The thermophysical properties of nanofluids have attracted the attention of researchers to a far greater extent than the heat transfer characteristics of nanofluids have. Contradictory results on the thermal-fluid behaviour of nanofluids have been numerically and experimentally reported on in the open literature. Natural convection has not been investigated experimentally as much as the other properties of nanofluids. In this study, the characteristics and stability of Al2O3-water nanofluids (d = 20 30 nm) were analysed using a Malvern zetasizer, zeta potential and UV-visible spectroscopy. The natural convection of Al2O3- water nanofluids (formulated with a single-step method) was experimentally studied in detail for the volume fractions 0, 0.05, 0.1, 0.2, 0.4 and 0.6% in a rectangular cavity with an aspect ratio of 1, heated differentially on two opposite vertical walls for the Rayleigh number (Ra) range 3.49 x 10⁸ to 1.05 x 10⁹. The viscosity of Al2O3-water nanofluids measured between 15 and 50 °C. The effect of temperature and volume fraction on viscosity was also investigated. A detailed study of the nanoparticle concentration effect on the natural convection heat transfer coefficient was performed. It was found that increasing the concentration of nanoparticles improves the heat transfer coefficient by up to 15% at a 0.1% volume fraction. Further increasing the concentration of nanoparticles causes the natural convection heat transfer coefficient to deteriorate. This research also supports the idea that "for nanofluids with thermal conductivity more than the base fluids an optimum concentration may exist that maximises heat transfer in an exact condition as natural convection, laminar force convection or turbulence force convection". / Dissertation (MEng)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

The exponential advancements in the field of electronics and power generation have resulted in increased pressure on the thermal management of these systems where the desire for enhanced heat transfer is prevalent. A technique for enhancing heat transfer that has gained sufficient attention over the past two decades is to suspend nano-sized metallic particles in a base fluid in order to enhance its thermophysical properties. Fluids produced in such a manner are commonly termed nanofluids. Due to the promising heat transfer capabilities of nanofluids, many industrial applications are beginning to implement these fluids in their thermal practices. One of the potential applications where nanofluids may be used which has received a great deal of research attention is jet-impingement heat transfer. Concerning the existing publications on nanofluid jet impingement, most works within the steady state regime are limited to the cooling of Al2O3-water nanofluids, while transient studies do not account for cooling without the effects of boiling phenomena and for surfaces other than steel. In this study, six particle volume fractions of TiO2-water ranging between 0.025 and 1% were prepared and characterised for appropriate cooling tests. The study was conducted within both the steady and transient state with the main objective of evaluating the thermal performance of the selected nanofluid and to determine the optimum particle concentration for jet-impingement cooling applications. Therefore, an experimental rig was designed and manufactured where a copper target surface of 42 mm was impinged upon by a 1.65 mm orifice nozzle at a non-dimensional nozzle-to-target height of 4. The results indicated that the use of nanofluids in impingement applications produced adverse effects, depending on the particle fraction considered. With respect to the steady-state cooling tests, the copper surface was subjected to a constant heat flux of 145 watt and cooled by the different fluids at Reynolds numbers ranging between approximately 10 000 and 30 000. A maximum enhancement of 14.75% was observed in the measured Nusselt numbers, which occurred at a particle volume concentration of 0.05%. When increasing the volume fraction above 0.1%, unfavourable effects were observed for the heat transfer of the system in comparison with the base case tests of DI-water. Such trends were characterised by the trade-off between the enhancement in thermal conductivity and viscosity, both of which were increased with an increase in particle concentration. As for the effect of Reynolds number on the resulting thermal performance, a directly proportional relation was shown and could be described by the forced convection effect. The transient impingement tests showed that particle concentrations less than 0.1% produced an enhancement in cooling efficiency, while those of higher volume fractions showed negative effects. According to these tests the maximum enhancement was also obtained at a volume fraction of 0.05% and produced an average cooling efficiency enhancement of 16%. The results of the investigation clearly showed that the use of TiO2-water nanofluids in jet-impingement cooling applications produced thermal enhancement depending on the selected particle concentration. / Dissertation (MEng)--University of Pretoria, 2019. / NRF / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Nanofluids are homogeneous mixture of dispersed solid particles in base fluids. These solid particles are usually smaller than 100nm. Suspended nanoparticles modify the properties of based fluids. It is claimed, in some literature, for nanofluids to have greater than expected heat transfer performance. Due to this, nanofluids have gained great attention from both research and development and industries active in cooling systems. This thesis reports several measurements of convective heat transfer coefficient in a horizontal open micro-tube test section under laminar flow regime. The test section has an inner diameter of 0.5mm made of stainless steel and it has a length of 30cm. Two different test sections have been built. The first one has 13 thermocouples attached on the wall and the second one has 10. These thermocouples are used to measure the wall temperature distribution along the tube. In addition, two more thermocouples are used inside the micro-tube, at the inlet and outlet, to measure the bulk temperature of the nanofluids. A syringe pump is used for injecting the nanofluids through the micro-tube. A DC power supply provides constant heat flux along the test section and a differential pressure transducer measures the pressure drop of the test section. Aqueous based Al2O3 (9 wt %), ZrO2 (9 wt %), TiO2(9 wt %), CeO2 (9wt %), CNT (0.15 wt %), and diamond (1 wt %) have been tested in this thesis. Local Shah’s correlation predicts very well the behaviour of these nanofluids. The results are compared with water in six different ways: heat transfer forconstant Reynolds numbers, volume and mass flow rates, pressure drops andpumping powers. Enhancement in heat transfer is recognisable only in thegraphs of Nu numbers for constant Reynolds numbers. This can be attributed to the higher viscosity for nanofluids. Moreover, friction factor for constant Reynolds numbers has been compared. All the nanofluids with the exception of Al2O3 and diamond suit quite well with Darcy-Weisbach correlation.

Nanofluids representing nanometer-sized solid particles dispersed in liquids are of interest in many fields of process and energy engineering, e.g., heat transfer, catalysis, and the design of functionalized materials [1]. The physical, chemical, optical, and electronic properties of nanofluids are strongly driven by the size, shape, surface potential, and concentration of the nanoparticles. For the analysis of diffusive processes in nanofluids allowing access to, e.g., particle size and its distribution, dynamic light scattering (DLS) is the state-of-the-art technique. It is based on the analysis of microscopic fluctuations originating from the random thermal movement of particles in the continuous liquid phase at macroscopic thermodynamic equilibrium. For anisotropic particles or particle aggregates, besides translational diffusion also rotational diffusion occurs. To obtain the sum of the orientation-averaged translational (DT) and rotational (DR) diffusivities by depolarized DLS [2], a homodyne detection scheme is usually applied which can hardly be fulfilled in the experimental realization. Furthermore, the experiments are restricted to limited ranges for temperature, particle concentration, and viscosity.

In recent years, the study of fluid flow with nanoparticles in base fluids has attracted the attention of several researchers due to its various applications to science and engineering problems. Recent investigations on convective heat transfer in nanofluids indicate that the suspended nanoparticles markedly change the transport properties and thereby the heat transfer characteristics. Convection in saturated porous media with nanofluids is also an area of growing interest. In this thesis, we study the effects of radiation on the heat and mass transfer characteristics of nanofluid flows over solid surfaces. In Chapter 2, an investigation is made into the effects of radiation on mixed convection over a wedge embedded in a saturated porous medium with nanofluids, while in Chapter 3 results are presented for the effects of radiation on convection heat transfer about a cone embedded in a saturated porous medium with nanofluids. The resulting governing equations are non-dimensionalized and transformed into a non-similar form and then solved by Keller box method. A comparison is made with the available results in the literature, and the results are found to be in very good agreement. The numerical results for the velocity, temperature, volume fraction, the local Nusselt number and the Sherwood number are presented graphically. The salient features of the results are analyzed and discussed for several sets of values of the pertinent parameters. Also, the effects of the Rosseland diffusion and the Brownian motion are discussed.

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<p>Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in numerous industrial sectors. Recently submicron and high aspect ratio particles (nanoparticles and nanotubes) were introduced into the heat transfer fluids to enhance the thermal conductivity of the resulting nanofluids. The aim of this project was to investigate the physico-chemical properties of nanofluids synthesized using submicron and high aspect ratio particles suspended in heat transfer fluids .</p>

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

Este trabalho trata da caracterização de propriedades termodinâmicas e de transporte de nanofluidos baseados em nanopartículas de alumina em água para diferentes concentrações. Suspensões estáveis foram elaboradas por meio de um agitador ultrassônico. As seguintes propriedades foram analisadas: i) condutividade térmica com o método da sonda-linear; ii) viscosidade dinâmica através do reômetro do tipo cone e placa e iii) ângulo de contato com base em registros fotográficos de gotas em uma superfície plana e o tratamento de imagem através de um programa elaborado em LabVIEW. Procedimentos foram utilizados visando validar os métodos experimentais adotados, entre eles a comparação com resultados para fluidos puros. Além do estudo experimental, foi realizada uma análise crítica da literatura sobre condutividade térmica e viscosidade dinâmica de nanofluidos. Com base nesta análise, os resultados experimentais foram comparados a dados empíricos da literatura e métodos de previsão de propriedades desenvolvidos para nanofluidos e para suspensões de particulado sólido em líquido. De uma maneira geral, os resultados levantados neste estudo para condutividade térmica e viscosidade dinâmica de nanofluidos foram significativamente superiores a maioria dos dados experimentais da literatura e aos resultados proporcionados pelos métodos de previsão. Entretanto, para nanofluidos com composições distintas de nanopartículas de alumina em água, comportamentos similares ao do presente estudo também são observados na literatura. No caso do ângulo de contato, verificou-se seu decréscimo com o incremento da concentração de nanopartículas. Tal resultado coincide com a bibliografia consultada, segundo a qual a molhabilidade do nanofluido se eleva com o incremento da concentração de nanopartículas. / The present study concerns the characterization of thermodynamic and transport properties of nanofluids based on alumina nanoparticles in deionized water. Stable suspensions were obtained using an ultrasonic homogenizer (Sonicator). The following properties were measured: i) thermal conductivity using the linear probe method, ii) dynamic viscosity through a cone-plate rheometer iii) contact angle, based on photographic of nanofluid drops on a flat surface and image processing through a program based on LabVIEW. The methods and experimental procedures were validated by performing measurements properties of pure fluids with well known characteristics. Besides the experimental study, it was performed a comprehensive literature review on thermal conductivity and dynamic viscosity of nanofluids. Experimental results were compared against the data from the literature and the respective predictive methods developed for suspensions of nanofluids and micro solid particles in liquid. Generally speaking, the nanofluid thermal conductivity and dynamic viscosity measured in the present study were higher than the empirical values from the literature and the values given by predictive methods. However, it should be highlighted that although for different compositions of nanofluids behaviors similar to the one observed in this study are also reported in the literature. In case of contact angle, it was found that its value decreases with increasing the nanoparticle volumetric concentration. Such results is coincident with literature reports according to which the nanofluid wettability, given in terms of the contact angle, increases with increasing the nanoparticle concentration.

Dissipadores de calor baseados em microcanais são apresentados como solução para a remoção de fluxos de calor elevados em espaços restritos, pois proporcionam elevados coeficientes de transferência de calor quando comparados a canais convencionais. Tais trocadores também proporcionam elevadas razões entre a área superficial em contato com o refrigerante por unidade de volume do dissipador. Além dos microcanais, a utilização de nanofluidos também se apresenta como tecnologia com potencial de incremento do coeficiente de transferência de calor. Os nanofluidos consistem na adição de nanopartículas a um fluido base visando alterar suas propriedades de transporte termodinâmicas. Neste contexto, o objetivo do presente estudo é avaliar o coeficiente de transferência de calor para escoamentos monofásicos e ebulição convectiva de nanofluidos aquosos no interior de microcanais. Para isto, foram realizados experimentos em canais com diâmetro de 1,1 mm e comprimento de 200 mm para água deionizada, nanofluidos de alumina com diâmetros de 20-30 e 40-80 nm, nanofluidos de dióxido de silício com diâmetros de 15 e 80 nm, e nanofluidos de cobre com diâmetro de 25 nm. Estas soluções foram ensaiadas para concentrações volumétricas de nanopartículas de 0,001, 0,01 e 0,1, velocidades mássicas de 200, 400 e 600 kg/m2s e fluxos de calor de 20 a 350 kW/m2. A análise dos resultados revelou que a adição de nanopartículas a água deionizada proporciona o incremento do número de Nusselt para escoamentos monofásicos, principalmente na região inicial do tubo. Concluiu-se que os efeitos da adição de nanopartículas a um fluido base no coeficiente de transferência de calor durante a ebulição convectiva estão relacionados ao recobrimento da superfície com uma camada porosa. A deposição de nanopartículas com diâmetro inferior a 30 nm resultou na redução do coeficiente de transferência de calor e das instabilidades térmicas do escoamento em relação a água deionizada. O coeficiente de transferência de calor e as instabilidades térmicas não apresentaram variações significativas da deposição de nanopartículas com diâmetro superior a 40 nm. Por meio da análise da textura das superfícies recobertas e do critério de nucleação proposto por Kandlikar et al. (1997) concluiu-se que tal comportamento encontra-se associado aos efeitos do acabamento superficial na densidade de cavidades de nucleação ativas. / Microchannels based heat exchangers were introduced as a solution to high heat flux removal in restrict spaces due to their high heat transfer coefficients compared to heat exchangers based on conventional channels. The high ratio of surface are per volume is an additional advantage to microchannels in relation to conventional channels. Beside the microchannels technology, the nanofluids also present itself as a technique with potential to increase the heat transfer coefficient. Nanofluids consist of a solution containing nanoparticles dispersed in a base fluid with the goal to improve its thermodynamic and transport properties. In this context, the objective of the present study is to evaluate the heat transfer coefficient for single-phase flow and convective boiling of aqueous nanofluids inside microchannels. Experiments were performed for channels with internal diameter of 1.1mm and 200 mm long for DI-water, nanofluids containing alumina- (nanoparticles diameters of 20-30 and 40-80 nm), silicon dioxide (nanoparticles diameters of 15 and 80 nm), and copper (nanoparticles diameter of 25 nm). These solutions were evaluated for volumetric concentrations of 0.001, 0.01 and 0.1%, mass velocities of 200, 400 and 600 kg/m2s and heat fluxes from 20 to 350 kW/m2. The analysis of the results revealed that the addition of nanoparticles to DI-water causes an increment in the Nusselt number for single phase flows, especially at the inlet of the tube. The results for flow boiling indicated that the effects of adding nanoparticles to the base fluid are related to the deposition on the heating surface of a nanoparticles porous layer due to the boiling process. The deposition of nanoparticles smaller than 30 nm promoted a reduction of the heat transfer coefficient compared to DI-water on a clean surface, and thermal instabilities were minimized. For the deposition of nanoparticles larger than 40 nm these parameters did not presented significant variations in comparison to DI-water. A combined analysis of the surfaces finishing and the criterion of Kandlikar et al. (1997) for bubble nucleation revealed that such behaviors are correlated to the effects of the surface texture associated to the boiling process on the density of active nucleation cavities.