[pt] É estudada a aplicação de nanofluidos como fluidos secundários em sistemas de refrigeração por compressão de vapor mediante o desenvolvimento de um modelo termodinâmico de parâmetros concentrados. Quando um nanofluido é usado como fluido térmico, sua condutividade e viscosidade aumentam com respeito às propriedades do fluido base correspondente. Como conseqüência, a irreversibilidade por transferência calor diminui enquanto que a por atrito aumenta. É aplicado o método dos coeficientes estruturais para determinar o efeito da concentração de nanopartículas no fluido secundário na irreversibilidade global do sistema, levando em consideração as inter-relações da estrutura analisada. Para estimar os limites práticos da redução da irreversibilidade térmica com o uso de nanofluidos é proposta uma otimização do custo operacional, a partir de análise termoeconômica, considerando a aplicação do novo fluido secundário no sistema, sem nenhuma outra modificação no mesmo. A partir do modelo proposto, verificado com dados experimentais do ciclo de refrigeração, simulou-se um caso particular de operação. Mediante uma otimização parcial, foi determinado o ponto de mínimo custo operacional, com a simples variação da concentração volumétrica de nanopartículas. Os resultados das otimizações fornecem diferentes valores da concentração ótima para diferentes cenários, caracterizados por vários comprimentos equivalentes do circuito secundário e diversos tempos de operação anual. Adicionalmente, o trabalho inclui um estudo sobre a aplicação de nanofluidos em um evaporador de casco e tubo, o qual foi simulado a partir de um modelo termodinâmico detalhado. Dados experimentais foram levantados para validar o modelo. / [en] The application of nanofluids as secondary fluids in vapor compression refrigeration systems is studied with the development of a lumped-parameter thermodynamic model. When a nanofluid is used as a heat transfer fluid, its thermal conductivity and viscosity increase, when compared with the corresponding properties of the base fluid. The irreversibilities due to heat transfer and due to friction decrease and increase, respectively. After irreversibility is calculated for each component, the method of structural coefficients of internal bonds is applied to determine the effect of the volumetric concentration of nanoparticles in the secondary fluid on the system s global irreversibility, taking into account the interrelations of the analyzed structure. To estimate the practical limits of thermal irreversibility reduction with nanofluid application, an optimization of operational cost was proposed, based on thermoeconomic analysis, and considering the application of the new secondary fluid on the system, without additional modifications. Based on the proposed model, which was verified by experimental data, an typical operation condition was simulated. Through partial optimization, the minimum operational cost is determined for a simple variation of volumetric concentration of nanoparticles. The results of the optimizations furnish different optimal concentration values for different scenarios. Additionally, an study of nanofluid application in a shell and tube evaporator was included. The evaporator was simulated from a detailed thermodynamic model. Experimental data were collected to validate the model.

This thesis explores colloidal semiconductor nanocrystal solutions as a feedstock for creating thin film semiconductor materials through printing processes. This thesis will span the synthesis of nanocrystals, ligand exchange chemistry, solution phase characterization methods, thin film device fabrication, thin film characterization methods, and device characteristics. We will focus extensively relating the structure of nanocrystals in solution and in thin films to their chemistry, optical properties and electronic properties. By way of introduction, the origin and nature of semiconductor nanocrystals will be explored. This discussion will place semiconductor nanocrystals in their historical context, namely the oil-shocks of the 1970s. The interest in II-VI semiconductor materials stemmed from a desire find photochemical synthetic routes to reduce the use of fossil fuels. As a result, II-VI semiconductor nanocrystal are far more developed synthetically. Additionally, our understanding of II-VI semiconductor nanocrystals is couched in the language of solid state physics rather than chemistry. This will lead into a discussion of their electronic structure and the iterative nature of nanocrystal synthetic development and our theoretical understanding of nanocrystals. The first chapter will discuss nanocrystal synthetic methods in a broad context, finally narrowing in on the synthesis chosen for this work. Following a description of the synthesis, we will then describe the ligand chemistry and the reactions which may be performed in the ligand shell. The final sections of the chapter will describe the synthetic routes to the three nanocrystal materials used in the rest of this work, namely CdSe-CdCl2/PBu3, CdSe-CdCl2/NH2Bu, and CdSe/NH2Bu. The second chapter will introduce the crystal structure of II-VI semiconductor nanocrystals and describe how the structure is measured. This will lead in to a discussion of pair distribution function analysis of X-ray data and examples of its application to the solution phase structure of semiconductor nanocrystals. Some size dependent structural properties, namely stain, will be demonstrated by PDF. At the end evidence for surface reconstruction in solution as ligands are removed will be presented. In the final chapter, techniques for film formation and ligand dissolution with be presented. Annealing of films produces electronic and structural changes which can be observed in the absorbance spectrum, electron microscopy, and X-ray scattering. I propose a three phase annealing model which includes 1) reversible desorption of the organic ligands, 2) irreversible particle fusion, and 3 ripening of grains. The temperature at which ripening occurs depends sensitively on the sample content, which increase chloride concentration decreasing the temperature at which ripening occurs. The ripening process is found to correlate with a phase transition from zinc blende to wurtzite, which indicates that grain boundary mobility is an important part of the ripening process. Finally thin film transistors are characterized electronically. Fused grains show superior electron mobility as high as 25 cm2/(Vs) and on/off ratios of 10\up5 and less than 0.5 V hysteresis in threshold voltage without the addition of indium. Surprisingly, the ripened grains show poorer transport characteristics. The manuscript concludes by noting the importance of the sintering process in achieving conductivity in thin films and discussing future directions to build upon this work.

Submitted by Renan Luis Fragelli null (renan.fragelli@gmail.com) on 2017-03-28T15:54:09Z No. of bitstreams: 1 Dissertação de Mestrado - Renan Fragelli.pdf: 5490064 bytes, checksum: 80c3b38563331cc9ef7e88ff192c5c8b (MD5) / Approved for entry into archive by Luiz Galeffi (luizgaleffi@gmail.com) on 2017-03-29T20:46:29Z (GMT) No. of bitstreams: 1 fragelli_rl_me_bauru.pdf: 5490064 bytes, checksum: 80c3b38563331cc9ef7e88ff192c5c8b (MD5) / Made available in DSpace on 2017-03-29T20:46:29Z (GMT). No. of bitstreams: 1 fragelli_rl_me_bauru.pdf: 5490064 bytes, checksum: 80c3b38563331cc9ef7e88ff192c5c8b (MD5) Previous issue date: 2017-03-16 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho surgiu a partir da necessidade de produzir avanços em projeto que trata do desenvolvimento de um porta-ferramentas refrigerado internamente através de um fluido em mudança de fase e, na tentativa de minimizar a alta temperatura na ferramenta de corte através desse sistema de circulação. A utilização de nanofluidos surgiu como uma alternativa para a otimização da transferência térmica entre fluido e ferramenta de corte. A pesquisa consiste em avaliar a influência da adição de nanopartículas de prata numa solução de etilenoglicol e água deionizada, e também, da adição de nanopartículas de hematita (Fe2O3) no fluido refrigerante R141b. Em ambos os casos, as nanopartículas possuíam formato esférico, diâmetro médio de 30nm e foram avaliadas em concentrações. Além disso, as duas soluções foram submetidas a um campo elétrico na região de transferência térmica para analisar a influência do efeito eletrohidrodinâmico e, por fim, considerando as propriedades magnéticas da hematita, este nanofluido foi testado sob influência de um campo magnético. Os testes mostraram que as nanopartículas realmente influenciaram as propriedades dos fluidos e, por consequência, a quantidade de calor transferido. O nanofluido Ag/ETG+H2O(l) (0,023 vol%) resultou num incremento de 11% no valor do coeficiente de transferência térmica convectivo (h) quando sujeito ao campo elétrico. Para o caso do nanofluido Fe2O3/R141b, o valor de h aumentou em 30,3%, porém, quando sob efeito do campo magnético ou elétrico, o coeficiente foi prejudicado, resultando num valor menor que o do controle. Ao final, tem-se a proposta de um possível modelo desse porta-ferramentas. / This work arose from the need to produce advances in design development of an internally cooled toolholder through a phase change fluid. In order to minimize the high temperature in the cutting tool by this circulation system, using nanofluids emerged as an alternative to optimize heat transfer between the fluid and the cutting tool. The research consists in evaluate the influence of addition of silver nanoparticles in an ethylene glycol and deionized water solution, and also the addition of hematite nanoparticles (Fe2O3) in the refrigerant R141b. In both cases, nanoparticles had spherical shape, diameter of 30nm, and they were evaluated in different concentrations. Moreover, both nanofluids were subjected to an electric field in the heat transfer region to evaluate the influence of electrohydrodynamic effect and, finally, considering the magnetic properties of hematite, this nanofluid was tested under the influence of a magnetic field. The tests have shown that the nanoparticles really influence the properties of the fluids and, therefore, the amount of heat transferred. The nanofluid Ag/ETG+H2O(l) also presented a positive influence of the electric field, further enhancing the value of the convective heat transfer coefficient (h) in 11% (0,039 vol%). In the case of Fe2O3/R141b nanofluid, the h value increased 30.3%. However, when the nanofluid was under magnetic or electric effect, the value of h was deteriorated, resulting in a lesser value than the control. As conclusion, a new toolholder prototype is presented.

Thesis (M. Sc. (Applied Mathematics)) -- University of Limpopo, 2022 / Nanofluids are a class of heat transfer fluids created by suspending nanoparticles in base fluids. Due to their enhanced thermal conductivity, nanofluids are fast replacing conventional heat transfer fluids like water, mineral oil, ethylene glycol and others. They contribute to advancement of technology and modernity through pertinent applications in fields such as biomedical, automotive industry, cooling technologies and many others. This study documents a survey of nanofluids and their applications and an investigation of peristaltic nanofluid flow through a two dimensional microchannel with and without slip effects. Peristaltic fluid transport plays an important role in engineering, technology, science and physiology. The Buongiorno model formulation is employed and the governing equations for peristaltic nanofluid flow in a two dimensional microchannel are non-dimensionalised and solved semi-analytically using the Adomian decomposition method. Series solutions for axial velocity, temperature and nanoparticle concentration profiles are coded into symbolic package MATHEMATICA for easy computation of the numerical solutions. The effects of the various parameters embedded in the model are simulated graphically and discussed quantitatively and qualitatively. The results are compared with those in literature that were obtained using other approximate analytical methods and the homotopy analysis method. The study revealed that the Brownian motion, thermophoresis, buoyance and the slip parameters have significant influence on the peristaltic flow axial velocity, temperature and nanoparticle concetration profiles. In the flow without slip, both the Brownian motion and thermophoresis parameters caused a cooling effect around the channel walls and a marginal temperature enhancement in the channel core region and significant flow reversal was noticed in the channel half-space with maximum axial velocity recording in the channel core region. In the slip flow, both Brownian motion and thermophorisis had a retardation effect on the nanoparticle concentration profile.

The use of nanofluids as heat transfer fluids has received a lot of attention from the heat transfer research community. Due to the increased thermal conductivity of nanofluids over their base fluids, the number of nanofluids scientific publications increased significantly in the past decade. The effects of the heated surface roughness, nanoparticles and surfactant concentrations on pool boiling of nanofluids have been thoroughly investigated. However, contradicting findings have been observed under what appeared to similar test conditions. In this experimental investigation, two boiling surfaces have been prepared with an average surface roughness of 6 and 60 nm using high precision machining. Alumina Oxide-Water based nanofluids have been used in this investigation. The initial nanoparticle size reported by the manufacturer is 10 nm. The nanoparticles concentration has been kept at 0.05 wt. %. A Sodium Dodecylbenzenesulfonate (SDBS) surfactant has been added to the nanofluids in order to improve its stability. Results showed that the nanofluids boiling performance depended on the boiling surface roughness. The heat transfer coefficient (HTC) obtained in the case of the smooth, mirror finished surface showed an enhancement of 205% with respect to pure water. This trend was reversed in the case of the rough surface which is believed to be due to significant nanoparticles deposition. The HTC obtained with the rough surface was 12% lower than that of pure water. The effect of the surfactant concentration on nanoparticles deposition has been investigated by changing the surfactant concentration from 0.1 to 1.0 wt. %. In the case of the rough surface, the increase of surfactant concentration was found to reduce the formation of the nanoparticles deposition layer. The HTC obtained with the higher surfactant concentration was increased by 46 %. The effect of nanoparticles concentration on the smooth surface shows an unexpected trend of 20 % reduction of the transfer rate of the nanofluids coupled with the increase of the nanoparticle concentration from 0.05 to 0.1 wt. %. However all concentrations showed heat transfer enhancement with respect to pure water. The minimum heat transfer coefficient ratio enhancement was 11 % using 0.1 wt. % nanofluids with respect to pure water. Since nanoparticles deposition has been observed and attributed to micro-layer evaporation, an investigation has been carried out to examine the nucleation process during the pure water and nanofluids pool boiling. The bubble growth rate in both cases was analyzed at different wall degrees of superheat ranging from 104.3 to 105.9 ºC. In addition, the bubble departure diameter and frequency have been measured and compared for both cases. The nanofluid bubble size was about 80 % smaller than that of pure water. The nanofluid bubble departure had almost constant frequency of 500 Hz over the range of wall superheats whereas the maximum bubble frequency in the case of pure water was 22.72 Hz. / Thesis / Master of Applied Science (MASc)

No description available.

<p>An experimental investigation has been carried out in order to investigate the effect of surface initial conditions, concentration, nanoparticles size and deposition pattern on pool boiling and jet impingement boiling of nanofluids. A flat copper surface with initial conditions of Ra = 420 nm, Ra = 80 nm and Ra = 20 nm has been used as the boiling surface. Al₂O₃ and CuO nanoparticles have been used with de-ionized water to prepare the nanofluids. At 0.01 vol. % concentration of Al₂O_3, the rate of heat transfer enhanced by 41% and 34% for the Ra = 80 nm and Ra = 20 nm, respectively. While, in the case of Ra = 420 nm, the rate of heat transfer deteriorated by 49%. At 0.005 vol. % concentration the rate of heat transfer deteriorated for all three surfaces. It is believed that the deterioration was due to the uniformity of the deposition. Using 0.01 vol. % concentration of CuO nanofluids resulted in the same trend, however, the rate of heat transfer is less compared to using Al₂O₃nanofluids. For example, in the case of Ra = 80 nm, the rate of heat transfer was reduced by 14%.</p> <p>The effect of nanoparticles size has been investigated by changing the nanoparticles size from 50 nm to 10 nm. The change in nanoparticles size resulted in a significant deterioration in the rate of heat transfer for all three surfaces. It is believed that the deterioration was due to the deposition uniformity. As the deposition uniformity has been found to be a major factor that affects the rate of heat transfer, new approach was introduced to quantify the effect of the rate of deposition on the pool boiling of nanofluids.</p> <p>An experimental investigation has been carried out in order to investigate using submerged impingement jet on the rate of heat transfer using nanofluids. At of 0.005 vol. % concentration of Al₂O₃, surface with Ra = 80 nm, jet to surface vertical distance of 3 mm and Reynolds number of 101311, the rate of heat transfer deteriorated by 19%.</p> <p>Comparing the pool boiling and jet impingement boiling of nanofluids showed that, in the case of jet impingement boiling, the rate of heat transfer was enhanced compared to the case of pool boiling and the deposition was less. However, jet impingement boiling experiments showed deterioration in the rate of heat transfer by 19% compared with pure water.</p> / Master of Applied Science (MASc)

Complex or smart fluids, made of evenly and stably suspended nanoparticles, have been an object of considerable research in the last decade due to their promising applications in a number of applications such as micro-electronic cooling, high power demands in nuclear plants, smaller and more efficient heat exchangers, oil recovery, and transportation. Nanofluids consist of typically less than 100nm sized particles dispersed in base fluids. The heat transfer characteristics of several nano-oxide suspensions in pool boiling with a suspended heating NiChrome wire have been analyzed. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions towards the suspended heated wire. Heat transfer in silica nanofluids at different acidity and base is measured for various ionic concentrations in a pool boiling experiment. Nanosilica suspension increases the critical heat flux by up to 300% compared to conventional fluids. The l0 nm particles possess a thicker double diffuse layer compared to 20 M particles. The catalytic properties of nanofluids decrease in the presence of salts, allowing the particles to cluster and minimize the potential increase in heat transfer. Nanofluids in a strong electrolyte, i.e., in high ionic concentration, allow a higher critical heat flux than in buffer solutions because of the difference in surface area. The formation and surface structure of the deposition affect the thermal properties of the liquid. When there is no particle deposition on the wire, the nanofluid increases CHF by about 50% within the uncertainty limits, regardless of pH of the base fluid or particle size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through inter-agglomerate pores, resulting in a nearly 3-fold increase in burnout heat flux. The pool boiling heat transfer has been even higher (- up to 4 times that of the base fluid) for Double Walled Carbon Nanotubes (DWNTs). A comparison study between Single and Double Walled Carbon Nanotube suspensions has been performed. A closed flow loop has been designed and fabricated to study the thermal transport characteristics of nanosilica suspensions in heated flow. The heat transfer coefficient and pressure drop data are provided for laminar and turbulent regimes (2000

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.

Comprehensive molecular design was used to identify new heat transfer fluids for direct immersion phase change cooling of electronic systems. Four group contribution methods for thermophysical properties relevant to heat transfer were critically evaluated and new group contributions were regressed for organosilicon compounds. 52 new heat transfer fluids were identified via computer-aided molecular design and figure of merit analysis. Among these 52 fluids, 9 fluids were selected for experimental evaluation and their thermophysical properties were experimentally measured to validate the group contribution estimates. Two of the 9 fluids (C6H11F3 and C5H6F6O) were synthesized in this work. Pool boiling experiments showed that the new fluids identified in this work have superior heat transfer properties than existing coolant HFE 7200. The radiative forcing and global warming potential of new fluids, calculated via a new group contribution method developed in this work and FT-IR analysis, were found to be significantly lower than those of current coolants. The approach of increasing the thermal conductivity of heat transfer fluids by dispersing nanoparticles was also investigated. A model for the thermal conductivity of nanoparticle dispersions (nanofluids) was developed that incorporates the effect of size on the intrinsic thermal conductivity of nanoparticles. The model was successfully applied to a variety of nanoparticle-fluid systems. Rheological properties of nanofluids were also investigated and it was concluded that the addition of nanoparticles to heat transfer fluids may not be beneficial for electronics cooling due to significantly larger increase in viscosity relative to increase in thermal conductivity.