Au cours des dernières années, les exigences croissantes en matière de dissipation thermique à haut rendement pour la microélectronique, les engins spatiaux, les réacteurs nucléaires, etc., encouragent le développement d'échangeurs de chaleur de nouvelle génération. Le caloduc est l’un des équipements de refroidissement efficaces et potentiels. La plupart du transfert de masse et de chaleur se fait au niveau de la micro-région près de la ligne triple de contact (solide, liquide, vapeur), qui est essentielle à l'amélioration de la performance thermique du caloduc. Cette étude se concentre sur le processus d'évaporation de gouttes sessiles de deux nouveaux fluides de travail (solution binaire et nanofluide), qui possèdent une micro-région similaire à celle du caloduc. Le flux de Marangoni induit par le gradient de concentration et la conductivité thermique exceptionnelle devraient améliorer significativement le débit evaporé du mélange alcool-eau et du nanofluide de graphène, respectivement. Une combinaison de techniques acoustiques et infrarouges est développée pour suivre la variation de la concentration d'alcool pendant l'évaporation des gouttes des mélanges 1-butanol-eau et éthanol-eau. Selon l'observation du comportement d'évaporation à différentes températures du substrat, une série d'équations empiriques est suggérée pour prédire le taux d'évaporation de la solution binaire de 1-butanol-eau en considérant l'effet Marangoni thermal et solutal. De plus, l'effet de la PEGylation, de la concentration des nanoparticules et de la température du substrat sur l'évaporation de gouttes de graphène nanofluide est étudié par des méthodes microscopiques, optiques et infrarouges. Les résultats expérimentaux et l'analyse thermodynamique peuvent contribuer à la compréhension complète du mécanisme impliqué concernant les performances d'évaporation du nanofluide de graphène. / In recent years, increasing requirement in high efficient heat dissipation for micro-electronics, spacecraft, nuclear reactors etc., encourage the development of next generation heat exchanger. Heat pipe is one of potential effective cooling equipments and most of mass and heat transfer take place at micro-region near triple phase (solid, liquid, vapor) contact line of working fluid, which is essential to thermal performance improvement of heat pipe. This study focuses on the evaporation process of sessile droplets of two novel working fluids (binary solution and nanofluid), which possess similar micro-region to that in heat pipe. Concentration gradient induced Marangoni flow and exceptional thermal conductivity are expected to significantly enhance evaporation rate of alcohol-water mixture and graphene nanofluid, respectively. A combination of acoustic and infrared techniques is developed to track alcohol concentration variation during evaporation of 1-butanol and ethanol aqueous droplets. According to observation of evaporation behavior at different substrate temperature, a series of empirical equations is suggested to predict evaporation rate of 1-butanol-water binary solution droplet considering thermal and solutal Marangoni effect. In addition, the effect of PEGylation, nanoparticle concentration and substrate temperature on drop evaporation of graphene nanofluid are investigated by microscopic, optical and infrared methods. Experimental results and thermodynamic analysis can contribute to the full understanding of involved mechanism concerning evaporation performance of graphene nanofluid.

Direct absorption solar collectors offer possible improvement in efficiency over traditional surface absorbing collectors, because they have fewer heat transfer steps and has the ability to utilise higher radiation fluxes. Carbon black based nanofluids, in a base fluid of salt water, were synthesised by a two-step method where the carbon black nanoparticles were treated with a surfactant, TWEEN-20, in a 1:2 mass ratio and sonicated for 60 minutes to break up agglomerates. The synthesised nanofluids showed stability for over 31 days. The different carbon black concentration nanofluids' solar irradiation absorption properties were compared with each other and with the base fluid of salt water in a concentrating, as well as non-concentration scenario. It was found that the carbon black nanofluids showed excellent absorption properties over the entire solar radiation spectrum. A 1 m2 concentrating unit using a two-axis tracking system, with two mirrors and a 1 m diameter circular Fresnel lens, was used to concentrate solar radiation on a direct absorption solar collector flow cell with a 10 cm2 collection area. An optimum concentration of 0.001 volume % carbon black was found to show a 42 % increase in heating rate, compared to that of salt water. The collector was, however, hampered by high energy losses and the maximum collector efficiency achieved was only 46 %, 23 % higher than that of salt water. The overall system efficiency was only 22 %. This low efficiency can be attributed to the high optical concentration losses (50 % - 70 %) present in the concentrating unit. / Dissertation (MEng)--University of Pretoria, 2016. / Chemical Engineering / MEng / Unrestricted

The superior properties of carbon nanotubes (CNTs) are best manifest in bulk materials when the CNTs are organized in tandem and embedded in a continuous matrix. Decorating the CNTs with magnetic nanoparticles (MNPs) facilitates their expedient organization with a magnetic field. One of the most convenient methods for their decoration is to first treat the CNTs with oxidative acids, and then coprecipitated MNPs in situ. This method results magnetized CNTs that are covalently functionalized with the MNPs. The associated destruction in the CNTs required running a comparative study of this protocol to identify the influence of the acid treatment on the decoration of multiwalled CNTs (MWNTs). Further, we explore means to tune the physical properties of these magnetized CNTs (mMWNTs) by varying the (1) MNP material composition, and (2) MNP:MWNT (w/w) magnetization weight ratio (γ). The resulted composite materials (mMWNTs) are utilized to synthesize a novel and hitherto unreported class of colloidal suspensions (MCCs) for which the dispersed phase, which consists of MWNTs decorated with MNPs, is both magnetoresponsive and electrically conductive. Synthesis of the dispersed phase merges processes for producing ferrofluids and mMWNTs. Later, these MCCs are adapted and engineered to produce a biological ink containing MWNTs that are twice functionalized, first with MNPs and thereafter with the anti-c-Myc monoclonal antibodies (Abs). The ink is pipetted and dynamically self-organized by an external magnetic field into a dense electrically conducting sensor strip that measures the decrease in current when a sample containing c-Myc antigens (Ags) is deposited on it. On the other side, a nondestructive methods to magnetize MWNTs and provide a means to remotely manipulate them is through the electroless deposition of magnetic nickel nanoparticles on their surfaces. The noncovalent bonds between Ni nanoparticles and MWNTs produce a Ni-MWNT hybrid material (NiCH) that is electrically conductive and has an enhanced magnetic susceptibility and elastic modulus. Raising γ (Ni:MWNT weight ratios) increases the coating layer thickness, which influences the NiCH magnetic properties and tunes its elastic modulus. The NiCH was used to fabricate Ni-MWNT macrostructures and tune their morphologies by changing the direction of an applied magnetic field. Leveraging the hydrophilic Ni-MWNT outer surface, a water-based conductive ink was created and used to print a conductive path that had an electrical resistivity of 5.9 Ωm, illustrating the potential of this material for printing electronic circuits. Further, the NiCHs are introduced into an epoxy matrix at low 0.25-1% volume fractions and aligned along the direction of an applied magnetic field, which produces anisotropic bulk properties. However, nanoparticles aligned in perpendicular directions in sequential layers result in an effectively isotropic composite material. Furthermore, the subsequent annealing of the NiCH in the presence of air oxidizes nickel to nickel oxide whereas carbon is released as gaseous carbon dioxide, which leads to a novel approach for the fabrication of nickel oxide nanotubes (NiONTs) based on MWNTs as a sacrificial template. New chelating polyelectrolytes are used as dispersing agents to achieve high colloidal stability both for NiCH and NiONTs. A gravimetric specific capacitance of 245.3 F g-1 and areal capacitance of 3.28 F cm-2 at a scan rate of 2 mV s-1 is achieved with an electrode fabricated using nickel oxide nanotubes as the active element with a mass loading of 24.1 mg/cm2. / Thesis / Doctor of Philosophy (PhD) / The superior properties of carbon nanotubes (CNTs) are best manifested in bulk materials when the CNTs are organized axially and in tandem, and embedded in a continuous matrix. Decorating the CNTs with magnetic nanoparticles (MNPs) facilitates their organization through “action from a distance” with a magnetic field. The attachment of MNPs to the surfaces of CNTs can be realized through covalent or non-covalent (i.e. physical) bonding. This work develops both methodologies to investigate how the physical properties of magnetized CNT (mCNT) can be tuned and produce new CNT-based nanostructures for particular applications. First, mCNTs are utilized to synthesize a hitherto unreported class of colloidal suspensions based on which a magnetic bio-ink is fabricated to print a fast-response biological sensor. Next, nickel-coated CNTs prepared using electroless deposition are used in the form of a filler at low volume fractions in an epoxy matrix, where they are aligned along multiple-direction using a magnetic field, producing either anisotropic or isotropic bulk properties on demand. Finally, subsequent annealing of nickel-coated CNTs in air oxidizes nickel to nickel oxide while carbon is released in the form of gaseous carbon dioxide. This leads to another novel approach for the fabrication of nickel oxide nanotubes, which are demonstrated to be an alternate viable material to fabricate electrodes for use in supercapacitors.

Nanofluids as an innovative class of heat transfer fluids created by dispersing nanometre-sizedmetallic or non-metallic particles in conventional heat transfer fluids displayed the potential toimprove the thermophysical properties of the heat transfer fluids. The main purpose of this study is toinvestigate the influence of the use of nanofluids on two-phase heat transfer, particularly on thethermal performance of the heat pipes. In the first stage, the properties of the nanofluids were studied,then, these nanofluids were used as the working fluids of the heat pipes. The thermal performance ofthe heat pipes when using different nanofluids was investigated under different operating conditionsexperimentally and analytically. The influences of the concentration of the nanofluids, inclinationangles and heat loads on the thermal performance and maximum heat flux of the heat pipes wereinvestigated.This study shows that the thermal performance of the heat pipes depends not only on thermophysicalproperties of the nanofluids but also on the characteristics of the wick structure through forming aporous coated layer on the heated surface. Forming the porous layer on the surface of the wick at theevaporator section increases the wettability and capillarity and also the heat transfer area at theevaporator of the heat pipes.The thermal performance of the heat pipes increases with increasing particle concentration in all cases,except for the heat pipe using 10 wt.% water/Al2O3 nanofluid. For the inclined heat pipe, irrespectiveof the type of the fluid used as the working fluid, the thermal resistance of the inclined heat pipes waslower than that of the heat pipes in a horizontal state, and the best performance was observed at theinclination angle of 60o, which is in agreement with the results reported in the literature. Otheradvantages of the use of nanofluids as the working fluids of the heat pipes which were investigated inthis study were the increase of the maximum heat flux and also the reduction of the entropy generationof the heat pipes when using a nanofluid.These findings revealed the potential for nanofluids to be used instead of conventional fluids as theworking fluid of the heat pipes, but the commercialization of the heat pipes using nanofluids for largescale industrial applications is still a challenging question, as there are many parameters related to thenanofluids which are not well understood. / <p>QC 20170228</p>

Cette thèse présente les résultats de deux études : la première concerne la convection naturelle turbulente dans une cavité rectangulaire chauffée uniformément par le bas et remplie d’un nanofluide et la seconde concerne l’investigation du transfert de chaleur conjugué dans un échangeur de chaleur à tubes ailetés.L’enceinte de la première étude a un faible rapport d’aspect. Ses parois gauche, droite et supérieure sont maintenues à une température relativement basse. Le fluide de travail est un nanofluide constitué d’eau et de nanoparticules, soient d’alumine (Al2O3), ou de cuivre (Cu) ou d’oxyde de cuivre (CuO). L’influence des paramètres tels que le nombre de Rayleigh (basé sur la hauteur H de la cavité et la densité de flux de chaleur), le type de nanofluide et la fraction volumique des nanoparticules sur la performance de refroidissement est présentée. Les équations de Navier-Stokes et les équations de conservation de la masse et de l'énergie sont résolues pour une géométrie bidimensionnelle par la méthode numérique des volumes finis. L'algorithme SIMPLE est utilisé pour le couplage pression-vitesse. La discrétisation des termes convectifs est faite avec le schéma QUICK. Le modèle de turbulence k-epsilon standard est utilisé. Le maillage du domaine simulé est généré par le code Gambit. Les résultats montrent que pour toutes les valeurs de Ra, le nombre de Nusselt moyen augmente d’une façon linéaire et monotone avec l’augmentation de la concentration des nanoparticules dans le fluide de base. Le flux de chaleur moyen prend des valeurs qui diminuent en fonction de l’ordre suivant : Cu, CuO et Al2O3.La deuxième étude est une investigation numérique des caractéristiques dynamique et thermique d'un échangeur de chaleur. Les calculs supposent un transfert de chaleur et un écoulement en régime permanent. Le nombre de Nusselt et le coefficient de frottement qui caractérisent l'échangeur de chaleur sont déterminés pour différentes valeurs du nombre de Reynolds. L’équation de conservation de l'énergie dans le fluide et l’équation de conduction de la chaleur dans le solide en trois dimensions ont été résolues avec les équations de la conservation de la masse et de la quantité de mouvement afin de déterminer ces caractéristiques. Les deux régimes d'écoulement laminaire et turbulent sont considérés. L'effet de la modélisation de la turbulence a été étudié en utilisant trois modèles différents (Spalart-Allmaras modèle de turbulence à une équation, le modèle k-epsilon ; standard et le modèle RSM). La validation du modèle a été effectuée en comparant les facteurs de frottement, f, et le facteur, j, de Colburn avec les données expérimentales trouvées dans la littérature. Les résultats tracés ont montré un bon accord qualitatif entre les résultats numériques et les données expérimentales. Les résultats montrent également que le plus simple des trois modèles de turbulence testés (à savoir. Spalart-Allmaras) donne les valeurs les plus proches des données expérimentales. / This thesis presents the results of two studies: the first concerns natural turbulent convection in a rectangular cavity heated from the bottom wall and filled with a nanofluid and the second relates to the investigation of conjugate heat transfer in a fin-and-tube heat exchanger.The cavity of the first study is tall and has a heat source embedded on its bottom wall, while its left, right and top walls are maintained at a relatively low temperature. The working fluid is a water based nanofluid having three nanoparticle types: alumina, copper and copper oxide. The influence of pertinent parameters such as the Rayleigh number, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. Steady forms of twodimensional Reynolds-Averaged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved by the volume control based on the discretisation method employing the SIMPLE algorithm for pressure-velocity coupling. Turbulence is modeled by using the standard k-epsilon model. The Rayleigh number, Ra, is varied from 2.49xE09 to 2.49xE11. The volume fractions of nanoparticles where varied too. Stream lines, isotherms, velocity profiles and temperature profiles are presented for various combinations of Ra, the type of nanofluid and solid volume fraction of nanoparticles. The results are reported in the form of average Nusselt number on the heated wall. It is shown that for all values of Ra, the average heat transfer rate from the heat source increases almost linearly and monotonically as the solid volume fraction increases. Finally the average heat transfer rate takes on values that decrease according to the ordering Cu, CuO and Al2O3.In the second study We determined the heat transfer and friction characteristics of a realistic fin-and-tube heat exchanger. The computations assume steady-state heat transfer and fluid flow. Nusselt number and friction factor characteristics of the heat exchanger are presented for various values of Reynolds numbers. The energy conservation and the heat conduction equations in 3 dimensions have been solved in the fluid and the solid respectivelyalong with the mass and momentum conservation equations in order to determine these characteristics. Both laminar and turbulent flow regimes are considered. The effect of turbulence modeling was investigated using three different models (the one equation Spalart-Allmaras turbulence model, the standard k-epsilon; model and the RSM model). The computations allowed the determination of the dynamic and thermal fields. Model validation was carried out by comparing the calculated friction factor f and Colburn j-factor to experimental results found in the literature. The plotted results showed a qualitatively good agreement between numerical results and experimental data. The results obtained also showed that the simplest of the three turbulence models tested(i.e. Spalart-Allmaras) gives the closest values to the experimental data.

A nanofluid is a new heat transfer fluid produced by mixing a base fluid and solid nano sized particles. This fluid has great potential in heat transfer applications, because of its increased thermal conductivity and even increased Nusselt number due to higher thermal conductivity, Brownian motion of nanoparticles, and other various effects on heat transfer phenomenon. In this work, the first aim is to predict convective heat transfer of nanofluids. A numerical code is created and run to obtain results in a pipe with two different boundary conditions, constant wall temperature and constant wall heat flux. The results for laminar flow for thermally developing region in a pipe are obtained for Al2O3/water nanofluid with different volumetric fraction and particle sizes with local temperature dependent conductivity approach. Various effects that influence nanofluid heat transfer enhancement are investigated. As a result, a better heat transfer performance is obtained for all cases, compared to pure water. The important parameters that have impact on nanofluid heat transfer are particle diameter of the nanoparticles, nanoparticle volumetric fraction, Peclet number, and viscous dissipation. Next, a heat transfer performance evaluation methodology is proposed considering increased pumping power of nanofluids. Two different criteria are selected for two boundary conditions at constant pumping power. These are heat transfer rate ratio of the nanofluid and the base fluid for constant wall temperature boundary condition and difference between wall temperature of the pipe at the exit and inlet mean temperature of the fluid ratio for constant wall heat flux case. Three important parameters that influence the heat transfer performance of nanofluids are extracted from a parametric study. Lastly, optimum particle size and volumetric fraction values are obtained depending on Graetz number, Nusselt number, heat transfer fluid temperature, and nanofluid type.

A nanofluid is a new heat transfer fluid produced by mixing a base fluid and solid nano sized particles. This fluid has great potential in heat transfer applications, because of its increased thermal conductivity and even increased Nusselt number due to higher thermal conductivity, Brownian motion of nanoparticles, and other various effects on heat transfer phenomenon. In this work, the first aim is to predict convective heat transfer of nanofluids. A numerical code is created and run to obtain results in a pipe with two different boundary conditions, constant wall temperature and constant wall heat flux. The results for laminar flow for thermally developing region in a pipe are obtained for Al2O3/water nanofluid with different volumetric fraction and particle sizes with local temperature dependent conductivity approach. Various effects that influence nanofluid heat transfer enhancement are investigated. As a result, a better heat transfer performance is obtained for all cases, compared to pure water. The important parameters that have impact on nanofluid heat transfer are particle diameter of the nanoparticles, nanoparticle volumetric fraction, Peclet number, and viscous dissipation. Next, a heat transfer performance evaluation methodology is proposed considering increased pumping power of nanofluids. Two different criteria are selected for two boundary conditions at constant pumping power. These are heat transfer rate ratio of the nanofluid and the base fluid for constant wall temperature boundary condition and difference between wall temperature of the pipe at the exit and inlet mean temperature of the fluid ratio for constant wall heat flux case. Three important parameters that influence the heat transfer performance of nanofluids are extracted from a parametric study. Lastly, optimum particle size and volumetric fraction values are obtained depending on Graetz number, Nusselt number, heat transfer fluid temperature, and nanofluid type.

The thermal efficiency of concentrated solar power (CSP) system depends on the maximum operating temperature of the system which is determined by the operating temperature of the TES device. Organic materials (such as synthetic oil, fatty acid, or paraffin wax) are typically used for TES. This limits the operating temperature of CSP units to below 400 degrees C. Increasing the operating temperature to 560 degrees C (i.e., the creeping temperature of stainless steel), can enhance the theoretical thermal efficiency from 54 percent to 63 percent. However, very few thermal storage materials are compatible for these high temperatures. Molten salts are thermally stable up to 600 degrees C and beyond. Using the molten salts as the TES materials confers several benefits, which include: (1) Higher operating temperature can significantly increase the overall cycle efficiency and resulting costs of power production. (2) Low cost of the molten salt materials can drastically reduce the cost. (3) The molten salts, which are environmentally safe, can also reduce the potential environmental impact. However, these materials suffer from poor thermo-physical properties. Impregnating these materials with nanoparticles can enhance these properties. Solvents doped with nanoparticles are termed as nanofluids. Nanofluids have been reported in the literature for the anomalous enhancement of their thermo-physical properties. In this study, the poor thermal properties of the molten salts were enhanced dramatically on mixing with nanoparticles. For example the specific heat capacity of these molten salt eutectics was found to be enhanced by as much as ~ 26 percent on mixing with nanoparticles at a mass fraction of ~ 1 percent. The resultant properties of these nanomaterials were found to be highly sensitive to small variations in the synthesis protocols. Computational models were also developed in this study to explore the fundamental transport mechanisms on the molecular scale for elucidating the anomalous enhancements in the thermo-physical properties that were measured in these experiments. This study is applicable for thermal energy storage systems utilized for other energy conversion technologies – such as geothermal energy, nuclear energy and a combination of energy generation technologies.

La controverse concernant les mécanismes proposés pour l’intensification de la conductivité thermique et de la forte augmentation de la viscosité suggère que les expériences avec des nanofluides bien dispersés et correctement caractérisés seraient intéressantes. Par conséquent, nous nous sommes fixés comme objectif la caractérisation de la conductivité thermique et la viscosité de deux nanofluides “eau-oxyde de silice“ et “eau-titanium“. Il a été observé que la conductivité thermique des deux nanofluides considérés concorde bien avec la théorie du milieu effectif, à savoir, le modèle de Maxwell, et ne montre aucune amélioration par rapport aux effets associés aux mécanismes proposés de l’intensification du transfert du nanofluide tels que le mouvement brownien ou l’effet de stratification. Pour confirmer ce résultat, nous avons également mesuré la conductivité thermique du nanofluide eau contenant une suspension de nanotubes de carbone NTC. Nous constatons que la conductivité thermique de ce nanofluide NTC est également en bon accord avec le modèle de Maxwell. Les disparités et les incohérences publiées par les différents groupes sur les résultats et modèles de la conductivité thermique ainsi que la viscosité du nanofluide se trouvent être principalement dues à la qualité du nanofluide telles que la stabilité colloïdale, la taille des particules, la formation des agrégats, etc… Par ailleurs, l’influence des incertitudes en raison de l’adoption de différents modèles sur le transfert de chaleur par convection naturelle a été étudiée. Il a été observé que les incertitudes dans les modèles prédictifs peuvent conduire à des évaluations erronées du transfert convectif. / The controversy regarding the proposed mechanisms of the exceptionally enhanced thermal conductivity of nanofluids, as well as sharp increase of nanofluid viscosity suggest that systematic experiment with well dispersed and well characterized nanofluids are highly desired. Therefore, on the basis of this suggestion, thermal conductivity and viscosity of silica-water and titania-water nanofluids were measured. It was observed that the thermal conductivity of both nanofluids agrees well with the effective medium theory, i.e., Maxwell model, and does not show any enhancement due to effects associated with the proposed mechanisms of thermal energy transfer in nanofluids like Brownian motion or liquid layering. To support these results, the thermal conductivity of water based nanofluid containing carbon nanotubes was measured. It was found that that thermal conductivity of CNTs nanofluids agrees well with Maxwell model up to 1 vol.%. The inconsistencies in the reported thermal conductivity and dynamic viscosity from different research groups are found to be mainly due to the characterization of the nanofluid, including determination of colloidal stability and particle size, (i.e, aggregates size) within nanofluid. The influence of uncertainties due to adopting various formulas for the dynamic viscosity on natural convection heat transfer was investigated. It was observed that uncertainties in the predictive models for the effective thermal conductivity and dynamic viscosity of nanofluids, leads to erroneous evaluation of the convective heat transfer with nanofluids, and this acts as a brake on research in the area.

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