Este trabalho trata do estudo teórico do ebulição convectiva de nanofluidos em canais de diâmetro reduzido (denominados de microcanais). Ele aborda, primeiramente, uma análise da literatura sobre a ebulição convectiva de fluidos convencionais em microcanais, na qual são discutidos critérios para a transição entre macro e microcanais e os padrões de escoamentos observados em canais de reduzido diâmetro. Métodos para a previsão das propriedades de transporte de nanofluidos foram levantados da literatura e estudos experimentais da convecção forçada, da ebulição nucleada e da ebulição convectiva de nanofluidos foram discutidos. Um método para a previsão do coeficiente de transferência de calor de nanofluidos em microcanais durante a ebulição convectiva foi proposto baseado em modelos convencionais da literatura ajustados para nanofluidos. O ajuste dos modelos convencionais foi realizado através de análise regressiva de dados experimentais para ebulição nucleada e convecção forçada de nanofluidos levantados da literatura, e da análise crítica de adimensionais que capturassem a influência das nanopartículas no processo de transferência de calor. De maneira geral o método proposto neste estudo apresenta concordância razoável com dados experimentais independentes, referente ao acréscimo do coeficiente de transferência de calor com o incremento da concentração volumétrica de nanopartículas. No entanto, a escassez de estudos experimentais sobre a ebulição convectiva de nanofluidos, especialmente em microcanais, impossibilitou uma análise mais aprofundada do método proposto. / The present work aims the theoretical study of convective boiling of nanofluids in small diameter channels (called microchannel). It discusses an analysis of the literature on convective boiling of conventional fluids in microchannels which presents criteria for the transition between conventional and microchannels and the flow patterns observed in small diameter channels. Methods for predicting the transport properties of nanofluids were compiled from the literature and experimental studies of forced convection, nucleate boiling and convective boiling of nanofluids were discussed. A method for predicting the heat transfer coefficient of nanofluids in microchannels during convective boiling was proposed based on conventional models from literature adjusted to nanofluids. The conventional models fitting was performed by regression analysis of experimental data for nucleate boiling and forced convection of nanofluids compiled from the literature and by critical analysis of dimensionless numbers which enable to capture the influence of nanoparticles on heat transfer process. In general the proposed method in this work presents reasonable agreement with independent experimental data regarding the increase in heat transfer coefficient with increasing nanoparticles volume fraction. However the scarcity of experimental studies on the convective boiling of nanofluids, especially in microchannels, precluded further analysis of the proposed method.

Heat transfer fluids are materials responsible for heat distribution, transfer and storage. Their significance is undeniable - many technological processes cannot be carried out without using heat transfer materials (for example due to overheating). These are usually mixtures of many compounds, for example glycols, silicones or water. Today's technologies constantly require more efficient, environmentally- and economically-friendly solutions for heat transfer applications. It is necessary to know the full physicochemical characteristics to design a new heat transfer fluid (mainly density, heat capacity, viscosity and thermal conductivity). Nanofluids (mixture of a basefluid and nanoparticles) were proposed as a solution for many industrial issues due to their enhanced thermophysical properties (i.e. thermal conductivity) than pure liquids. Moreover, these enhancements exhibit unusual features which make this group of materials interesting from molecular and industrial point of view. Ionic liquids, task specific materials with tuneable properties were repeatedly recommended as heat transfer fluids due to their specific properties (mainly low vapour pressure, wide liquidus range, or non-flammability) caused by the ionic structure. A very interesting material can be obtained by mixing ionic liquids and nanoparticles where specific properties of ionic liquids are preserved, and thermophysical properties are enhanced due to nanoparticles dispersion. In this work, we investigated ionic liquid - based nanofluids from the experimental and theoretical point of view, including imidazolium-, pyrrolidinium- and phosphonium-based ionic liquids with several different anions, and multiwalled carbon nanotubes, graphite, boron nitride and mesoporous carbon as nanoparticles, and also in mixtures with water. As a final result, we assessed the molecular recognition of the thermophysical properties enhancements in ionanofluids, developed the predictive models for physical properties, compared all investigated systems to commercial heat transfer fluids. The project was supported by King Faisal University (Saudi Arabia) through a research fund from the International Cooperation and Knowledge Exchange Administration department at KFU. Cytec are thanked for the generous donation of the trihexyl(tetradecyl)phosphonium chloride sample.

The focus of this research was to create and characterize high temperature alumina and nitrate salt eutectic nanofluids for use in thermal energy storage (TES) systems. The nitrate eutectic was originally used in the TES system demonstrated as part of the Solar Two power tower and is currently employed as the TES material at Andasol 1 in Spain. Concentrations of alumina nanoparticles between 0.1% and 10% by weight were introduced into the base material in an effort to create nanofluids which would exhibit improved specific heat capacity to reduce the $/kWht thermal energy storage system costs. The composite materials were created using an aqueous mixing method in which both the nanoparticles and nitrate eutectic were placed into solution using acidic water. This solution was then sonicated in an ultrasonic bath in an effort to reduce nanoparticle agglomeration and to improve homogeneity. After boiling off the excess water, the nanoparticle-nitrate eutectic composite was recovered for characterization. The thermal properties of both the composite and base materials were characterized using the differential scanning calorimetry techniques outlined in ASTM E 1269. The created nanofluids were not stable and did not offer a cost-effective alternative to the current nitrate eutectic TES material. Despite these setbacks, a positive correlation between alumina concentration and nanofluid specific heat was demonstrated. Additionally, the specific heat capacities of the created nanofluids exceeded that predicted by the current theoretical models. These findings suggest that further work in the field of high temperature nanofluids for use in TES systems is warranted.

The present work contains two parts: nanofluids and bioheat transport, both involving multiscales and sharing some common features. The former centers on addressing the three key issues of nanofluids research: (i) what is the macroscale manifestation of microscale physics, (ii) how to optimize microscale physics for the optimal system performance, and (iii) how to effectively manipulate at microscale. The latter develops an analytical theory of bioheat transport that includes: (i) identification and contrast of the two approaches for developing macroscale bioheat models: the mixture-theory (scaling-down) and porous-media (scaling-up) approaches, (ii) rigorous development of first-principle bioheat model with the porous-media approach, (iii) solution-structure theorems of dual-phase-lagging (DPL) bioheat equations, (iv) practical case studies of bioheat transport in skin tissues and during magnetic hyperthermia, and (v) rich effects of interfacial convective heat transfer, blood velocity, blood perfusion and metabolic reaction on blood and tissue macroscale temperature fields. Nanofluids, fluid suspensions of nanostructures, find applications in various fields due to their unique thermal, electronic, magnetic, wetting and optical properties that can be obtained via engineering nanostructures. The present numerical simulation of structure-property correlation for fourteen types of two/three-dimensional nanofluids signifies the importance of nanostructure’s morphology in determining nanofluids’ thermal conductivity. The success of developing high-conductive nanofluids thus depends very much on our understanding and manipulation of the morphology. Nanofluids with conductivity of upper Hashin-Shtrikman bounds can be obtained by manipulating structures into an interconnected configuration that disperses the base fluid and thus significantly enhancing the particle-fluid interfacial energy transport. The numerical simulation also identifies the particle’s radius of gyration and non-dimensional particle-fluid interfacial area as two characteristic parameters for the effect of particles’ geometrical structures on the effective thermal conductivity. Predictive models are developed as well for the thermal conductivity of typical nanofluids. A constructal approach is developed to find the constructal microscopic physics of nanofluids for the optimal system performance. The approach is applied to design nanofluids with any branching level of tree-shaped microstructures for cooling a circular disc with uniform heat generation and central heat sink. The constructal configuration and system thermal resistance have some elegant universal features for both cases of specified aspect ratio of the periphery sectors and given the total number of slabs in the periphery sectors. The numerical simulation on the bubble formation in T-junction microchannels shows: (i) the mixing enhancement inside liquid slugs between microfluidic bubbles, (ii) the preference of T-junctions with small channel width ratio for either producing smaller microfluidic bubbles at a faster speed or enhancing mixing within the liquid phase, and (iii) the existence of a critical value of nondimensional gas pressure for bubble generation. Such a precise understanding of two-phase flow in microchannels is necessary and useful for delivering the promise of microfluidic technology in producing high-quality and microstructure-controllable nanofluids. Both blood and tissue macroscale temperatures satisfy the DPL bioheat equation with an elegant solution structure. Effectiveness and features of the developed solution structure theorems are demonstrated via examining bioheat transport in skin tissues and during magnetic hyperthermia. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Heat exchange systems used in everything from cars to microelectronics have rapidly advanced in recent years to offer high heat transfer rates in increasingly smaller sizes. However, these systems have become essentially optimized using conventional heat transfer fluids. To test the viability of nanofluids as a new heat transfer fluid, an experimental investigation was designed using a constant pressure drop configuration to drive flow into a heated square microchannel test section. The experimental trials included seven different test fluids tested over varying concentrations and surfactant use. Two identical test sections were used to collect results on heat transfer rates, pressure drop, mass flowrate and pumping power for all fluids. These results show a heat transfer improvement for nanofluids of 8-16% over pure water, with no meaningful increase in pumping power. This result is highly desirable, as it indicates an easily obtainable heat transfer improvement without an associated pumping cost increase. Importantly, the experiment shows the potential viability of nanofluids for heat transfer applications, while acknowledging limitations such as long term nanofluid stability. / text

Thesis (M.S.) University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 29, 2007) Includes bibliographical references.

Philosophiae Doctor - PhD / 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. / South Africa

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2011. / Nanofluid, which was first discovered by the Argonne laboratory, is a nanotechnology- based heat transfer fluid. This fluid consists of particles which are suspended inside conventional heat transfer liquid or base fluid. The purpose of this suspension is for enhancing thermal conductivity and convective heat transfer performance of this base fluid. The name nanofluid came about as a result of the nanometer- sized particles of typical length scales 1-100nm which are stably suspended inside of the base fluids. These nanoparticles are of both physical and chemical classes and are also produced by either the physical process or the chemical process. Nanofluid has been discovered to be the best option towards accomplishing the enhancement of heat transfer through fluids in different unlimited conditions as well as reduction in the thermal resistance by heat transfer liquids. Various manufacturing industries and engineering processes such as transportation, electronics, food, medical, textile, oil and gas, chemical, drinks e.t.c, now aim at the use of this heat transfer enhancement fluid. Advantages such organisations can obtain from this fluid includes, reduced capital cost, reduction in size of heat transfer system and improvement of energy efficiencies. This research has been able to solve numerically, using Maple 12 which uses a fourth- fifth order Runge -kutta- Fehlberg algorithm alongside shooting method, a set of nonlinear coupled differential equations together with their boundary conditions, thereby modelling the heat and mass transfer characteristics of the boundary layer flow of the nanofluids. Important properties of these nanofluids which were considered are viscosity, thermal conductivity, density, specific heat and heat transfer coefficients and microstructures (particle shape, volume concentration, particle size, distribution of particle, component properties and matrixparticle interface). Basic fluid dynamics equations such as the continuity equation, linear momentum equation, energy equation and chemical species concentration equations have also been employed.

Este trabalho trata do estudo teórico do ebulição convectiva de nanofluidos em canais de diâmetro reduzido (denominados de microcanais). Ele aborda, primeiramente, uma análise da literatura sobre a ebulição convectiva de fluidos convencionais em microcanais, na qual são discutidos critérios para a transição entre macro e microcanais e os padrões de escoamentos observados em canais de reduzido diâmetro. Métodos para a previsão das propriedades de transporte de nanofluidos foram levantados da literatura e estudos experimentais da convecção forçada, da ebulição nucleada e da ebulição convectiva de nanofluidos foram discutidos. Um método para a previsão do coeficiente de transferência de calor de nanofluidos em microcanais durante a ebulição convectiva foi proposto baseado em modelos convencionais da literatura ajustados para nanofluidos. O ajuste dos modelos convencionais foi realizado através de análise regressiva de dados experimentais para ebulição nucleada e convecção forçada de nanofluidos levantados da literatura, e da análise crítica de adimensionais que capturassem a influência das nanopartículas no processo de transferência de calor. De maneira geral o método proposto neste estudo apresenta concordância razoável com dados experimentais independentes, referente ao acréscimo do coeficiente de transferência de calor com o incremento da concentração volumétrica de nanopartículas. No entanto, a escassez de estudos experimentais sobre a ebulição convectiva de nanofluidos, especialmente em microcanais, impossibilitou uma análise mais aprofundada do método proposto. / The present work aims the theoretical study of convective boiling of nanofluids in small diameter channels (called microchannel). It discusses an analysis of the literature on convective boiling of conventional fluids in microchannels which presents criteria for the transition between conventional and microchannels and the flow patterns observed in small diameter channels. Methods for predicting the transport properties of nanofluids were compiled from the literature and experimental studies of forced convection, nucleate boiling and convective boiling of nanofluids were discussed. A method for predicting the heat transfer coefficient of nanofluids in microchannels during convective boiling was proposed based on conventional models from literature adjusted to nanofluids. The conventional models fitting was performed by regression analysis of experimental data for nucleate boiling and forced convection of nanofluids compiled from the literature and by critical analysis of dimensionless numbers which enable to capture the influence of nanoparticles on heat transfer process. In general the proposed method in this work presents reasonable agreement with independent experimental data regarding the increase in heat transfer coefficient with increasing nanoparticles volume fraction. However the scarcity of experimental studies on the convective boiling of nanofluids, especially in microchannels, precluded further analysis of the proposed method.

Nanofluidics is an emerging field with many science and engineering applications. The physics of material transport through nanochannels are of interest in filtration, sensing, device miniaturization, and biomimetics. To address such ambitions with nanofluidic tools will require advancements in our understanding and control over nanofluidic systems. This work contributes to electrokinetic phenomena, characterization techniques, and applications in nanofluidics. Ion transport data through nanopipettes are used to validate a finite element model for nonlinear electrokinetic flows. With the model, we conclude that asymmetric surfaces induce fluid vortices and provide insight into supporting mathematical techniques. We then establish nanobubble-plugged nanopipettes as promising ionic devices due to the electrokinetic effects of three-phase interfaces. Using cryogenic transmission electron microscopy, ion current measurements, and extensive physical modeling, we conclude that nanobubble plugs are metastable, slow-growing, and induce strong current rectification and enhancement. All these insights let us study microbial surfaces using electrokinetic phenomena detected by a scanned nanopipette. Over immobilized Pseudomonas aerugonsa cells and Δphz-type biofilms, we detect topographic and surface charge properties due to voltage-dependent signals through a scanned nanopipette probe. Our efforts establish a fast hopping probe scanning ion conductance microscopy technique for long-range surface charge detection. Finally, we use an integrated carbon nanotube channel to demonstrate how solid-state charge can drive electrokinetic flows through Coulomb drag coupling.