Thesis (M.S.)--State University of New York at Binghamton, Thomas J. Watson School of Engineering and Applied Science, Department of Mechanical Engineering, 2009. / Includes bibliographical references.

Thesis (MSc (Applied Mathematics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Macroscopic diffusion through porous media is considered in systems where this process does not occur along with or induce bulk convective flow of the diffusing species. The diffusion coefficient present in the governing equations of suchmacroscopic diffusion is unique to a pair of species in a binary system. This coefficient may be determined experimentally, but such experimentation must be carried out for every different pair of species. Taking this into consideration, a deterministic pore-scale model is proposed to predict the effective diffusivity of homogeneous and unconsolidated porous media which ultimately depends solely on the porosity of the media. The approach taken is to model a porous medium as either a fibre bed or an array of granules through which the diffusive process is assumed to be homogenous and transversally isotropic. The fibre bed and granular modelsmay be viewed as two-dimensional and three-dimensional models respectively, and may also be combined to form a weighted average model which adjusts to differing diffusive behaviour at different porosities. The model is validated through comparison with published analytical and numerical models as well as experimental data available in the literature. A numerical program is implemented to generate further data for various arrangements of homogeneous, anisotropic and transversely isotropic porous media. The numerical results were validated against an analytical model from the literature which proved to be inapplicable to a specific case. The weighted average analytical model is proposed for this case, instead. The results of this study indicate that the weighted average analytical model is in good agreement with the numerical and experimental data and as such may be applied directly to a binary system of which the porosity is known in order to predict the effective diffusivity. / AFRIKAANSE OPSOMMING: Makroskopiese diffusieprosesse deur poreuse media word oorweeg in sisteme waar geen konveksie van die diffunderende stof plaasvind of geïnduseer word nie. Die wiskundige beskrywing van hierdie prossese bevat die sogenaamde diffusiekoëffisïent, ’n konstante wat uniek is tot ’n tweeledige sisteem. Dié konstante kan eksperimenteel bepaal word, maar as gevolg van die uniekhied daarvan tot verskillende sisteme moet dit vir elke tweeledige sisteem bepaal word. Op grond hiervan word ’n deterministiese model voorgestel om die effektiewe diffusiwiteit vir diffusie deur homogene en losstaande poreuse media te voorspel. Die model hang slegs af van die porositeit van die poreuse medium wat benader word as ’n veselbed of korrelstruktuur. Die diffusieproses deur dergelike strukture word beskou as homogeen en isotroop in die dwarsstroomrigting. Die veselbed- en korrelmodelle word beskou as twee- en driedimensionele modelle onderskeidelik en word gekombineer om ’n geweegde gemiddelde model te vorm wat dus by enige porositeit die verlangde porositeit gee. Die model is geverifieer deur vergelyking met analitiese- en numeriese modelle asook eksperimentele data vanuit die literatuur. ’n Numeriese program is gebruik om verdere resultate te verkry vir verskeie skikkings van homogene, anisotrope en dwarsverskuifde poreuse media. Die numeriese resultate is gekontroleer deur vergelyking met ’n analitiese model vanuit die literatuur. ’n Spesifieke geval is uitgewys waarvoor hierdie model nie toepasbaar is nie, maar waarvoor die voorgestelde geweegde gemiddelde model goeie resultate lewer. Die uitkomste dui aan dat die analitiese model goed ooreenstem met die numeriese en eksperimentele data en kan dus direk toegepas word om die effektiewe diffusiwiteit te verkry van ’n tweeledige sisteem waarvan die porositeit bekend is.

abstract: Early-age cracks in fresh concrete occur mainly due to high rate of surface evaporation and restraint offered by the contracting solid phase. Available test methods that simulate severe drying conditions, however, were not originally designed to focus on evaporation and transport characteristics of the liquid-gas phases in a hydrating cementitious microstructure. Therefore, these tests lack accurate measurement of the drying rate and data interpretation based on the principles of transport properties is limited. A vacuum-based test method capable of simulating early-age cracks in 2-D cement paste is developed which continuously monitors the weight loss and changes to the surface characteristics. 2-D crack evolution is documented using time-lapse photography. Effects of sample size, w/c ratio, initial curing and fiber content are studied. In the subsequent analysis, the cement paste phase is considered as a porous medium and moisture transport is described based on surface mass transfer and internal moisture transport characteristics. Results indicate that drying occurs in two stages: constant drying rate period (stage I), followed by a falling drying rate period (stage II). Vapor diffusion in stage I and unsaturated flow within porous medium in stage II determine the overall rate of evaporation. The mass loss results are analyzed using diffusion-based models. Results show that moisture diffusivity in stage I is higher than its value in stage II by more than one order of magnitude. The drying model is used in conjunction with a shrinkage model to predict the development of capillary pressures. Similar approach is implemented in drying restrained ring specimens to predict 1-D crack width development. An analytical approach relates diffusion, shrinkage, creep, tensile and fracture properties to interpret the experimental data. Evaporation potential is introduced based on the boundary layer concept, mass transfer, and a driving force consisting of the concentration gradient. Effect of wind velocity is reflected on Reynolds number which affects the boundary layer on sample surface. This parameter along with Schmidt and Sherwood numbers are used for prediction of mass transfer coefficient. Concentration gradient is shown to be a strong function of temperature and relative humidity and used to predict the evaporation potential. Results of modeling efforts are compared with a variety of test results reported in the literature. Diffusivity data and results of 1-D and 2-D image analyses indicate significant effects of fibers on controlling early-age cracks. Presented models are capable of predicting evaporation rates and moisture flow through hydrating cement-based materials during early-age drying and shrinkage conditions. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2011

Les réacteurs chimiques impliquant une phase liquide et une phase gazeuse sont couramment utilisés dans l'industrie pétrochimique et biologique car les écoulements à bulles ont de très bonnes propriétés de transfert et de mélange. Cela permet de mêler intimement différents composés et d'optimiser les réactions chimiques. Néanmoins, les mécanismes et les phénomènes mis en jeu dans le mélange au sein d'un écoulement à bulles restent encore mal connus. Ce travail a donc consisté à identifier les différents mécanismes de mélange en écoulement à bulles pour réviser le modèle physique de transport des espèces chimiques. Afin de distinguer et séparer les différents mécanismes, le mélange d'un traceur passif a été étudié dans différentes configurations expérimentales. Premièrement, l'étude du mélange dans un écoulement à bulles fortement confiné dans une cellule de Hele-Shaw a permis de mettre en évidence le mélange par capture du traceur dans les sillages. Ce mécanisme de mélange, fortement intermittent et convectif, s'est révélé être incompatible avec un processus purement diffusif. Deuxièmement, l'étude du mélange dans un essaim de bulles homogène tridimensionnel a été entreprise. Au contraire du cas confiné, le mélange, qui est causé par l'agitation induite par les bulles dans le liquide, est bien de nature diffusive. Nous avons donc pu mesurer les coefficients de diffusion effectifs en fonction de la fraction volumique de gaz. Ces coefficients sont différents dans les directions verticale et horizontale, ce qui traduit le caractère anisotrope du mélange. De plus, ils deviennent constants au-delà d'une certaine valeur de fraction volumique. Pour finir, nous avons considéré le mélange dans un essaim inhomogène de bulles, où se développe une boucle de recirculation du liquide. Dans le cas d'une recirculation modérée, la dispersion du traceur peut être estimée en combinant le mélange résultant de l'agitation des bulles avec l'advection par le mouvement moyen du fluide. / Bubble columns are commonly used for chemical processes because of their good mixing and transfer capabilities. This work aims at understanding and modelling the mixing induced by bubbles. In order to distinguish the differents mixing mechanisms, the dispersion of a low-diffusive scalar has been investigated in various experimental configurations. The first one is a bubbly flow in a Hele-Shaw cell where the confinement prevents from the developpement of turbulence. In this case, the mixing is controlled by the capture and the transport by the bubble wakes. This mechanism, which cannot be described by an effective diffusivity, has been modelled by considering the intermittent transport of finite volumes of dye. The second configuration is a homogeneous swarm of rising bubbles where the mixing results from the dispersion by the bubble-induced turbulence. It can therefore be modelled by an anistropic effective diffusivity, which becomes independent of the gas volume fraction beyond a certain value. Finally, an inhomogenous bubbly flow, where a liquid recirculation loop is present, has been considered. In the case of a moderate inhomogeneity, shear induced-turbulence is not generated by the gradients of the mean flow and the mixing can be modelled by the sum of the bubble-induced dispersion and the advection by the mean flow.

Made available in DSpace on 2014-06-11T19:25:32Z (GMT). No. of bitstreams: 0 Previous issue date: 2002-12-19Bitstream added on 2014-06-13T19:32:43Z : No. of bitstreams: 1 ferrari_pr_me_rcla.pdf: 475478 bytes, checksum: 988b82c4b0d86391822af94333d12689 (MD5) / O descarte de resíduos industriais constitui-se num problema para os agentes produtores e para a sociedade. O volume de resíduo produzido é bastante grande e, normalmente, são poluidores ambientais. Pesquisadores afirmam que a determinação das propriedades térmicas de um solo e do resíduo, pode colaborar na decisão de incorporar este ao solo, visando melhorar sua capacidade de difundir a energia necessária ao processo de crescimento das plantas. O presente trabalho foi desenvolvido com o propósito de obter a medida da difusividade térmica de um solo incorporado um resíduo industrial de silicato, em várias proporções. O método utilizado foi aquele proposto por CLIVATI (1991), adaptado para as condições da pesquisa. A partir dos valores obtidos para a temperatura com o tempo, as difusividades térmicas das amostras foram determinadas, comparadas entre si e com a literatura. Dos resultados concluiu-se que amostras de solo com resíduo incorporado na proporção de 10%, em massa, apresentaram valores de difusividade térmica muito próximos daqueles obtidos para o solo. Tais resultados indicam a possibilidade de se descartar esse resíduo no solo, em condições semelhantes àquelas propostas neste trabalho. / The dump of industrial residue, constitutes a problem for the agents that produce them and for the society. The amount of residue produced is quite voluminous and they are generally environment polluters. Researchers state that the determination of the thermal properties of the soil and of the residue can cooperate in the decision to the incorporate the residue into the soil, thus improving its capacity to diffuse the necessary energy for the growth process of the plants. This work has been developed with the purpose of determining the value of the thermal diffusivity of a soil in which an industrial residue of silicate was incorporated in various proportions. The method used was the one proposed by CLIVATI (1991) and adequate for the actual conditions. From the values obtained for the temperature in time function, the thermal diffusivities of the samples were determined, compared among themselves and with the current literature. The obtained results allow us to conclude that the soil samples with the residue incorporated at the proportion of 10% in weight, presented values of thermal diffusivities very close to those obtained from the barren soil. These results point out for the possibility of dumping this residue into the soil, in the conditions closed to those proposed in this work.

Made available in DSpace on 2014-10-09T12:26:53Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:48Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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Made available in DSpace on 2015-05-08T14:59:42Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-07-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This work proposes the thermal characterization of an opaque solid using the Flash Method as an estimation technique of thermophysical properties. Thermal characterization corresponds to the identification of heat transfer properties. The properties that most interest among researchers are: specific heat, thermal capacity, conductivity and thermal diffusivity. The characterization techniques have been evolving in parallel with the rise of new materials, as well as the necessity and accuracy of the intrinsic characteristics of each one. The use of the equations of heat and mass transfer is subject to the availability of data on the intrinsic characteristics of the materials involved, the thermophysical properties in particular. Estimates of energy exchange can be performed from the values of density, thermal diffusivity, specific heat and thermal conductivity of materials. In this context, there are countless failures in thermal projects by the unavailability of reliable data of thermodynamic and transport properties of the studied object. It is proposed in this research a numerical analysis, comparing it with experimental data on thermal characterization. Among the available experimental techniques, a transient pulse method was chosen, in particular, the Flash method, universally recognized for its accuracy and easy usage. Results are presented in terms of thermal diffusivity, thermal conductivity and specific heat. / Este trabalho propõe a caracterização térmica de um sólido opaco utilizando o Método Flash como técnica de estimação de propriedades termofísicas do mesmo. A caracterização térmica de materiais corresponde à identificação das propriedades de transporte de calor. As propriedades que despertam maior interesse entre os pesquisadores são: calor específico, capacidade térmica, condutividade, e difusividade térmica. As técnicas de caracterização vêm evoluindo paralelamente com o surgimento de novos materiais, assim como, a necessidade e a precisão das características intrínsecas de cada um. O uso das equações de transferência de calor e massa é condicionado à disponibilidade de dados sobre as características intrínsecas dos materiais envolvidos, em particular das propriedades termofísicas. Estimativas de trocas energéticas podem ser realizadas a partir dos valores de massa específica, difusividade térmica, calor específico e condutividade térmica do objeto de estudo. Neste contexto, inúmeros são os fracassos em projetos térmicos pela não disponibilidade de dados confiáveis das propriedades termodinâmicas e de transportes dos materiais. Propõe-se nesta pesquisa uma análise numérica, comparando-a com os dados obtidos experimentalmente na caracterização térmica. Entre as técnicas experimentais disponíveis, um método transiente de pulso, foi escolhido, em particular, o Método Flash, universalmente conhecido pela sua precisão e facilidades de uso. Os resultados serão apresentados em termos da difusividade térmica, da condutividade térmica e calor específico.

Made available in DSpace on 2015-05-08T15:00:07Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1329035 bytes, checksum: 466183a4f40b6f6df088719837cbee35 (MD5) Previous issue date: 2010-03-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Biofuels obtained from the avocado have become most promising because of the great potential of oil that can be extracted from the pulp of the fruit and the ethanol that can be obtained from the fruit stone. In order to be able to take out oil from the avocado pulp, one must dry up the fruit first. The purpose of the present work has been to study the drying process used for sliced avocado based on mathematical models so as to simulate mass transfer during this procedure. The equations applied to the mathematical models were numerically solved, and, by using the method of the least squares, it was possible to obtain the Effective Diffusivity and the Mass Transfer Convective Coefficient. The experimental conditions studied during the drying operation required that the velocity set at 3m/sec at temperatures of 50, 60 and 70º C were taken into account. A thorough knowledge of the mass transfer coefficients was of paramount importance for the drying process, as the Coefficient of Convective Mass Transfer is associated to the product s organic and geometric influences as well as to the thermodynamic properties of the air. On analyzing the results, it was observed that a rise in temperature favored the process of mass transfer, increasing these coefficients. It was also noted that the Fick model did not fit the experimental data well; whereas the Newton model showed better results, being capable of predicting the mass transfer on the product surface. The results were most significant because they made known the mass transfer during the convective drying of the avocado pulp. / A obtenção de biocombustíveis extraídos do abacate tem-se mostrado promissores pelo grande potencial de se extrair óleo da popa e etanol do caroço. Para se obter o óleo da polpa do abacate faz-se necessário o processo de secagem da mesma. O objetivo deste trabalho foi estudar o processo de secagem do abacate cortado em fatias e de se utilizar modelos matemáticos para simular a transferência de massa durante este processo. As equações dos modelos matemáticos resultantes foram resolvidas numericamente e com o método dos mínimos quadrados foram identificados à Difusividade Efetiva e o Coeficiente Convectivo de Transferência de Massa ao longo do processo. As condições experimentais estudadas no processo de secagem levam em consideração a velocidade do ar fixada em 3m/s e temperaturas usadas de 50, 60 e 70ºC. O conhecimento dos coeficientes de transferência de massa é de suma importância para a secagem, sendo que o Coeficiente Convectivo de Transferência de Massa está associado às influências orgânicas e geométricas do produto e as propriedades termodinâmicas do ar. Analisando os resultados, observou-se que o aumento da temperatura favoreceu o processo de transferência de massa e com isso o aumento desses coeficientes. Observou-se também que o modelo de Fick não teve um bom ajuste aos dados experimentais, enquanto que o de Newton demonstrou ser capaz de predizer melhor a transferência de massa na superfície do produto. Os resultados obtidos foram importantes, pois permitiram o conhecimento da transferência de massa durante o processo de secagem convectiva da polpa do abacate.