The spectacular heat transfer enhancement revealed experimentally in nanofluids suspensions is being investigated theoretically at the macro-scale level aiming at explaining the possible mechanisms that lead to such impressive experimental results. In particular, the possibility that thermal wave effects via hyperbolic heat conduction could have been the source of the excessively improved effective thermal conductivity of the suspension is shown to provide a viable explanation although the investigation of alternative possibilities is needed prior to reaching an ultimate conclusion. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

This study investigates of the effect of nanoparticle volume fractions, NaCl concentration and pH on size of agglomerates, electrical conductivity and zeta potential of alumina nanofluids. The volume fractions used were 1, 2, 3 and 5 vol%. Different base fluids were prepared by adding NaCl (100, 300 and 500 ppm) or adjusting the pH (9, 7, 5 and 3). The results showed that the size of nanoparticle agglomerates was increased with an increase in nanoparticles concentration and NaCl concentration. Also, the electrical conductivity was increased with an increase in nanoparticle concentration and NaCl concentration. The size of nanoparticle agglomerates was 110 nm and the electrical conductivity was 290.2 μS/cm at pH 3 and 0 ppm, which was the highest value of electrical conductivity and smallest agglomerates nanoparticle size at 1 vol% with no salt. The highest value was 1830 μS/cm at pH 9 and 500 ppm of NaCl with 5 vol% of nanoparticles.

Sulfuric acid hydrolysis of native cellulose fibers produces stable suspensions of cellulose nanocrystals. Within a specific concentration range, the suspensions spontaneously form an anisotropic chiral nematic liquid crystal phase. This thesis examines the phase separation behaviour of these suspensions, alone and in the presence of added macromolecules. Initially, the effect of hydrolysis conditions on the nanocrystal and phase separation properties for hydrolyzed softwood pulp were investigated and compared to suspensions prepared from hardwood pulp. The macromolecules studied, blue dextrans of varying molecular weights and dye ligand densities, were synthesized and characterized with a number of techniques. The polyelectrolytic nature of these macromolecules was found to strongly influence their physico-chemical properties. Added blue dextran causes separation of an isotropic phase from highly concentrated, completely anisotropic suspensions. The observed phase separation was found to be associated with the charged dye molecules attached to the dextran. The partitioning behaviour of blue dextrans in biphasic aqueous suspensions of native cellulose nanocrystals was also studied with regard to the effect of total concentration of blue dextran, degree of dye substitution and dextran molecular weight on the blue dextran partition coefficient. Electrostatic and entropic contributions to the partition coefficient of blue dextran were discussed. Triphase isotropic-isotropic-nematic (I1--I 2--N) equilibria are observed in suspensions containing both neutral dextrans and polyelectrolytic blue dextrans of varying molecular weight. Based on these results, phase diagrams for cellulose nanocrystal suspensions with different combinations of dextran and blue dextran are presented.

Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 124-133). Also available on the World Wide Web.

Nanoluids are colloidal solutions made up of particles of the nanometric scale suspended in a fluid. This type of solution has widespread great interest since the discovery of their particular properties. The Poisson-Nernst Planck system of equations (PNP) is one of the known models for the description of ion transport. This thesis aims to develop a method to solve the PNP equations in space and time for these nanoluids. Additionally, a simulation platform (C++) is developed using an iterative scheme to solve the nonlinear equations resulting from the discretization of the system. After an overview of the literature on the subject, a discussion on the validity of the results obtained through the simulation platform through its comparison with literature and a commercial software package, COMSOL.

No description available.

This work deals with a study of enhanced critical heat flux (CHF) and burnout heat flux (BHF) in pool boiling of water with suspended silica nanoparticles using ribbon-type and wire heaters. Previously our group and other researchers have reported three-digit percentage increase in critical heat flux in silica nanofluids. This study investigates the effect of various heater surface dimensions and cross-sectional shapes on pool boiling heat transfer characteristics of water and water-based nanofluids. CHF and BHF were analyzed for circular and rectangular cross-section nichrome wires and ribbons of increasing sizes in the range of 0.32mm to 2.38mm width, approaching a flat-plate scenario. Experimental trends showed that the CHF and BHF in water pool boiling decrease as heater surface area increases, and for similar surface area, the wire had a 25% higher CHF than that of the ribbon. For concentrations from 0.1vol% to 2vol%, various properties such as viscosity, pH, and surface tension as well as silica deposition on surface and glowing length of ribbon were measured in order to study the possible factors in the heat transfer behavior of nanofluids. The deposition of the particles on the wire allows high heat transfer through inter-agglomerate pores, resulting in a nearly 3-fold increase in burnout heat flux at very low concentrations. Results have shown a maximum of up to 340% CHF enhancement for ribbon-type heaters, and the relationship of CHF with respect to nanoparticle concentration has been found to be non-monotonic with a peak around 0.2vol% to 0.4vol%. Visualization of boiling experiments aided with determination of relative bubble sizes, nucleation, and flow regimes. The surface morphology of the heater was investigated using SEM and EDS analyses, and it was inferred that the 2vol% concentration deposition coating had higher porosity and rate of deposition compared with 0.2vol% case.

No description available.

The thermal conductivities of nanofluids were measured as a function of temperature, particle size, and concentration. These nanofluids consisted of alumina, titania, or ceria dispersed in deionized water, ethylene glycol, or a mixture of the two. The results indicated that the temperature behavior of the thermal conductivity of the base fluid dominates that of the nanofluid. It was also discovered that decreasing nanoparticle size lowers the thermal conductivity of the nanofluid. None of the existing thermal conductivity models for heterogeneous materials was capable of predicting all of the observed relationships between thermal conductivity and temperature, particle size, volume fraction, and the thermal conductivities of the individual conductivities. Thus, a semi-empirical predictive model was proposed to predict the thermal conductivity of nanofluids. This model consists of the volume fraction-weighted geometric mean of the liquid and solid thermal conductivities where the solid conductivity is a function of particle size. The model provided predictions within 2.3 % of measured values in this work.

Les transferts de chaleur constituent la base de nombreux processus industriels qui sont présents dans notre vie quotidienne. L’intensification de ces échanges et l’amélioration du rendement sont devenues aujourd’hui une problématique majeure dans le monde industriel, des organismes de réglementation mais aussi de la société dans son ensemble, qui prend conscience de l’épuisement progressif des ressources énergétiques et qui se soucie de l’avenir en matière énergétique. Face à ces enjeux énergétiques et environnementaux, Le défi technologique réside dans le développement de nouveaux processus pour une meilleure gestion de l’énergie. Ce travail de thèse s’inscrit dans ce cadre, et concerne particulièrement les problèmes liés à l’intensification des échanges thermiques dans les échangeurs de chaleur. Les améliorations des échanges thermiques dites ‘passives’ sont une voie déjà largement élaborée et atteignent leurs limites. De nouvelles stratégies d’optimisation doivent donc être étudiées. Une de ces stratégies consiste à améliorer les propriétés thermiques des fluides caloporteurs utilisés dans les systèmes thermiques, notamment dans les échangeurs de chaleur. Des progrès importants en chimie ont permis dès la fin des années 90 de synthétiser des particules de taille nanométrique, qui, dispersées dans un liquide porteur, constituent des nanofluides. Leur synthèse répond au besoin d’améliorer les propriétés thermiques des fluides caloporteurs en y insérant une phase solide de conductivité thermique très élevée. Le fil directeur de ce travail consiste donc à caractériser de manière approfondie le comportement thermique et rhéologique des nanofluides à base de nanotubes de carbone NTC utilisés tout au long de ce travail afin de quantifier les principaux paramètres influençant leurs propriétés thermo-physiques et les phénomènes physiques régissant l’intensification des transferts thermiques induits par ces nanofluides. Une analyse des travaux de recherche antérieurs a été menée dans le but de s’affranchir des différents paramètres qui peuvent influencer le comportement thermique et rhéologique des nanofluides dont on citera les paramètres liés à la composition des nanofluides (fraction volumique des NTC, type de surfactant, rapport d’aspect des NTC), la température, le fluide de base… Suite à cette étude, nous avons mené une étude expérimentale sur les propriétés thermo-physiques des nanofluides testés (conductivité thermique, viscosité dynamique, masse volumique) et sur les performances thermiques dans un échangeur de chaleur. Nous avons présenté également une analyse des résultats de façon à étudier l’influence des paramètres évoqués ci-dessus. Les résultats obtenus sont comparés et discutés vis-à-vis des modèles classiques existants, en proposant des améliorations et des interprétations selon les tendances obtenues. Les résultats prometteurs de cette étude sont très encourageants et montrent que l’utilisation des nanofluides à base de nanotubes de carbone offre clairement une amélioration des performances thermiques par rapport aux fluides de base classiques. Les nanofluides à base de NTC peuvent constituer ainsi un débouché prometteur des transferts thermiques et présentent de bonnes perspectives et développement. / Heat transfer is one of the most important industrial processes in our daily lives. Nowadays, the intensification of the heat transfer and the improving of the energy efficiency have become a major problem in industry, regulatory agencies, and also the society that becomes conscious of the progressive exhaustion of the world’s energy resources and cares about the future of energy. Due to these energy and environmental issues, the technological challenge is to develop new processes for better energy management. This work fits in that context and applies particularly the problems associated to the improvements of heat exchanger’s energy efficiency. The conventional methods for increasing the heat transfer in heat exchangers have already been extensively explored and have reached their objective limits. There is therefore an urgent need for new strategies with improved performances. The novel concept of improving the thermal properties of the working fluids used in thermal system, especially in heat exchangers, has been proposed as a means of meeting these challenges. The innovative concept of nanofluids heat transfer fluids consisting of suspended of nanoparticles with very high thermal conductivities has been proposed for these challenges. The aim of this work is therefore to characterize profoundly the thermal and the rheological behavior of nanofluids containing carbon nanotubes CNTs used throughout of this work. This is in order to quantify the main parameters influencing their thermophysical properties and physical phenomena governing the intensification of heat transfer induced by these nanofluids. An analysis of previous researches has been conducted for the purpose of establishing various parameters that may influence the thermal and rheological behavior of nanofluids, which including the parameters related to the composition of nanofluids (volume fraction of CNTs, type of surfactant, aspect ratio of CNTs), the temperature, the base fluid... Following this study, experiments have been carried out on the thermal physical properties of tested nanofluids (thermal conductivity, dynamic viscosity, density) and thermal performances in a heat exchanger. Analyses of the results have been presented in order to study the influence of the abovementioned parameters. The results obtained are compared and discussed vis-à-vis the existing conventional models, suggesting improvements and interpretations according to the trends obtained. The promising results of this study are very encouraging and show that the use of nanofluids containing carbon nanotubes clearly improved the thermal performances compared to the conventional base fluids. The CNT-based nanofluids can thus be a promising candidate for heat transfer and presents good perspective and development.