Numerical simulation of microwave thawing using a coupled solver approach

Thawing of a frozen food product in a domestic microwave oven is numerically simulated using a coupled solver approach. The approach consists of a dedicated electromagnetic FDTD solver and a closely coupled UFVM multi-physics package. Two overlapping numerical meshes are defined; the food material and container were meshed for heat transfer and phase change solution, whilst the microwave oven cavity and waveguide were meshed for the microwave irradiation. The two solution domains were linked using a cross-mapping routine. This approach allowed the rotation of the food load to be captured. Power densities obtained on the structured FDTD mesh were interpolated onto the UFVM mesh for each timestep/turntable position. The UFVM solver utilised the power density data to advance the temperature and phase distribution solution. The temperature-dependant dielectric and thermo-physical properties of the food load were updated prior to revising the electromagnetic solution. Changes in thermal/electric properties associated with the phase transition were fully accounted for as well as heat losses from product to cavity. Two scenarios were investigated: a centric and eccentric placement on the turntable. Developing temperature fields predicted by the numerical solution are validated against experimentally obtained data. Presented results indicate the feasibility of fully coupled simulations of the microwave heating of a frozen product. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of the Physical Properties of Room Temperature Ionic Liquids Using a Love Wave Device

In this work, we have shown that a 100 MHz Love wave device can be used to determine whether room temperature ionic liquids (RTILs) are Newtonian fluids and have developed a technique that allows the determination of the density-viscosity product, rho eta of a Newtonian RTIL. In addition, a test for a Newtonian response was established by relating the phase change to insertion loss change. Five concentrations of a water-miscible RTIL and seven pure RTILs were measured. The changes in phase and insertion loss were found to vary linearly with the square root of the density-viscosity product for values up to (rho eta)(1/2) similar to 10 kg m(-2) s(-1/2). The square root of the density-viscosity product was deduced from the changes in either phase or insertion loss using glycerol as a calibration liquid. In both cases, the deduced values of rho eta agree well with those measured using viscosity and density meters. Miniaturization of the device, beyond that achievable with the lower-frequency quartz crystal microbalance approach, to measure smaller volumes is possible. The ability to fabricate Love wave and other surface acoustic wave sensors using planar metallization technologies gives potential for future integration into lab-on-a-chip analytical systems for characterizing ionic liquids.

Optical quantum memory for polarization qubits with V-type three-level atoms

We investigate an optical quantum memory scheme with V-type three-level atoms based on the controlled reversible inhomogeneous broadening (CRIB) technique. We theoretically show the possibility to store and retrieve a weak light pulse interacting with the two optical transitions of the system. This scheme implements a quantum memory for a polarization qubit - a single photon in an arbitrary polarization state - without the need of two spatially separated two-level media, thus offering the advantage of experimental compactness overcoming the limitations due to mismatching and unequal efficiencies that can arise in spatially separated memories. The effects of a relative phase change between the atomic levels, as well as of phase noise due to, for example, the presence of spurious electric and magnetic fields are analyzed.

Argamassas funcionais para uma construção sustentável

A aposta na sustentabilidade tem conduzido o mercado da construção a procurar novas soluções técnicas e novos materiais, que por serem mais eficientes, conseguem dar resposta aos requisitos cada vez mais exigentes deste sector. A aplicação dos conceitos de sustentabilidade não se pode restringir a novas construções, tendo que prever também as renovações e a reabilitação de edifícios antigos. Assim, novos materiais que sejam desenvolvidos, devem contemplar todas estas vertentes de utilização. Neste trabalho desenvolveram-se argamassas com novas funcionalidades, que contribuem para melhorar os níveis de sustentabilidade dos edifícios, através da incorporação de nanomateriais para armazenamento de calor latente e degradação de poluentes do ar interior. Estudou-se não só o impacto da incorporação destas nanopartículas no estado fresco e endurecido, mas também o seu desempenho do ponto de vista funcional quando integrados na estrutura da argamassa. É possível obter argamassas com capacidade para armazenar calor latente através da incorporação de um material de mudança de fase. Este material constituído por uma mistura de parafinas, consegue armazenar calor e libertálo posteriormente. As composições desenvolvidas podem ser aplicadas em novos projectos ou na reabilitação de edifícios contribuindo para reduzir o consumo energético, melhorando o conforto térmico no interior. Com a redução da factura energética obtém-se uma efectiva diminuição do impacto ambiental, energético e económico do edifício. Para além do armazenamento de calor latente, também se desenvolveram argamassas capazes de eliminar poluentes do ar interior e, simultaneamente, com capacidade de auto-limpeza. Utilizaram-se nanopartículas de dióxido de titânio como aditivo fotocatalítico, tendo-se analisado o efeito da introdução deste aditivo nas argamassas. As composições testadas demonstraram elevada capacidade fotocatalítica e de auto-limpeza, sem comprometer as suas propriedades no estado endurecido. Ao aplicar estas composições na camada de acabamento interior melhora-se a qualidade do ar no interior das habitações e reduz-se a necessidade de utilização de sistemas de ventilação. As argamassas funcionais contribuem para melhorar os níveis de sustentabilidade da construção, tendo impacto económico e ambiental em todo o ciclo de vida do edifício.

Chemical vapour deposition of vanadium oxide thermochromic thin films

Thermochromic materials change optical properties, such as transmittance or reflectance, with a variation in temperature. An ideal intelligent (smart) material will allow solar radiation in through a window in cold conditions, but reflect that radiation in warmer conditions. The variation in the properties is often associated with a phase change, which takes place at a definite temperature, and is normally reversible. Such materials are usually applied to window glass as thin films. This thesis presents the work on the development of thermochromic vanadium (IV) oxide (VO2) thin films – both undoped and doped with tungsten, niobium and gold nanoparticles – which could be employed as solar control coatings. The films were deposited using Chemical Vapour Deposition (CVD), using improved Atmospheric Pressure (APCVD), novel Aerosol Assisted (AACVD) and novel hybrid AP/AACVD techniques. The effects of dopants on the metalto- semiconductor transition temperature and transmittance/reflectance characteristics were also investigated. This work significantly increased the understanding of the mechanisms behind thermochromic behaviour, and resulted in thermochromic materials based on VO2 with greatly improved properties.

Cinética da transferência de calor em materiais com mudança de fase

É extensa a bibliografia dedicada a potenciais aplicações de materiais com mudança de fase na regulação térmica e no armazenamento de calor ou de frio. No entanto, a baixa condutividade térmica impõe limitações numa grande diversidade de aplicações com exigências críticas em termos de tempo de resposta curto ou com requisitos de elevada potência em ciclos de carga/descarga de calor latente. Foram desenvolvidos códigos numéricos no sentido de obter soluções precisas para descrever a cinética da transferência de calor com mudança de fase, com base em geometrias representativas, i.e. planar e esférica. Foram igualmente propostas soluções aproximadas, sendo identificados correspondentes critérios de validação em função das propriedades dos materiais de mudança de fase e de outros parâmetros relevantes tais como as escalas de tamanho e de tempo, etc. As referidas soluções permitiram identificar com rigor os fatores determinantes daquelas limitações, quantificar os correspondentes efeitos e estabelecer critérios de qualidade adequados para diferentes tipologias de potenciais aplicações. Os referidos critérios foram sistematizados de acordo com metodologias de seleção propostas por Ashby e co-autores, tendo em vista o melhor desempenho dos materiais em aplicações representativas, designadamente com requisitos ao nível de densidade energética, tempo de resposta, potência de carga/descarga e gama de temperaturas de operação. Nesta sistematização foram incluídos alguns dos compósitos desenvolvidos durante o presente trabalho. A avaliação das limitações acima mencionadas deu origem ao desenvolvimento de materiais compósitos para acumulação de calor ou frio, com acentuada melhoria de resposta térmica, mediante incorporação de uma fase com condutividade térmica muito superior à da matriz. Para este efeito, foram desenvolvidos modelos para otimizar a distribuição espacial da fase condutora, de modo a superar os limites de percolação previstos por modelos clássicos de condução em compósitos com distribuição aleatória, visando melhorias de desempenho térmico com reduzidas frações de fase condutora e garantindo que a densidade energética não é significativamente afetada. Os modelos elaborados correspondem a compósitos de tipo core-shell, baseados em microestruturas celulares da fase de elevada condutividade térmica, impregnadas com o material de mudança de fase propriamente dito. Além de visarem a minimização da fração de fase condutora e correspondentes custos, os modelos de compósitos propostos tiveram em conta a adequação a métodos de processamento versáteis, reprodutíveis, preferencialmente com base na emulsificação de líquidos orgânicos em suspensões aquosas ou outros processos de reduzidas complexidade e com base em materiais de baixo custo (material de mudança de fase e fase condutora). O design da distribuição microestrutural também considerou a possibilidade de orientação preferencial de fases condutoras com elevada anisotropia (p.e. grafite), mediante auto-organização. Outros estágios do projeto foram subordinados a esses objetivos de desenvolvimento de compósitos com resposta térmica otimizada, em conformidade com previsões dos modelos de compósitos de tipo core-shell, acima mencionadas. Neste enquadramento, foram preparados 3 tipos de compósitos com organização celular da fase condutora, com as seguintes características e metodologias: i) compósitos celulares parafina-grafite para acumulação de calor, preparados in-situ por emulsificação de uma suspensão de grafite em parafina fundida; ii) compósitos celulares parafina-Al2O3 para acumulação de calor, preparados por impregnação de parafina em esqueleto cerâmico celular de Al2O3; iii) compósitos celulares para acumulação de frio, obtidos mediante impregnação de matrizes celulares de grafite com solução de colagénio, após preparação prévia das matrizes de grafite celular. Os compósitos com esqueleto cerâmico (ii) requereram o desenvolvimento prévio de um método para o seu processamento, baseado na emulsificação de suspensões de Al2O3 em parafina fundida, com adequados aditivos dispersantes, tensioactivos e consolidantes do esqueleto cerâmico, tornando-o auto-suportável durante as fases posteriores de eliminação da parafina, até à queima a alta temperatura, originando cerâmicos celulares com adequada resistência mecânica. Os compósitos desenvolvidos apresentam melhorias significativos de condutividade térmica, atingindo ganhos superiores a 1 ordem de grandeza com frações de fase condutora inferior a 10 % vol. (4 W m-1 K-1), em virtude da organização core-shell e com o contributo adicional da anisotropia da grafite, mediante orientação preferencial. Foram ainda preparados compósitos de armazenamento de frio (iii), com orientação aleatória da fase condutora, obtidos mediante gelificação de suspensões de partículas de grafite em solução aquosa de colagénio. Apesar da estabilidade microestrutural e de forma, conferida por gelificação, estes compósitos confirmaram a esperada limitação dos compósitos com distribuição aleatória, em confronto com os ganhos alcançados com a organização de tipo core-shell.

Quasi steady state and dynamic hygrothermal performance of fibrous Hemp and Stone Wool insulations: Two innovative laboratory based investigations

Moisture and heat management properties of Hemp and Stone Wool insulations were studied by mounting them between a hot and a cold climate chamber. Both insulations were exposed to identical hygrothermal boundary conditions. Quasi steady state and dynamic tests were carried out at a range of relative humidity exposures. The likelihood of interstitial condensation was assessed and equivalent thermal conductivity values of the insulations were determined. The adsorption-desorption isotherms of the insulations were also determined in a dynamic vapour sorption (DVS) instrument. It was observed that the likelihood of condensation was higher in Stone Wool insulation than in Hemp insulation. Hemp insulation performed better in managing moisture due to its high hygric inertia and water absorption capacity. It was observed that the equivalent thermal conductivity of Stone Wool insulation was dependent on enthalpy flow and phase change of moisture. The equivalent thermal conductivity of Hemp insulation was close to its declared thermal conductivity in dynamic conditions when high relative humidity exposures were transient. In quasi steady state boundary conditions, when the insulation was allowed to reach the equilibrium moisture content at ranges of relative humidity, there was a moisture dependent increase of thermal conductivity in Hemp insulation.

Materiais de mudança de fase em revestimentos

Dissertação para obtenção do grau de Mestre em Engenharia Civil na Área de especialização em Edificações

CuxSnSx+1 (x = 2, 3) thin films grown by sulfurization of metallic precursors deposited by dc magnetron sputtering

We report the results of the growth of Cu-Sn-S ternary chalcogenide compounds by sulfurization of dc magnetron sputtered metallic precursors. Tetragonal Cu2SnS3 forms for a maximum sulfurization temperature of 350 ºC. Cubic Cu2SnS3 is obtained at sulfurization temperatures above 400 ºC. These results are supported by XRD analysis and Raman spectroscopy measurements. The latter analysis shows peaks at 336 cm-1, 351 cm-1 for tetragonal Cu2SnS3, and 303 cm-1, 355 cm-1 for cubic Cu2SnS3. Optical analysis shows that this phase change lowers the band gap from 1.35 eV to 0.98 eV. At higher sulfurization temperatures increased loss of Sn is expected in the sulphide form. As a consequence, higher Cu content ternary compounds like Cu3SnS4 grow. In these conditions, XRD and Raman analysis only detected orthorhombic (Pmn21) phase (petrukite). This compound has Raman peaks at 318 cm-1, 348 cm-1 and 295 cm-1. For a sulfurization temperature of 450 ºC the samples present a multi-phase structure mainly composed by cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4. For higher temperatures, the samples are single phase and constituted by orthorhombic (Pmn21) Cu3SnS4. Transmittance and reflectance measurements were used to estimate a band gap of 1.60 eV. For comparison we also include the results for Cu2ZnSnS4 obtained using similar growth conditions.

Reabilitação energética de um edifício de serviços: convergência para NZEB

Dissertação de Natureza Científica para obtenção do grau de Mestre em Engenharia Civil na Área de Especialização de Edificações

Impact of considering the effects of latent heat on caldera-hosted hydrothermal systems

To investigate the thennal effects of latent heat in hydrothennal settings, an extension was made to the existing finite-element numerical modelling software, Aquarius. The latent heat algorithm was validated using a series of column models, which analysed the effects of penneability (flow rate), thennal gradient, and position along the two-phase curve (pressure). Increasing the flow rate and pressure increases displacement of the liquid-steam boundary from an initial position detennined without accounting for latent heat while increasing the thennal gradient decreases that displacement. Application to a regional scale model of a caldera-hosted hydrothennal system based on a representative suite of calderas (e.g., Yellowstone, Creede, Valles Grande) led to oscillations in the model solution. Oscillations can be reduced or eliminated by mesh refinement, which requires greater computation effort. Results indicate that latent heat should be accounted for to accurately model phase change conditions in hydrothennal settings.

Variation des biomarqueurs dans le spectre visible non résolu de la Terre

L’évolution rapide des technologies de détection et de caractérisation des exoplanètes depuis le début des années 1990 permet de croire que de nouveaux instruments du type Terrestrial Planet Finder (TPF) pourront prendre les premiers spectres d’exoplanètes semblables à la Terre d’ici une ou deux décennies. Dans ce contexte, l’étude du spectre de la seule planète habitée connue, la Terre, est essentielle pour concevoir ces instruments et analyser leurs résultats. Cette recherche présente les spectres de la Terre dans le visible (390-900 nm), acquis lors de 8 nuits d’observation étalées sur plus d’un an. Ces spectres ont été obtenus en observant la lumière cendrée de la Lune avec le télescope de 1.6 m de l’Observatoire du Mont-Mégantic (OMM). La surface de la Lune réfléchissant de manière diffuse la lumière provenant d’une portion de la Terre, ces spectres sont non résolus spatialement. L’évolution de ces spectres en fonction de la lumière réfléchie à différentes phases de Terre est analogue à celle du spectre d’une exoplanète, dont la phase change selon sa position autour de l’étoile. L'eau, l'oxygène et l'ozone de l’atmosphère, détectés dans tous nos spectres, sont des biomarqueurs dont la présence suggère l’habitabilité de la planète et/ou la présence d’une activité biologique. Le Vegetation Red Edge (VRE), une autre biosignature spectrale, dû aux organismes photosynthétiques à la surface, est caractérisé par l’augmentation de la réflectivité autour de 700 nm. Pour les spectres de 5 nuits, cette augmentation a été évaluée entre -5 et 15% ±~5%, après que les contributions de la diffusion de Rayleigh, des aérosols et d’une large bande moléculaire de l’ozone aient été enlevées. Les valeurs mesurées sont cohérentes avec la présence de végétation dans la phase de la Terre contribuant au spectre, mais s’étendent sur une plage de variations plus large que celles trouvées dans la littérature (0-10%). Cela pourrait s’expliquer par des choix faits lors de la réduction des données et du calcul du VRE, ou encore par la présence d’autres éléments de surface ou de l’atmosphère dont la contribution spectrale autour de 700 nm serait variable.

Microwave Dielectric Properties of RETiTaO6 (RE = La, Ce,Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) Ceramics

Microwave dielectric ceramics based on RETiTaO6 (RE = La, Cc, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) were prepared using a conventional solid-state ceramic route. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscopy techniques. The sintered samples were characterized in the microwave frequency region. The ceramics based on Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy, which crystallize in orthorhombic aeschynite structure, had a relatively high dielectric constant and positive T f while those based on Ho, Er, and Yb, with orthorhombic euxenite structure, had a low dielectric constant and negative Tf. The RETiTaO6 ceramics had a high-quality factor. The dielectric constant and unit cell volume of the ceramics increased with an increase in ionic radius of the rare-earth ions, but density decreased with it. The value of Tf increased with an increase in RE ionic radii, and a change in the sign of Tf occurred when the ionic radius was between 0.90 and 0.92 A. The results indicated that the boundary of the aeschynite to euxenite morphotropic phase change lay between DyTiTaO6 and HoTiTaO6. Low-loss ceramics like ErTiTaO6 (Er = 20.6, Qxf = 85,500), EuTiTaO6 (Er = 41.3, Qxf = 59,500), and YTiTaO6 (Er = 22.1, Q„xf = 51,400) are potential candidates for dielectric resonator applications

Development of solar air heaters & thermal energy storage system for drying applications in food processing industries

In the present work, the author has designed and developed all types of solar air heaters called porous and nonporous collectors. The developed solar air heaters were subjected to different air mass flow rates in order to standardize the flow per unit area of the collector. Much attention was given to investigate the performance of the solar air heaters fitted with baffles. The output obtained from the experiments on pilot models, helped the installation of solar air heating system for industrial drying applications also. Apart from these, various types of solar dryers, for small and medium scale drying applications, were also built up. The feasibility of ‘latent heat thermal energy storage system’ based on Phase Change Material was also undertaken. The application of solar greenhouse for drying industrial effluent was analyzed in the present study and a solar greenhouse was developed. The effectiveness of Computational Fluid Dynamics (CFD) in the field of solar air heaters was also analyzed. The thesis is divided into eight chapters.

Synthesis, Characterization and Applications of Hybrid Nanocomposites of TiO2 With Conducting Polymers

A nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm), or structures having nano-scale repeat distances between the different phases that make up the material. In the broadest sense this definition can include porous media, colloids, gels and copolymers, but is more usually taken to mean the solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite will differ markedly from that of the component materials. Size limits for these effects have been proposed, <5 nm for catalytic activity, <20 nm for making a hard magnetic material soft, <50 nm for refractive index changes, and <100 nm for achieving superparamagnetism, mechanical strengthening or restricting matrix dislocation movement. Conducting polymers have attracted much attention due to high electrical conductivity, ease of preparation, good environmental stability and wide variety of applications in light-emitting, biosensor chemical sensor, separation membrane and electronic devices. The most widely studied conducting polymers are polypyrrole, polyaniline, polythiophene etc. Conducting polymers provide tremendous scope for tuning of their electrical conductivity from semiconducting to metallic region by way of doping and are organic electro chromic materials with chemically active surface. But they are chemically very sensitive and have poor mechanical properties and thus possessing a processibility problem. Nanomaterial shows the presence of more sites for surface reactivity, they possess good mechanical properties and good dispersant too. Thus nanocomposites formed by combining conducting polymers and inorganic oxide nanoparticles possess the good properties of both the constituents and thus enhanced their utility. The properties of such type of nanocomposite are strongly depending on concentration of nanomaterials to be added. Conducting polymer composites is some suitable composition of a conducting polymer with one or more inorganic nanoparticles so that their desirable properties are combined successfully. The composites of core shell metal oxide particles-conducting polymer combine the electrical properties of the polymer shell and the magnetic, optical, electrical or catalytic characteristics of the metal oxide core, which could greatly widen their applicability in the fields of catalysis, electronics and optics. Moreover nanocomposite material composed of conducting polymers & oxides have open more field of application such as drug delivery, conductive paints, rechargeable batteries, toners in photocopying, smart windows, etc.The present work is mainly focussed on the synthesis, characterization and various application studies of conducting polymer modified TiO2 nanocomposites. The conclusions of the present work are outlined below, Mesoporous TiO2 was prepared by the cationic surfactant P123 assisted hydrothermal synthesis route and conducting polymer modified TiO2 nanocomposites were also prepared via the same technique. All the prepared systems show XRD pattern corresponding to anatase phase of TiO2, which means that there is no phase change occurring even after conducting polymer modification. Raman spectroscopy gives supporting evidence for the XRD results. It also confirms the incorporation of the polymer. The mesoporous nature and surface area of the prepared samples were analysed by N2 adsorption desorption studies and the mesoporous ordering can be confirmed by low angle XRD measurementThe morphology of the prepared samples was obtained from both SEM & TEM. The elemental analysis of the samples was performed by EDX analysisThe hybrid composite formation is confirmed by FT-IR spectroscopy and X-ray photoelectron spectroscopyAll the prepared samples have been used for the photocatalytic degradation of dyes, antibiotic, endocrine disruptors and some other organic pollutants. Photocatalytic antibacterial activity studies were also performed using the prepared systemsAll the prepared samples have been used for the photocatalytic degradation of dyes, antibiotic, endocrine disruptors and some other organic pollutants. Photocatalytic antibacterial activity studies were also performed using the prepared systems Polyaniline modified TiO2 nanocomposite systems were found to have good antibacterial activity. Thermal diffusivity studies of the polyaniline modified systems were carried out using thermal lens technique. It is observed that as the amount of polyaniline in the composite increases the thermal diffusivity also increases. The prepared systems can be used as an excellent coolant in various industrial purposes. Nonlinear optical properties (3rd order nonlinearity) of the polyaniline modified systems were studied using Z scan technique. The prepared materials can be used for optical limiting Applications. Lasing studies of polyaniline modified TiO2 systems were carried out and the studies reveal that TiO2 - Polyaniline composite is a potential dye laser gain medium.