Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2006. / Garimella, Srinivas, Committee Chair ; Ghiaasiaan, S. Mostafa, Committee Member ; Graham, Samuel, Committee Member ; Breedveld, Victor, Committee Member ; Fuller,Tom, Committee Member.

Thesis (Ph.D.)--Ohio University, November, 2002. / Title from PDF t.p. Includes bibliographical references (leaves 175-177)

[pt] O duto estudado era composto por duas laterais arco cilíndricas convexas isotérmicas de mesmo raio por uma base plana adiabática. Obteve-se os coeficientes de transporte de calor para convecção forçada em regime laminar deste canal. Para tanto, optou-se pelo emprego do método da analogia de transparência de massa e calor. A técnica recorrida foi a de sublimação de naftaleno, cuja utilização exigiu instrumentos de precisão e condições ambientais controladas. A variação da massa, devido à sublimação, das laterais de naftaleno análogas às superfícies isotérmicas fornecia, juntamente com outros parâmetros, o conhecimento dos coeficientes médios de troca de massa. Estes eram adimensionados como número de sherwood. Apartir dos pontos experimentais encontrados determinou-se a equação do comportamento do número de Sherwood, para perfil não desenvolvido, em função do comprimento adimensional, definido como o inverso do número de Graetz. Aplicada à analogia, chegou-se aos coeficientes médios de transferência de calor, adimensionalizados como número de Nusselt. Em posse destes resultados, fez-se a análise teórico da performance de um coletor solar simulado utilizando o canal em estudo. / [en] The duct analyzed was made up of two cylindrical side walls and anadiabatic base plate. Heat transfer coefficients for forced convection in laminar flow were obtained. The heat and mass transfer analogy method was used by applying the naphtalene sublimation tecnique. The side walls mass change, due to sublimation, supplied the overall mass transfer coefficients. These were stated in dimensionless form as sherwood’s number. An equation giving Sh as a function of the dimensionless length was obtained from the experimental points. The profile was not fully developed. In accordance with the analogy the overall heat transfer coefficients were found. A theoretical study of a solar energy collector employing these duct was made.

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.

La torréfaction est une étape nécessaire pour la production de gazoles à partir de biomasse lignocellulosique par voie thermochimique (chaîne Biomass To Liquid). Il s'agit d'un traitement thermique dans le domaine de température compris entre 200 et 300°C en milieu non oxydant ; le but de cette étape est de modifier la structure de la biomasse afin d'en faciliter le transport pneumatique après broyage. Cependant, des réactions exothermiques ont été observées et peuvent mener à un mauvais contrôle de la température au sein du réacteur et nuire à la qualité des produits, voire endommager l'installation. L'objectif de cette thèse est de quantifier la chaleur émise par les réactions exothermiques de torréfaction de la biomasse lignocellulosique, et d'en étudier les impactes lors du changement d'échelle, où les phénomènes de transfert de masse et de chaleur ne sont plus négligeables. Durant nos travaux, des mesures de perte de masse et de flux de chaleur ont été réalisées à l'échelle de la poudre (microparticule) sur trois types d'essence de bois (robinier, épicéa et eucalyptus) ainsi que sur les principaux constituants de la matière lignocellulosique (cellulose, xylane, glucomannane et lignine). Un modèle cinétique capable de reproduire la perte de masse ainsi que le flux de chaleur généré par les réactions exothermiques, a été développé. Il utilise le concept de distribution d'énergie d'activation. Tous les paramètres du modèle ont été identifiés par méthode inverse sur un ensemble de tests isothermes d'une durée de 10 heures. Cela permet de proposer des paramètres cinétiques robustes et des valeurs fiables d'énergie d'activation. Par la suite, des mesures de température pendant des essais de torréfaction sur des planches de bois (méso-échelle) et sur un lit fixe de particules (échelle macroscopique) ont permis de mesurer la propagation d'une onde thermique générée par les réactions exothermiques. Une modélisation macroscopique qui intègre le modèle cinétique développé permet de propager l'effet des réactions exothermiques à l'échelle de la macro particule. L'analyse de l'ensemble des résultats permet de mettre en exergue l'importance de l'échelle lit sur l'emballement thermique observé expérimentalement. L'ensemble du travail, mené à différentes échelles spatiales et complété par une analyse permettant de relier ces échelles entre-elles, constitue une avancée significative vers la prédiction de l'exothermicité de la torréfaction afin d'assurer la sécurité et la faisabilité à l'échelle industrielle. / Lignocellulosic biomass torrefaction is an important step for diesel production through the BTL (Biomass To Liquid) chain. Torrefaction is a non-oxidative thermal treatment in the temperature range from 200 to 300°C. The aim of this process is to modify biomass structure in order to facilitate pneumatic transportation after grinding. However, some exothermic reactions are triggered in this temperature range which can lead to a lack of temperature control inside the reactor with detrimental effects on the product quality or even destructive effects on the facility. The purpose of this study is to contribute to the development of a multi-scale model for simulating these thermal runaway phenomena. Starting with the smallest scale, anhydrous weight loss combined with heat flux measurement from powders have been performed in a TGA-DSC. A data base was developed from three types of woody biomass, namely locust, spruce and an eucalyptus and from the main lignocellulsic components which are cellulose, xylan, glucomannan and lignin. A thermo-kinetic model able to reproduce the measured mass loss and heat flux during torrefaction has been developed by using the Distributed Activation Energy Method. Kinetic parameters and reaction enthalpies have been identified by inverse method taking into account the comprehensive set of data over several isothermal conditions with residence times of up to ten hours. Proceeding to larger scales, temperature measurements under torrefaction conditions have been performed separately in individual macro-particles and in a large scale packed bed of wood chips in order to test models at these scales. Thermal excursions were observed both within the particles and the bed due to the exothermic reactions. In the fixed bed an actual amplifying thermal wave was observed to propagate along the axial direction have been performed. A macroscopic heat and mass transfer model coupled with the kinetic model developed in this work allowed to simulate the temperature field at the macro-particle scale. Further model developmental work is needed to simulate the bed scale, Experimental observations and modelling carried out in this work represent an important improvement for the prediction of the heat released by torrefaction reactions in order to make this thermal pre-treatment safer and economically valuable.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / Devido à degradaÃÃo progressiva dos corpos hÃdricos em geral, tem-se dado bastante atenÃÃo a alternativas para a recuperaÃÃo da qualidade de suas Ãguas. TÃcnicas de aeraÃÃo e oxigenaÃÃo artificial via injeÃÃo de ar ou oxigÃnio puro na Ãgua (plumas de bolhas) podem ser utilizadas para tal finalidade. Nesse contexto, o presente trabalho desenvolveu um modelo integral para avaliaÃÃo da transferÃncia de massa de plumas de bolhas circulares em ambientes nÃo estratificados, o qual à baseado em uma distribuiÃÃo radial do tipo Gaussiana das propriedades do escoamento e em relaÃÃes matemÃticas para o coeficiente de entrada turbulenta e o fator de amplificaÃÃo da quantidade de movimento devido à turbulÃncia. O impacto da transferÃncia de massa na hidrodinÃmica de plumas de bolhas foi investigado considerando diferentes diÃmetros de bolha, vazÃes de gÃs e profundidades em sistemas de aeraÃÃo e oxigenaÃÃo. Os resultados revelaram impacto significativo quando bolhas finas sÃo consideradas mesmo em profundidades moderadas. Bolhas mÃdias apresentaram, em geral, comportamento semelhante ao de bolhas grossas. As simulaÃÃes tambÃm indicaram que, em condiÃÃes de vazÃes relativamente baixas e elevadas profundidades, dissoluÃÃo e turbulÃncia podem afetar a hidrodinÃmica da pluma de bolhas, o que demonstra a importÃncia de se levar o fator de amplificaÃÃo da quantidade de movimento em consideraÃÃo. SimulaÃÃes utilizando o modelo proposto e modelos clÃssicos disponÃveis na literatura resultaram em boa concordÃncia tanto para processos de aeraÃÃo quanto de oxigenaÃÃo. Finalmente, foram apresentados estudos de caso para os dois processos. / Due to the progressive degradation of water bodies in general, alternatives have been studied so as to restore their water quality. Artificial aeration/oxygenation by injecting air/pure oxygen in the water (bubble plumes) can be used for this purpose. Hence, this study presents an integral model to evaluate gas transfer from circular bubble plumes in unstratified environments which is based on a radial Gaussian type distribution of plume properties and functional relationships for the entrainment coefficient and factor of momentum amplification due to turbulence. The impact of gas-liquid mass transfer on bubble plume hydrodynamics is investigated considering different bubble sizes, gas flow rates and water depths. Also simulations were run for aeration and oxygenation systems in order to provide the analysis of these effects. The results revealed a significant impact when fine bubbles are considered, even for moderate water depths. Medium bubbles present overall similar behavior as coarse bubbles. Additionally, model simulations also indicate that for bubble plumes with relatively low gas flow rates and high water depths, both dissolution and turbulence can affect bubble plume hydrodynamics, which demonstrates the importance of taking the momentum amplification factor relationship into account. For deeper water conditions, simulations of bubble dissolution using the present model and classical models available in the literature resulted in a very good agreement for both aeration and oxygenation processes. Finally, case studies involving those processes are presented.

Orientador: Marco Aurélio Cremasco / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química / Made available in DSpace on 2018-08-21T17:25:48Z (GMT). No. of bitstreams: 1 VergelZapata_CarlosFernando_M.pdf: 3787074 bytes, checksum: c52cfd4ba974066436ff88b0a07c8b76 (MD5) Previous issue date: 2012 / Resumo: Os modelos matemáticos são utilizados como instrumentos de suporte operacional para melhorar e facilitar a otimização de um processo. No presente trabalho foi proposto um modelo simplificado para determinar a concentração de ozônio na água destilada por meio de um processo de ozonização em uma coluna de borbulhamento, em condições isotérmicas (21°C) e isobáricas (pressão atmosférica). O modelo foi baseado em uma cinética de reação de segunda ordem para a decomposição de ozônio na fase líquida. Para o desenvolvimento da modelagem a resistência do gás à transferência de massa e os efeitos da dispersão para as duas fases (gasosa-líquida) foram considerados desprezíveis. Além disso, assumiu-se composição constante de ozônio na fase gasosa. O modelo foi resolvido segundo as condições de contorno do processo, pelo método numérico Runge-Kutta de quarta ordem com a ajuda do software Matlab 7.0. Durante o processo de ozonização estudado o pH da solução não foi mantido constante, portanto no presente trabalho foi proposto uma cinética de reação de primeira ordem para a variação do pH, a qual representou satisfatoriamente. Diante dos resultados, observou-se que um aumento na concentração de ozônio na fase gasosa como condição inicial do processo e uma diminuição do pH da solução ao longo do tempo aumenta a concentração de ozônio na fase líquida. As distribuições de concentração de ozônio na água destilada obtidos da modelagem apresentaram concordância com os dados experimentais encontrados na literatura / Abstract: Mathematical models are used as operational support tools which improve and facilitate processes optimization. In this work, it was suggested a simplified model to determine the ozone concentration in distillated water, through an ozonation process in a bubble column under isothermal (21°C) and isobaric (atmospheric pressure) conditions. The model was based in second-order reaction kinetics of ozone decomposition in the liquid phase. For the modeling development, gas resistance to mass transfer and the dispersion effects for both phases (liquid-gas), were considered negligible. In addition, it was assumed a constant composition of ozone in the gaseous phase. The model was solved according the boundary conditions of the process, through the fourth-order Runge-Kutta numerical method supported on the software Matlab 7.0. During the ozonation process studied, the solutions pH wasn't maintained constant, therefore, it was proposed a first-order kinetics of reaction for the pH variation, which successfully represents. The results showed that an increase in the concentration of ozone in the gaseous phase as initial condition and a decrease of pH of solution through time increases the liquid phase ozone concentration. The ozone concentration distributions obtained of the modeling in distilled water, present agreement with the experimental data found in literature / Mestrado / Engenharia de Processos / Mestre em Engenharia Química

[pt] Neste trabalho é feita uma análise teórica e numérica da evaporação não isotérmica de um líquido contido em um recipiente cilíndrico parcialmente cheio, com paredes adiabáticas. Postula-se que a evaporação acontece em presença de convecção natural impulsionada por diferenças de massa específica, associadas com gradientes de temperatura e composição da mistura. Esta consiste de um gás e o vapor do líquido. Embora a formulação seja geral, o presente trabalho focaliza a evaporação de água para o ar. Estudou-se três situações. Um caso isotérmico, variante do clássico problema de difusão de Stefan, um Caso em que a temperatura do líquido é maior do que a temperatura ambiente e um terceiro caso no qual a temperatura do líquido é menor do que a do ambiente. Duas diferentes condições de contorno foram usadas na abertura do recipiente de modo a explorar a sensibilidade do escoamento às condições no topo. A distância entre a superfície do líquido e o topo variou de duas a dez vezes o raio do recipiente. Duas diferenças de temperatura entre o líquido e o ambiente foram investigadas, 3 graus Celsius e - 2 graus Celsius. O ar ambiente foi considerado como sendo muito seco ou muito úmido. Encontrou-se que, quando a temperatura do líquido é maior do que a temperatura ambiente, a taxa de evaporação alcança valores até quatro vezes maiores do que para o caso isotérmico. Para o caso em que a temperatura do líquido é menor do que a temperatura ambiente, a taxa de evaporação decresce para valores até duas vezes menores do que para o caso isotérmico. / [en] This work reports a theoretical and numerical analysis of the non-isothermal evaporation of a liquid contained in a partially filled cylinder vessel, with adiabatic walls. It is assumed that the evaporation occurs in the presence of natural convection driven by differences in specific mass associated with gradient of temperature and mixture composition. The mixture consist of a gas and the vapor of the evaporating liquid. Although the formulation is general, the specific focus of the present work is on the evaporation of water into air. Three situations were studied. An isothermal case, which is a variant of the classical Stefan diffusion problem, a case where the liquid temperature is higher than the ambient temperature, and a third case in which the liquid temperature is lower than the ambient. Two different boundary conditions were used at the openning of the vessel in a way to explore the sensitivity of the flow to the conditions on the top. The distance between the liquid surface and the top of the vessel varied from two to ten times the vessel radius. Two temperature differences between the liquid and the ambient were investigated, 3 degrees Celsius and - 2 degrees Celsius. The environmental air was considered to be either very dry or very wet. It was found that, when the liquid temperature is higher than the ambient temperature, the rate of evaporation can reach values up to four times larges than that for the isothermal case. For the case where the liquid temperature. is lower than the ambient temperature, the rate of evaporation decreases to values down to half of theisothermal case.

Recent developments have indicated high oxygen consumption rates of about 35 g-mole/m³-min during oxidative pressure leaching. At such high oxygen consumption rates the mass transfer of dissolved oxygen at the gas-liquid interface may become rate-limiting. The objective of this study was to obtain an understanding of the gas-liquid mass transfer processes that take place in mechanically agitated pressure leaching systems. The classical reaction between sodium sulphite and dissolved oxygen to form sulphate at atmospheric pressure was used to determine the oxygen mass transfer rates in a 200-liter asymmetrical plastic tank, modelled after the shape of the first compartment of the zinc pressure leach. The effect of this asymmetry was compared with the work of Swiniarski who used a cylindrical symmetrical tank of similar volume. A number of process variables such as the impeller type and size, the impeller speed, the impeller immersion depth and the effect of full baffles that affect mixing were investigated. Also, the volumetric power consumption associated with the mass transfer rates were measured. The results indicate that the asymmetrical tank is at least 3.6 times more efficient in mass transfer than the symmetrical tank. There is a critical speed below which the mass transfer parameter, K[formula omitted], is almost zero and above which K[formula omitted] increases almost linearly with impeller tip speed. A simple energy balance model for bubble creation can predict the critical tip speed. It is shown that K[formula omitted] is enhanced at shallow depths, with a corresponding high mass transfer to energy ratio. The relative effectiveness of impeller types and sizes with regard to the use of power for gas-liquid mass transfer was established. Full baffles degrade the mass transfer rate at increased depth of impeller immersion. The results also add substantial support to the findings provided by DeGraaf [5] that: (i) The dimensionless correlations used in liquid mixing systems do not accurately predict dispersion rates by agitators. (ii) The optimum conditions for gas dispersion and the consequent generation of gas-liquid interfacial area are different from fluid mixing. (iii) The classical mixing power equations for impellers markedly overestimate power requirements during impeller gas dispersion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The mass and thermal transport in porous media play an important role in many engineering and geological processes. The hydrodynamic and thermal effects are two interesting aspects arising in the research of porous media. This thesis is concerned with numerical investigations of double-diffusive natural convective heat and mass transfer in saturated porous cavities with Soret and Dufour effects. An in-house FORTRAN code, named ALFARHANY, was developed for this study. The Darcy-Brinkman-Forchheimer (generalized) model with the Boussinesq approximation is used to solve the governing equations. In general, for high porosity (more than 0.6), Darcy law is not valid and the effects of inertia and viscosity force should be taken into account. Therefore, the generalized model is extremely suitable in describing all kinds of fluid flow in a porous medium. The numerical model adopted is based on the finite volume approach and the pressure velocity coupling is treated using the SIMPLE/SIMPLER algorithm as well as the alternating direction implicit (ADI) method was employed to solve the energy and species equations. Firstly, the model validation is accomplished through a comparison of the numerical solution with the reliable experimental, analytical/computational studies available in the literature. Additionally, transient conjugate natural convective heat transfer in two-dimensional porous square domain with finite wall thickness is investigated numerically. After that the effect of variable thermal conductivity and porosity investigated numerically for steady conjugate double-diffusive natural convective heat and mass transfer in two-dimensional variable porosity layer sandwiched between two walls. Then the work is extended to include the geometric effects. The results presented for two different studies (square and rectangular cavities) with the effect of inclination angle. Finally, the work is extended to include the Soret and Dufour effects on double-diffusive natural convection heat and mass transfer in a square porous cavity. In general, the results are presented over wide range of non-dimensional parameters including: the modified Rayleigh number (100 ≤ Ra* ≤ 1000), the Darcy number (10-6 ≤ Da ≤ 10-2), the Lewis number (0.1 ≤ Le ≤ 20), the buoyancy ratio (-5 ≤ N ≤ 5), the thermal conductivity ratio (0.1 ≤ Kr ≤ 10), the ratio of wall thickness to its height (0.1 ≤ D ≤ 0.4), the Soret parameter (-5 ≤ Sr ≤ 5), and the Dufour parameter (-2 ≤ Df ≤ 2).