PbO-Ge02), dopadas com nanopartículas de prata e com os íons terras raras Érbio, Ytérbio e Túlio na forma trivalente. Foi verificado que ocorreu um aumento de 20 % e 8 % na difusividade térmica dos vidros germanatos e teluritos, respectivamente, devido a adição de nanopartículas de prata e que o tratamento térmico provocou a quebra de anéis de tetraedros formados nos vidros germanato. Os filmes finos contendo nanopartículas de ouro e prata foram crescidos em um substrato através da técnica de co-sputtering utilizando a matriz vítrea de germanato (Ge02 - PbO) como alvo. Os resultados indicam a formação de estruturas diferentes daquelas detectadas no vidro, além de apresentar mudanças no comportamento térmico em relação a matriz vítrea. Nos nanofluidos de ouro e prata foram observados um aumento de 20% e 16%, respectivamente, na difusividade térmica em relação a difusividade térmica água pura. / In this work, three different materials (glasses, thin films and nanofluids) were analysed to check the changes in their thermal and optical properties due to the presence of noble metal nanoparticles. In the glasses and films, the nucleation of nanoparticles was induced by thermal treatment of the samples while in the nanofluids the nanoparticles were obtained through one-step chemical reaction. The presence of metallic nanoparticles was confirmed by electron microscopy images and by surface plasmon peaks detected via absorption technique in the ultraviolet and visible region of the electromagnetic spectrum. In addition, Raman spectra were obtained to check structural changes. Thermal lens measurements were performed in all samples to obtain the respective thermal diffusivity. Studied vitreous system were germanate (Ge02 - PbO) and tellurite (Te02 - PbO-Ge02) doped with silver nanoparticles, rare earth ions, erbium, thulium and ytterbium in the trivalent form. It was found an increase of 20% and 8% in the thermal diffusivity of germanate and tellurite glasses, respectively, due to the addition of silver nanoparticles. It also verified that the thermal treatment has caused the break of Ge04 tetrahedra rings. Thin films containing gold and silver nanoparticles were grown by the co-sputtering technique using the germanate vitreous matriz (Ge02 - PbO) as target. Raman results show structural differences between thin films and the corresponding glasses. This was reflected in a different thermal behavior. In gold and silver nanofluids were observed an increase of 20% and 16% in the thermal diffusivity in comparison with the thermal diffusivity of pure water, respectively.

<div>Thermal desalination of high salinity water resources is crucial for increasing freshwater supply, but efficiency enhancements are badly needed. Nanomaterial enhancements and novel condensation regimes offer enormous potential for improving promising technologies like membrane distillation (MD). In this work, we first examined nanofluids for MD, including the role of nanoscale physics, and model system-level energy efficiency enhancements. Our model included the dominant micro-mixing from Brownian motion in fine particle nanofluids (copper oxide) and the unusually high axial conduction from phonon resonance through Van der Waals interaction in carbon nanotube nanofluids. Carbon nanotubes resulted in a consistent, wide range of improvements; while copper oxide particles showcased diminishing returns after a concentration of 0.7%, where Brownian motion effects reduced. However, the enhancements at higher concentrations from liquid layering around nanoparticles were impractical in MD, since the related high surfactant levels compromised the membrane hydrophobicity and promoted fouling. Dilute solutions of metallic nanofluids can be actively integrated to enhance the performance of MD, whereas stronger nanofluid solutions should be limited to heat exchangers that supply thermal energy to MD systems. We then investigated slippery liquid infused porous surfaces (SLIPS) for enhanced condensation rates in MD. Dropwise condensation heat transfer was modelled considering the effects of the departing, minimum droplet radii and the interfacial thermal resistances. Effective droplet shedding from these surfaces led to an experimental thermal efficiency of 95%. Alternatively, porous condensers with superior wicking properties and conductive heat transfer offered a robust solution to high salinity desalination. We modelled the onset of flooding in porous condensers using Darcy’s law for porous media, including the effects of the condenser permeability and determined the optimal condenser thickness at varying system length scales. The increased active area of condensation resulted in a significant enhancement (96.5%) in permeate production and 31.7% improvement in experimental thermal efficiency. However, porous condensers were only compatible with flat plate module designs limiting their practicality.</div>