Numerical simulation of frost growth and densification using deformable and static grids

The present thesis aims at developing a basis for the numerical simulation of the growth and densification of macroscopic frost sheets. Notwithstanding the fact that icing has still a long way to fully understand its physics, this work is potentially important, since a broad range of empirical correlations as well as the implementation of formerly neglected physical effects are studied herewith. In particular, this research is focused on developing methodologies for three-dimensional meshes, capable to predict the frost growth and densification of complex geometries. This work comprises five chapters. The first one is an introduction to icing and frost formation. The main motives that urge to numerically simulate frosting are detailed. Moreover, the different methods and approaches to model frosting are presented. From these, the main objectives and outline of the thesis are derived. The next two chapters are the core of this dissertation, which comprehend the two developed methods. In detail, the second chapter introduces a model that simulates the frost growth and densification using a moving mesh method. First, the most relevant empirical correlations used to describe the frost layer conductivity and diffusivity among others are tested by means of parametric studies. A thorough discussion on the performance of such parameters is made, emphasizing the fact of using diffusion resistance factors above 1.0 in order to capture the frost growth. The best-fitted solutions validate the model input combinations which give good agreements against experimental data under certain experimental conditions. Furthermore, the method is tested against a 2D numerical case, highlighting the main advantages of using a deformable grid, i.e. the accurate tracking of the air-frost interface. The third chapter introduces a fixed-grid-porous-media method capable of simulating the growth and densification of frost sheets. In pursuance of finding out possible explanations to the needed artificially enhanced diffusion resistance factors, a velocity field is calculated across all the domain. A porous media treatment is given to the frost layer, whereas the transported temperature and vapour density are used to define the thermophysical state of each cell, which might enable phase change. The method is tested with a study case of a duct flow with a non-homogeneously cooled lower boundary. The influence of accounting for the convection, as well as the enhanced diffusion resistance factors within the frost layer, are studied. The last two chapters contain the concluding remarks, as well as ideas on how the present work could be continued.La present tesi desenvolupa una base per a la simulació numèrica del creixement i densificació de capes de gebre macroscòpiques. Malgrat que la física de la formació de gel té encara un llarg camí per recórrer, aquest treball és potencialment important, ja que s'hi estudia un ampli rang de correlacions empíriques i la implementació d'efectes físics anteriorment negligits. En particular, aquesta recerca se centra en el desenvolupament de metodologies per a malles tridimensionals, capaces de predir la formació de gebre en geometries complexes. Aquest treball està format per cinc capítols. El primer és una introducció a la formació de gel. En aquest s'hi detallen els motius principals pels quals són necessàries eines numèriques per a la predicció d'aquest fenomen. A més, es presenten els mètodes existents per modelar el gebre. A partir d'aquests, es deriven els principals objectius de la tesi. El segon capítol introdueix un model que simula el creixement i densificació de gebre mitjançant un mètode de malla mòbil. En primer lloc, les correlacions empíriques més rellevants utilitzades per descriure la conductivitat de la capa de gel i la seva difusió entre d'altres són provades mitjançant estudis paramètrics. Es realitza una discussió exhaustiva del rendiment d'aquests paràmetres, fent èmfasi en l'ús de factors de resistència de difusió superiors a 1.0 per tal de capturar el creixement del gebre. A més, el mètode es prova amb un cas numèric 2D, destacant els principals avantatges d'utilitzar una malla deformable, és a dir, el seguiment precís de la interfície gel-aire. El tercer capítol introdueix un mètode de medi porós amb malla fixa, capaç de simular el creixement i densificació de capes de gebre. Amb la intenció de cercar explicacions que verifiquin la necessitat d'utilitzar factors de resistència de difusió artificialment augmentats, es calcula el camp de velocitat a tot el domini. Es realitza un tractament de medi porós en la zona del gebre, i la temperatura i densitat del vapor transportat s'utilitzen per definir l'estat termofísic de cada cel·la, que pot desencadenar un canvi de fase. El mètode es prova amb un cas d'estudi d'un canal de vent amb la paret inferior refredada de forma no homogènia. S'estudia la influència de tenir en compte la convecció, així com els factors de resistència augmentats dins de la capa de gel. Els dos darrers capítols contenen comentaris finals, així com idees sobre com es podria continuar el treball realitzat

Quantitative imaging of gas adsorption equilibrium and dynamics by X-ray Computed Tomography

We present the development and application of X-ray Computed Tomography (CT) for the determination of the adsorption properties of microporous adsorbents and the study of breakthrough experiments in a laboratory fixed-bed adsorption column. Using the model system CO2/helium on activated carbon, equilibrium and dynamic adsorption/desorption measurements by X-ray CT are described, and the results are successfully compared to those obtained from conventional methods, including the application of a one-dimensional dynamic column breakthrough model. The study demonstrates the practical feasibility of applying X-ray CT to measure internal and transient concentration profiles in adsorbent systems on the length-scales from a single adsorbent pellet to a packed column

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

Parameter estimation and modeling of lithium and lithium-ion batteries

Specific characteristics of Li-ion batteries (LIBs) make them promising candidates for energy storage systems when compared with the others. High working voltage and energy density as well as green technology of LIBs are the reasons for increasing interest to use these electrochemical systems as the substitute of conventional combustion engine of automobiles. Consequently, the interest to study these technologies has increased recently and several models have been introduced to simulate their behavior. However, it is difficult to model all multiphysics phenomena happening inside such rechargeable batteries. Some important choices need to be made when one wants to select an appropriate model for considering the main physics elements and yet be simple enough for large time scale studies. Combining chemical/electrochemical kinetics and transport phenomena, electrochemical models have been introduced to tackle most important principles inside the cell. These models, however, require known electrochemical parameters which most of the time are hard to get experimentally. Parameter estimation (PE) techniques simplify extracting these representative parameters of the cell behaviour. In this study, a PE methodology has been introduced to estimate the most influential electrochemical parameters of LIBs considering different positive electrode materials. The methodology starts with simplifying the well-known pseudo-two-dimensional (P2D) model, the most complex and the most popular electrochemical engineering models for simulating porous electrodes and introducing an enhanced single particle model (SPM). Neglecting the micro-structure of LIB, major electrochemical parameters are detected at the cell level. Next, the best time domains and discharge current rates to estimate each parameter are estimated by virtue of sensitivity analyses. Owing to the fact that the behavior of LIBs depends on the active materials employed in the electrode, the proposed methodology is verified for three different positive electrode active materials including LiCoO2, LiMn2O4 and LiFePO4. Furthermore, focusing on LiFePO4 (LFP), as the most promising positive electrode active material, a new modification is proposed to the model to address special features of this material. In this regard, a simplified electrochemical model is equipped with a variable resistance equation whose coefficients are estimated by means of PE.Résumé : Les batteries au Li-ion (BLI) figurent parmi les technologies les plus prometteuses pour le design de systèmes de stockage d’énergie à cause de leurs caractéristiques intrinsèques. Leur grand voltage de travail, leur grande densité énergétique et leur impact écologique positif expliquent l’intérêt soutenu de l’utilisation des BLI pour remplacer par exemple les moteurs à explosion dans les applications de transport terrestre. Il n’est donc pas surprenant de constater que ces technologies ont eu une attention scientifique importante et que plusieurs auteurs ont développé des modèles numériques simulant leur comportement. Il reste cependant difficile de représenter tous les phénomènes multiphysiques qui se déroulent à l’intérieur des batteries rechargeables par des modèles mathématiques. Des compromis importants doivent être faits lorsqu’on doit choisir un modèle représentant les principaux phénomènes physico-chimiques tout en restant assez simple pour pouvoir l’utiliser dans des études s’échelonnant sur de larges périodes temps. Représentant à la fois la cinétique électrochimique et le transport de masse, les modèles électrochimiques ont été introduits pour prendre en compte les phénomènes les plus importants. Ces modèles demandent cependant de connaître tous les paramètres électrochimiques, des données qui sont difficiles à obtenir expérimentalement. Les techniques d’estimation de paramètres simplifient l’obtention de ces données critiques pour représenter le comportement de la pile. Dans cette étude, une méthode d’estimation de paramètres a été introduite pour estimer les paramètres électrochimiques des BLI les plus influents, en prenant en compte différents matériaux d’électrode positive. La méthode proposée repose sur une amélioration du modèle à particule unique, qui représente lui-même une simplification du modèle pseudo-2D, le modèle électrochimique le plus connu et le plus complexe dans le domaine de la simulation de piles à électrodes poreuses. Les paramètres électrochimiques les plus importants ont été identifiés en négligeant la micro-structure de la batterie au Li-ion. Une étude de sensibilité a ensuite permis d’identifier les domaines temporels et les courants de décharge les plus favorables pour l’identification de chaque paramètre. Étant donné que le comportement des BLI dépend fortement des matériaux actifs utilisés pour la fabrication des électrodes, la méthodologie proposée a été testée sur 3 matériaux actifs différents (LiCoO2, LiMn2O4 and LiFePO4) employés dans la fabrication industrielle d’électrodes positives. Finalement, une autre amélioration du modèle à particule unique a été proposée et testée afin de mieux représenter le comportement spécifique du LiFePO4 (LFP), un matériau actif parmi les plus prometteurs pour l’électrode positive. Plus précisément, un modèle électrochimique simplifié incluant une équation représentant la variation de résistance en fonction du degré de décharge a été développé et les coefficients de cette équation ont été déterminés au moyen de la méthode d’estimation de paramètres proposée

Numerical simulation of frost growth and densification using deformable and static grids

The present thesis aims at developing a basis for the numerical simulation of the growth and densification of macroscopic frost sheets. Notwithstanding the fact that icing has still a long way to fully understand its physics, this work is potentially important, since a broad range of empirical correlations as well as the implementation of formerly neglected physical effects are studied herewith. In particular, this research is focused on developing methodologies for three-dimensional meshes, capable to predict the frost growth and densification of complex geometries. This work comprises five chapters. The first one is an introduction to icing and frost formation. The main motives that urge to numerically simulate frosting are detailed. Moreover, the different methods and approaches to model frosting are presented. From these, the main objectives and outline of the thesis are derived. The next two chapters are the core of this dissertation, which comprehend the two developed methods. In detail, the second chapter introduces a model that simulates the frost growth and densification using a moving mesh method. First, the most relevant empirical correlations used to describe the frost layer conductivity and diffusivity among others are tested by means of parametric studies. A thorough discussion on the performance of such parameters is made, emphasizing the fact of using diffusion resistance factors above 1.0 in order to capture the frost growth. The best-fitted solutions validate the model input combinations which give good agreements against experimental data under certain experimental conditions. Furthermore, the method is tested against a 2D numerical case, highlighting the main advantages of using a deformable grid, i.e. the accurate tracking of the air-frost interface. The third chapter introduces a fixed-grid-porous-media method capable of simulating the growth and densification of frost sheets. In pursuance of finding out possible explanations to the needed artificially enhanced diffusion resistance factors, a velocity field is calculated across all the domain. A porous media treatment is given to the frost layer, whereas the transported temperature and vapour density are used to define the thermophysical state of each cell, which might enable phase change. The method is tested with a study case of a duct flow with a non-homogeneously cooled lower boundary. The influence of accounting for the convection, as well as the enhanced diffusion resistance factors within the frost layer, are studied. The last two chapters contain the concluding remarks, as well as ideas on how the present work could be continued.La present tesi desenvolupa una base per a la simulació numèrica del creixement i densificació de capes de gebre macroscòpiques. Malgrat que la física de la formació de gel té encara un llarg camí per recórrer, aquest treball és potencialment important, ja que s'hi estudia un ampli rang de correlacions empíriques i la implementació d'efectes físics anteriorment negligits. En particular, aquesta recerca se centra en el desenvolupament de metodologies per a malles tridimensionals, capaces de predir la formació de gebre en geometries complexes. Aquest treball està format per cinc capítols. El primer és una introducció a la formació de gel. En aquest s'hi detallen els motius principals pels quals són necessàries eines numèriques per a la predicció d'aquest fenomen. A més, es presenten els mètodes existents per modelar el gebre. A partir d'aquests, es deriven els principals objectius de la tesi. El segon capítol introdueix un model que simula el creixement i densificació de gebre mitjançant un mètode de malla mòbil. En primer lloc, les correlacions empíriques més rellevants utilitzades per descriure la conductivitat de la capa de gel i la seva difusió entre d'altres són provades mitjançant estudis paramètrics. Es realitza una discussió exhaustiva del rendiment d'aquests paràmetres, fent èmfasi en l'ús de factors de resistència de difusió superiors a 1.0 per tal de capturar el creixement del gebre. A més, el mètode es prova amb un cas numèric 2D, destacant els principals avantatges d'utilitzar una malla deformable, és a dir, el seguiment precís de la interfície gel-aire. El tercer capítol introdueix un mètode de medi porós amb malla fixa, capaç de simular el creixement i densificació de capes de gebre. Amb la intenció de cercar explicacions que verifiquin la necessitat d'utilitzar factors de resistència de difusió artificialment augmentats, es calcula el camp de velocitat a tot el domini. Es realitza un tractament de medi porós en la zona del gebre, i la temperatura i densitat del vapor transportat s'utilitzen per definir l'estat termofísic de cada cel·la, que pot desencadenar un canvi de fase. El mètode es prova amb un cas d'estudi d'un canal de vent amb la paret inferior refredada de forma no homogènia. S'estudia la influència de tenir en compte la convecció, així com els factors de resistència augmentats dins de la capa de gel. Els dos darrers capítols contenen comentaris finals, així com idees sobre com es podria continuar el treball realitzat.Postprint (published version

Centralized Thermal Storage Systems Model for Buildings of the Future: Development and Validation

Thermal energy storage system (TES) is a promising technology for buildings heating and cooling applications. Energy storage systems have been widely used for reducing energy use from peak-demand to off-peak times. Among the various thermal storage technologies, phase change materials (PCMs) are the most commonly used approaches for storing thermal energy for buildings heating and cooling application. These materials enable buildings to store and retrieve a considerable amount of energy, typically by being integrated into structural components through a wide variety of TES techniques. A centralized energy storage system can provide a part of the heating and cooling requirements of a low-energy building. Relatively little general information pertaining to the thermal characteristics of latent heat thermal energy storage (LHTES) systems are available; further investigation is required to analyze the thermal performance of centralized LHTES systems in buildings. In this dissertation, a 3-dimensional mathematical model of a centralized LHTES system is conducted and validated for both a quasi-steady state and a transient conjugate heat transfer problem. The model is then used to carry out a parametric study to investigate the effect of geometrical parameters, charging and discharging times and mass flow rates on the long-term system performance. Based on the parameters that could affect the long-term system performance, artificial neural networks (ANN) are developed not only to reduce the computational time but also to relate the outlet air-temperature to the inlet air-temperature of LHTES. The database obtained from the numerical solution is first used to train the ANN and then utilized to evaluate the accuracy of the trained ANN. The developed model is then integrated with a building’s mechanical ventilation system to investigate the potential improvement in occupants’ thermal comfort level and energy efficiency arising from the integration of the LHTES. It was found that the temperature difference between the air as a heat transfer fluid (HTF) and the PCM melting point has a significant effect on the performances of a LHTES system. The thermal energy retrieved from the centralized LHTES system is the highest when the inlet air temperature is about 10K higher than the PCM mean melting temperature

Modeling of Corrugated Graphite Foam Heat Exchangers

A new manufacturing process was recently developed by the Oak Ridge National Laboratory for the production of graphite foam. The high thermal conductivity and heat transfer area of the foam make it desirable for thermal management applications such as compact heat exchangers. The heat transfer capabilities of the foam are especially useful in internal forced convection applications. However, due to the very low permeability of graphite foam, pressure-drop is very high compared to other porous media, and pressuredrop reduction schemes are necessary to ensure its practical application. Pressure-drop reduction can be achieved through the strategic machining of graphite foam into complex geometries. In this study, a corrugated configuration is analyzed. Computational methods are employed to determine optimal geometric parameters of corrugated graphite foam heat exchangers that result in a significant reduction in pressure-drop without severely affecting heat transfer performance. The results indicate that pressure-drop in optimal corrugated configurations is reduced by nearly two orders of magnitude when compared to a full graphite foam block of the same overall size. These optimal cases are characterized by relatively uniform flow in the transverse direction across the foam, and this uniformity is reduced by slot widths that are too long and narrow and by flowrates that are too high. These optimal configurations decrease the heat transfer coefficients by nearly fifty percent when compared to a full block, but the reduction of pressure-drop is much more significant. Therefore, corrugating the foam is shown to be an effective method to significantly reduce the pressure-drop without severely affecting the heat transfer performance of graphite foam heat exchangers

Pore structure characterization of low permeability rocks

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2014.Hoje as pesquisas em rochas de baixa permeabilidade (grande tendência no mundo e em breve na indústria petrolífera brasileira) se voltam à escala de poros seja para investigação petrofísica, morfológica, de distribuição de tamanhos de grãos ou poros ou escoamento de fluidos, prática descrita pelos valores de permeabilidade. A avaliação destas propriedades por sua vez, é essencial ao desenvolvimento e exploração de reservas de hidrocarbonetos. No entanto, a determinação de parâmetros do sistema poroso nessas rochas, arenitos de baixa permeabilidade (TGS) e rochas selantes (SR), continua a ser um grande desafio devido à extrema variabilidade de ambientes deposicionais e complexa microestrutura composta por argilas e tamanhos de poros de submícrons a ångströms. Nesta tese empregou-se um conjunto de técnicas experimentais para a caracterização da estrutura porosa de TGS e SR. De tal modo, o trabalho foi dividido em dois tópicos principais: (i) Caracterização do sistema poroso e propriedades petrofísicas em TGS utilizando-se as técnicas de permeabilidade por decaimento de pulso (PDP), NMR de baixo campo, adsorção gasosa N2 (N2GA), porosimetria por intrusão Hg (MICP), nano- e microtomografia de raios X (res. Abstract : Nowadays, significant research effort in low-permeability rocks (a wide tendency elsewhere and soon in the Brazilian petroleum industry) has been focused on pore-scale petrophysics, morphologies and distributions, as well as fluid flow circulation described by the values of permeability. The evaluation of these properties in turn is essential for the assessment and exploitation of hydrocarbon reserves; however, determining pore system parameters in such rocks as tight gas sandstones (TGS) and seal rocks (SR) remains challenging because of the extreme variability in depositional environments resulting in complex pore structures comprised by clays and length scales from sub-microns to Angstroms. In this work we applied a set of techniques to characterize submicron-pore structures in TGS and SR. Therefore it was divided into two main topics of interest: (i) Characterization of petrophysical properties and pore systems in very low permeability TGS using Pulse-Decay Permeability (PDP), Low Field Nuclear Magnetic Resonance (LFNMR), Nitrogen Gas Adsorption (N2GA), Mercury Intrusion Capillary Pressure (MICP) and Multi-scale 3D X-ray Nano- and MicroCT (down to 0.7 µm resolution) techniques; (ii) Study of Photoacoustic Spectrometry (PAS) for determining thermal diffusivity (TD) and porosity in three seal rocks originating from dissimilar fields as a key issue for safe exploration, storage purposes (CO2 sequestration) and developments in shale characterization. The values obtained for TD were between 0.01667 and 0.09298 (cm2/s) while porosity ranged from 1.42 to 9%. For the analyzed TGS the 3D pore-structure characterization lead to pore tortuosity and shape factors ranges of 2.19-5.47 and 3.2-8.5, respectively, and pore size distributions tended to be bimodal for MICP, trimodal for 3D multi-scale and tetramodal for LFNMR measurements. The porosity values ranged from 1.94 to 11.96% obtained by the combination of N2GA and MICP techniques and permeability from 0.036 to 0.00066 mD by PDP technique. The measured pore-structure parameters were also used to predict empirical permeability in TGS (using e.g. Carman-Kozeny (Dullien, 1992) and Coates (1999) models). The set of applied methods has shown to be a useful tool for the unconventional reservoir characterization since it allows obtaining pore morphological and quantitative parameters which account for the permeability values

Double population cascaded lattice boltzmann method

Lattice Boltzmann Methods (LBM) are powerful numerical tools to simulate heat and mass transfer problems. Instead of directly integrating the N-S equations, LBM solves the discretized form of the Boltzmann Transport Equation (BTE), keeping track of the microscopic description of the systems. Therefore, LBM can solve fluid flows with great stability and computational efficiency, especially complex geometry fluid flows. For thermal flows, double distribution function (DDF) LBM scheme is the most popular and successful approach. But it is evident from the literature that existing double distribution function (DDF) LBM approaches, which use two collision operators, involve collision schemes which violate Galilean invariance, therefore producing instabilities for flows with high Re and Ra numbers. In this thesis, a double population cascaded lattice Boltzmann method is developed to improve the DDF LBM scheme from this drawback. The proposed method reduces the degree of violation of Galilean invariance, increasing the stability and accuracy of the LBM scheme. The scheme was implemented to simulate advection-diffusion, forced convection and natural convection heat transfer problems. The proposed scheme was also successfully tested for turbulent flow regimes and 3-D fluid flow in porous media. The results obtained from this work are in strong agreement with those available in the literature obtained through other numerical methods and experiments.Os métodos de ”Lattice”Boltzmann (LBM) são potentes ferramentas numéricas para simular problemas de transferência de massa e calor. Ao invés de integrar diretamente as equações de Navier-Stokes, o método LBM resolve, de forma discretizada, a equação de transporte de Boltzmann, acompanhando a descrição microscópica dos sistemas. O método LBM pode solucionar fluxo de fluidos com grande estabilidade e eficiência computacional, especialmente fluxos em geometrias complexas. Para fluxos térmicos, o esquema LBM de dupla função de distribuição (DDF) é a abordagem mais popular e bem sucedida. Mas é evidente, a partir da literatura, que as abordagens LBM de dupla função de distribuição (DDF), as quais utilizam dois operadores de colisão, envolvem esquemas de colisão que violam a invariância de Galileu, produzindo instabilidades para fluxos com números Re e Ra altos. Nesta tese, o método de ”Lattice”Boltzmann em cascata de dupla população em cascata é desenvolvido para corrigir o esquema DDF LBM. O método proposto reduz o grau de violação da invariância de Galileu, aumentando a estabilidade e acurácia do método LBM. O método foi implementado para simular problemas de advecção, difusão, convecções natural e forçada típicos de transferências de calor. O esquema proposto foi também bem sucedido em regimes de fluxo turbulento e em escoamentos 3-D em meios porosos. Os resultados obtidos neste trabalho estão fortemente de acordo com experimentos e métodos numéricos disponíveis na literatura

Direct Pore Level Simulation of Heat Transfer in Open Cell Reticulated Porous Ceramics

The project involved in studying the fluid transport, heat and mass transport inside various ceramic porous inserts by Direct Pore Level Simulations (DPLS). The geometric grid data required for the simulations are reconstructed from the computer tomographic scan images of the real porous media. The simulation results are used to study the influence of the structural properties of porous media on the fluid flow, heat transfer and mass transfer