Submitted by Maike Costa (maiksebas@gmail.com) on 2017-05-23T14:15:11Z No. of bitstreams: 1 arquivo total.pdf: 1081749 bytes, checksum: 9f58d6baeffd1f939a659b2d2e3102be (MD5) / Made available in DSpace on 2017-05-23T14:15:11Z (GMT). No. of bitstreams: 1 arquivo total.pdf: 1081749 bytes, checksum: 9f58d6baeffd1f939a659b2d2e3102be (MD5) Previous issue date: 2015-08-08 / With the great technological advances experienced by humanity becomes providential depth knowledge about real processes of heat transfer, as well as a need arises to analyze them quantitatively. In the present work is studied the heat transfer in laminar forced convective in the entrance region of a circular tube considering the effects of axial conduction into the fluid and radiation, since in low Peclet numbers play an important role in heat transfer problems and its omission offers a significant error in the computation of the heat transfer rate. In the first part of this work is considered a slug-flow, whose exact analytical solution was discussed. In the second part of the work is considered a flow in the power law model, proposes an approximate analytic solution and numerical solution, as well as the comparison of these solutions. In this paper is used the hybrid numericanalytical method named Generalized Integral Transform Technique (GITT) to solve the energy equation. The temperature field and local Nusselt number are calculated for several values of Peclet numbers and with a boundary condition of first kind. The results presented in the form of tables and graphs permit to analyze the influence that the Peclet number and the power law index exercise in the temperature profile and the Nusselt number. The results of this study are presented in full compliance with the scientific literature. / Com o grande avanço tecnológico experimentado pela humanidade torna-se providencial um conhecimento aprofundado sobre os processos reais de transferência de calor, bem como surge uma necessidade de analisá-los quantitativamente. No presente trabalho estuda-se a transferência de calor na convecção forçada laminar na região de entrada térmica de um tubo circular considerando os efeitos de condução axial no fluido e de radiação, uma vez que em baixos números de Peclet desempenham um papel importante nos problemas de transferência de calor e sua omissão oferece um erro apreciável na computação da taxa de transferência de calor. Na primeira parte deste trabalho considerase um escoamento tipo pistão, cuja solução analítica exata foi discutida. Na segunda parte do trabalho considera-se um escoamento no modelo lei de potência, apresentando uma proposta de solução analítica aproximada e uma solução numérica, bem como a comparação entre as duas soluções. Neste trabalho utiliza-se o método híbrido numéricoanalítico denominado Técnica de Transformada Integral Generalizada (GITT) para resolver a equação da energia. O campo de temperatura e o número de Nusselt local são avaliados para vários valores do número de Peclet e uma condição de contorno do primeiro tipo. Os resultados, apresentados na forma de tabelas e gráficos, permitem analisar a influência que o número de Peclet e o índice lei de potência exercem no perfil de temperatura e no número de Nusselt. Os resultados obtidos neste trabalho se apresentam em total concordância com as literaturas científicas.

This thesis attempted to determine whether precipitation at CFB Borden, Ontario Canada is derived from recycled or marine source water using δ18O and to explore the enrichment of leaf water in three temperate forest trees. The d-excess values and rainfall intensity were used as indicators of recycled water and were found to suggest that precipitation at Borden is not derived from recycled water. The leaves of ash, aspen and maple showed significant enrichment in δ18O over xylem water. Maple leaves were significantly more enriched over the leaves of ash and aspen (p-value = 0.0019), despite source water not being significantly different (p-value = 0.1782). Modelling leaf water demonstrated the need for the inclusion of the Peclet effect in the steady state model for ash and aspen leaves, however for maple, the Peclet effect is insignificant. The non-steady state model used measured values of δET, which were uncertain and lead to poor model predictions. / NSERC Strategic Project #351040

[pt] O fenômeno de retorno de chama em tubos é conhecido e estudado há várias décadas. Sua análise clássica é baseada na determinação do gradiente de velocidade crítico, Gc, que o delimita como função das propriedades das misturas combustíveis. Entretanto, não é conhecido o efeito da diluição por CO2, importante para a previsão da segurança das instalações do pré-sal. Por isto, são aqui desenvolvidos estudos específicos do retorno de chamas pré-misturadas em escoamentos laminares. O objetivo geral deste trabalho é determinar experimentalmente a influência da diluição por CO2 sobre o retorno de chamas (flashback) em misturas de hidrocarbonetos (CH4 ou propano) e de ar. O levantamento do estado da arte permitiu especificar as características da instalação experimental para o estudo deste fenômeno e, também, identificar as principais questões a serem abordadas. Foi projetado e construído um aparato experimental para o estudo do flashback em escoamentos laminares. Os resultados originais obtidos mostram como a propensão ao retorno de chama é influenciada pela natureza do combustível, pela estequiometria da mistura e pela diluição. Misturas de propano possuem maior propensão ao flashback e maiores valores de Gc do que as de metano. Também foi mostrado que há uma redução da propensão ao flashback com o aumento da diluição. Esta propensão foi relacionada aos números adimensionais que caracterizam a combustão, isto é, os números de Lewis, Péclet, Karlovitz e Zel dovich. Para este último, uma proposta original visando sua determinação é apresentada, que envolve uma expressão da taxa de liberação de calor da reação química global controlada por uma variável de progresso. Esta formulação permite resolver o problema da singularidade na região da estequiometria. / [en] The flashback phenomenon in tubes has been known and studied for several decades. Its classical analysis is based on the determination of the critical velocity gradient, Gc, which delimits it as a function of the fuel mixture properties. However, the effect of the CO2 dilution is not known, which is important for predicting the safety of pre-salt facilities. For this reason, specific studies of premixed flame flashback in laminar flows are developed here. The general objective of this work is to experimentally determine the influence of CO2 dilution on flame flashback in mixtures of hydrocarbons (CH4 and propane) and air. The state of the art research made it possible to specify the characteristics of the experimental installation for this phenomenon study and, also, to identify the main issues to be addressed. An experimental apparatus was designed and built to study the flame flashback in laminar flows. The original results obtained show how the propensity of the flame flashback is influenced by the nature of the fuel, the stoichiometry of the mixture, and the dilution. Propane mixtures have a greater propensity for flashback and higher values of Gc than those of methane. It has also been shown that there is a reduction in the propensity of flashback with increasing dilution. This propensity was related to the dimensionless numbers that characterize combustion, that is, the Lewis, Péclet, Karlovitz, and Zel dovich numbers. For the latter, an original proposal aimed at its determination is presented, which involves an expression of the heat release rate from the global chemical reaction controlled by a progress variable.

Fouling in a nanofitration membrane module is usually a result of concentration polarization. The effect of permeate suction on the slightly negatively charged spiral wound nanofiltration membrane is investigated. According to the film theory, the mass transfer coefficient is inversely proportional to concentration polarization. The effect of permeate suction destabilizes the boundary layer. This will decrease the concentration polarization layer, and consequently will increase mass transfer through the membrane's surface. To validate the hypothesis, experiments were carried out on a NF membrane that can be described by the solution-diffusion model. This model has coefficients that can be measured experimentally. Using the membrane wall concentration in this model instead of the bulk feed concentration can help estimating the mass transfer coefficient more appropriately. Two experimental studies were carried out, one with a standard high pressure pump, and another one with the added effect of suction pressure applied to the permeate collector tube. Three different concentrations of binary dilute solutions of NaCl, MgSO4, and MgCl2, at three different pressures (low, medium, and high) were tested. For all tested solutions, permeate suction increased the diffusive Peclet number as a function of the feed concentration (x) according to the equation Pe = a1x²+b1x+c1, with R²>0.99, where x is the feed concentration in Mol/l, and a1, b1, and c1 are coefficients dependent on feed pressure for every salt solution. With the increase of the Peclet number, it was observed that the concentration polarization decreased, and both the product flow and the product quality were improved. Suction had the greatest impact at the range of 100 to 110 psi feed pressure, where the concentration polarization reduced approximately 14 to 20 %. ANOVA for the concentration polarization showed that suction was significant in reducing the calculated concentration polarization layer for all tested solutions. It was concluded that permeate suction reduced concentration polarization, increased product flow rate, and improved product quality. Thus, adding permeate suction has beneficial consequences because it reduces membrane fouling and extends its useful service life.

No description available.

L'objectif de cette thèse a été d'étudier, sous différentes conditions d'écoulement, la rhéologie et l'électrorhéologie de suspensions de nanoparticules (taille comprise entre 10 nm et 200 nm) appelées plus couramment nanofluides, afin de quantifier l'importance des principaux paramètres impliqués dans la valeur de leur viscosité. Une approche inédite, la rhéologie à très fort taux de cisaillement, a permis de distinguer pour les nanofluides le régime d'agrégation péricinétique dominé par le mouvement Brownien, du régime d'agrégation orthocinétique dominé par les interactions hydrodynamiques. La transition entre ces deux régimes a pu être atteinte grâce à la conception de microrhéomètres capillaires à capteurs de pression intégrés, dont la réalisation ont constitué une partie importante de ce travail. Les résultats de ces recherches mettent en évidence une corrélation entre l'augmentation du nombre de Peclet Pe quantifiant l'importance des interactions hydrodynamiques par rapport aux interactions Browniennes et la taille des agrégats au passage de la transition Pe=1. Des conclusions nettes ont été tirées sur la prépondérance du rôle des agrégats dans la rhéologie des nanofluides et sur l'influence des conditions d'écoulement sur leur forme.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

A numerical model for describing the kinetics of intracellular water transport during cryopreservation was developed. As ice is formed outside the cell, depleting the extracellular liquid of water, the cell will experience an osmotic pressure difference across its membrane, which causes cell dehydration and concomitant shrinkage. Although Mazur (1963) has previously modeled this phenomenon as a two-compartment system with membrane limited transport, the assumption of well-mixed compartments breaks down at large Biot numbers. Therefore, we have developed a numerical solution to this moving-boundary problem, including diffusive transport in the intracellular liquid, in addition to the osmotically driven membrane flux. Our model uses a modified Crank-Nicolson scheme with a non-uniform Eulerian-Lagrangian grid, and is able to reproduce predictions from Mazurs model at low Biot numbers, while generating novel predictions at high Biot numbers. Given that cell damage may result from excessive water loss, our model can be used to predict freezing methods that minimize the probability of cell injury during the cryopreservation process.

Simulated moving bed chromatography process (SMBCP) is the technical realisation of a countercurrent adsorption process through the cyclic port switching. SMB technology reduces the cost of packing material with high loading capacity and provides high purity and high recovery in a very short time. Major commodity applications have been found in the petroleum, food, biotechnology, pharmaceutical and fine chemical industries. The industrial applications bring an emergent demand to improve the SMBCP operation for higher product quality, productivity, efficiency and robustness. However, for this particular process, we encounter several challenges. Firstly, the interplay of the effects of strong nonlinearities, competition of solutes, mass transfer resistance and fluid dynamic dispersion produces steep concentration fronts. Mathematical model accounted for this particular property constitutes a serious difficulty for the solution procedure. Secondly, a dynamic SMB model consists of a set of partial differential, ordinary differential and algebraic equations, which are highly coupled. The large size is a problem due to its intensive computation when on-line optimisation and real-time control are necessary. Thirdly, the SMB unit operation exhibits complex dynamics. Process metrics for design and operation can be determined only when a cyclic steady state is reached after a certain number of switching. Achieving this steady state by solving the PDE models cycle after cycle involves expensive calculation. Studies have been carried out to solve these problems through process analysis, investigation on spatial discretisation techniques, and development of an accelerated integration scheme. / Through a systematic study on the advances of SMB modelling, design and control, a set of functionally equivalent models for SMBCP are identified and summarized for their practical applications. The limitations of the existing modelling techniques in industrial applications are also identified. Furthermore, structural analysis of the existing models is conducted for a better understanding of the functionality and suitability of each model. Suggestions are given on how to choose an appropriate model with sufficient accuracy while keeping the computational demand reasonably low for real time control. / Effort is made on to the systematic investigation of different numerical methods for the solution of PDEs to circumvent the steep gradients encountered in chromatographic separation. Comprehensive studies are conducted on a single column chromatographic process represented by a transport-dispersive-equilibrium linear model. Numerical solutions from the upwind-1 finite difference, wavelet-collocation, and high resolution methods are evaluated by quantitative comparisons with the analytical solution for a range of Peclet numbers. It reveals that for a PDE system with a low Peclet number, all existing numerical methods work well, but the upwind finite difference method consumes the most time for the same degree of accuracy of the numerical solution. The high resolution method provides an accurate numerical solution for a PDE system with a medium Peclet number. The wavelet collocation method is capable of catching up steep changes in the solution, and thus can be used for solving PDE models with high singularity. / The advantages and disadvantages of the wavelet based approaches are further investigated through several case studies on real SMBCP system. A glucose-fructose separation process is firstly chosen with its relatively simple isotherm representations. Simulations are conducted using both wavelet collocation and upwind finite difference methods. For more complicated applications, an enantiomers separation process is selected. As the PDEs model exhibit a certain degree of singularity, wavelet collocation and high resolution methods are adopted for spatial discretisation. It is revealed that both the wavelet based approaches and high resolution methods are good candidates in terms of computation demand and prediction accuracy on the steep front. This is the first time that these two frontier numerical methods are used for such a complex SMB system models and our results are encouraging for the development of model-based online control scheme. / In developing a new scheme to rapidly obtain the solution at steady state for any arbitrary initial condition, the concept of Quasi-Envelope (QE) is adopted under the consideration that a SMBCP can be treated as a pseudo-oscillatory process because of a large number of continuous switching. The scheme allows larger steps to be taken to predict the slow change of starting state within each switching. Combined with previously developed wavelet-based technique, this method is successfully applied to the simulation of a SMB sugar separation process. Investigations are also carried out on the location of proper starting point for the algorithm and on the effect of changing stepsize to the convergence of iteration method. It is found that if the starting state of Quasi-Envelope is chosen to be the same as the original function, the multivalue algorithm would require similar computational effort to achieve the steady state prediction, regardless of the integration stepsize. If using constant stepsize, launching QE later is helpful when quasi-envelope displays steep change at the start-up period. A changing stepsize produces slow convergence compared to the constant stepsize strategy, thus increasing the work load where the stepsize change is occurring. Other iteration method is required to be imposed to achieve faster convergence right from the beginning. Potential applications can be seen for other chemical engineering processes with inherent cyclic behaviour.