Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work had the purpose to analyze the heat transfer, promoted by buoyancy forces, of colloidal dispersions of nanoparticles in transformer oil inside closed cavities. To achieve this purpose, measurements of some properties of the dispersions of diamond and carbon nanotubes in transformer oil, produced by a two-step method, were made: thermal conductivity, by the transient hot wire method; dynamic viscosity, by rheometers of concentric cylinders with shear stress controlled; and dielectric strength, with a dielectric strength analyzer of manual adjust. The results of these measurements were used in numerical simulations of the ows of the dispersions and of the transformer oil inside closed cavities. The numerical results were obtained via two numerical codes, based on Finite Volume Method (FVM), one for 2D ows, that had its development as part of this work, and the other for 3D ows, previously developed. The results showed that, in terms of heat transfer, the nanoparticles dispersions have increased about 23% the convection heat transfer coe_cient of the pure oil for a same Grashof number. However, the use of these nanoparticles led a reduce of 95% in the dielectric strength of the pure oil. Therefore, these results are very important in terms of the applicability of nanoparticles dispersions in electrical transformers. / Este trabalho teve o objetivo de analisar a transferência de calor, por convecção natural, de dispersões coloidais de nanopartículas em óleo de transformador elétrico no interior de cavidades fechadas. No cumprimento deste objetivo foram realizadas medições de algumas propriedades das dispersões de nanopartículas de diamante e nanotubos de carbono em óleo de transformador elétrico, produzidas por um método de dois passos: condutividade térmica, pelo método transiente do fio quente; viscosidade dinâmica, por meio de reômetros de cilindros concêntricos com tensão cisalhante controlada e rigidez dielétrica, com um analisador de rigidez dielétrica de ajuste manual. Os resultados destas medições foram utilizados nas simulações numéricas dos escoamentos das dispersões e do óleo de transformador elétrico em cavidades fechadas. Os resultados numéricos foram obtidos por meio de dois códigos numéricos, baseados no Método dos Volumes Finitos (MVF), um para escoamentos 2D, que teve seu desenvolvimento como parte integrante deste trabalho, e o outro para escoamentos 3D, desenvolvido anteriormente. Os resultados mostraram que, em termos da transferência de calor, as dispersões de nanopartículas incrementaram o coeficiente de transferência de calor do óleo puro em ate 23% para um mesmo numero de Grashof. Porem, o uso de nanopartículas levou a uma redução de 95% na rigidez dielétrica do óleo puro. Portanto, estes resultados são de grande importância em termos da aplicabilidade de dispersões de nanopartículas em transformadores elétricos. / Mestre em Engenharia Mecânica

FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Atualmente nas indústrias é notória a presença de problemas envolvendo a dinâmica dos fluidos com transferência de energia térmica. Neste contexto, problemas onde a transferência de energia térmica ocorre sobre cilindros aquecidos merece um destaque especial. No presente trabalho objetivou-se desenvolver uma análise numérica tridimensional sobre o problema de transferência de energia térmica por convecção natural em cilindros concêntricos na presença de pares discretos de fonte-sumidouro de energia térmica, com ênfase no estudo do regime estável-instável do escoamento (fluido de trabalho ar). O código numérico foi desenvolvido em coordenadas cilíndricas, discretizado utilizando a técnica dos volumes finitos e esquemas temporais e espaciais de segunda ordem, onde o acoplamento pressão velocidade é feito através do método do passo fracionado. Através dos campos de velocidades, de temperatura e de vorticidade obtidos, verificou-se como a transferência de energia térmica é afetada pelas primeiras instabilidades no regime instável. Foi possível também, evidenciar a desestabilização do escoamento estudado, não sendo necessário para tal o uso de uma malha fina. Além disso, os dados obtidos apresentaram uma excelente concordância com os resultados experimentais da literatura, sobretudo no regime estável. / In the current industry, is notorious the presence of problems involving fluid dynamics with heat transfer. In this context, problems where the heat transfer occurs over heated cylinders deserves a special attention. The present work study aimed to develop a three-dimensional numerical analysis of the heat transfer by natural convection in concentric cylinders in the presence of discrete heat source-sink pairs, with emphasis on the study of stable-unstable regime flow (air working fluid). The numerical code was developed in cylindrical coordinates, discretized using the finite volume technique and temporal/ spatial schemes of second order, where the pressure-velocity coupling is done through the fractional step method. Through velocity, temperature and vorticity fields, it was found that the heat transfer is affected by the first instabilities in the unstable regime. It was also possible to verify the destabilization of the flow with a thin mesh. Moreover, the data obtained showed an excellent agreement with the experimental data found in the literature, especially in the stable regime. / Dissertação (Mestrado)

Made available in DSpace on 2014-10-09T12:43:21Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:33Z (GMT). No. of bitstreams: 1 05066.pdf: 8948400 bytes, checksum: 15fab2b2e51c4072c160d3e9ae523bd7 (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Orientador : Amauri Garcia / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-04T03:13:50Z (GMT). No. of bitstreams: 1 Spinelli_JoseEduardo_D.pdf: 12716637 bytes, checksum: 79131dc23b1f9de6e4d77299eeff6973 (MD5) Previous issue date: 2005 / Resumo: Poucos estudos têm analisado os efeitos da convecção no líquido interdendrítico, bem como a influência da direção de crescimento em relação à gravidade nas variáveis térmicas e estruturais durante a solidificação. Neste trabalho, foi utilizado um dispositivo de solidificação unidirecional vertical descendente, constituído por uma lingoteira de aço inoxidável com diâmetro interno de 60 mm, 157 mm de comprimento e 9 mm de espessura. Na parte superior, foi posicionada uma câmara de refrigeração a água do mesmo material do molde e com espessura útil de 3,0 mm. Após a obtenção dos lingotes e registrados os respectivos perfis térmicos experimentais, foram determinadas as variáveis térmicas de solidificação: coeficiente de transferência de calor metal/molde, velocidades de deslocamento da isoterma liquidus, gradientes térmicos e taxas de resfriamento para a solidificação unidirecional descendente de ligas hipoeutéticas do sistema Sn-Pb (Sn5%Pb, Sn15%Pb e Sn20%Pb). Estas variáveis térmicas são confrontadas com as previsões teóricas de um modelo numérico de solidificação e, em seguida, correlacionadas com os parâmetros estruturais: espaçamentos dendríticos primários, secundários e transição colunar/equiaxial (TCE). Dessa forma, são determinadas equações experimentais de crescimento para a solidificação descendente e verificado um critério de previsão da TCE. Realiza-se também uma análise comparativa dos presentes resultados com aqueles obtidos para solidificação vertical ascendente de ligas de mesma composição. Os resultados comparativos mostram que o efeito convectivo estimula a ocorrência da TCE e ainda é responsável por uma sensível redução dos espaçamentos dendríticos primários. As previsões teóricas de modelos de crescimento dendrítico representativos da literatura são confrontadas com os resultados experimentais / Abstract: Only a few studies have reported influences of interdendritic convection and growth direction on dendrite arm spacings. A downward directional solidification apparatus was used having a stainless steel split mold with an internal diameter of 60 mm, height 157 mm and a 9 mm wall thickness. The upper part of the split mold was closed with a water-cooling chamber made of stainless steel, with a wall thiclmess of 3 mm. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat transfer coefficients, tip growth rates, thermal gradients and cooling rates during downward unsteady state solidification ofhypoeutectic Sn-Pb alloys. The resulting thermosolutal convection can start in the melt both within the interdendritic region and ahead of the dendrite array. The experimental results have shown that melt convection may be causing pileup of fractioned dendritic arms, which must stimulate the CET occurrence. The results have supported a criterion recently proposed based on a critical tip cooling rate. For upward unidirectional condition this critical value was found to be about 0.014 K/s for hypoeutectic Sn-Pb alloys. In the present study, in conditions of downward solidification, melt convection seems to favor the structural transition, which is anticipated and occurs for a critical cooling rate of about 0.03 K/s, for any of three hypoeutectic alloys experimentally examined. Primary dendritic arm spacings have been affected by the direction of growth, decreasing in conditions of downward vertical solidification when compared with those grown vertically upwards. The main predictive theoretical models for dendritic spacings are compared with the experimental observations / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

La réduction de la consommation des bâtiments passe par : l'économie d'énergie, l'efficacité énergétique et l'utilisation des énergies renouvelables. Sur ce dernier point, l'intégration à grande échelle de composants photovoltaïques (PV) est une solution. Le rendement et la durée de vie des cellules PV en silicium cristalin diminuant avec l'augmentation de leur température de fonctionnement, il est essentiel de mettre au point des configurations d'intégration limitant leur échauffement. L'intégration en configuration de double-peau – la surface PV est séparée du bâtiment par une lame d'air – est une solution prometteuse. Sous l'effet de la chaleur, un écoulement de convection naturelle se met en place entre les deux parois, refroidissant ainsi les panneaux PV. Cet écoulement peut également servir de moteur pour la ventilation en été, et de préchauffage de l'air en hiver. Cette étude, expérimentale, fait partie d'un projet visant à comprendre le fonctionnement des double-peaux PV en analysant séparément les différents phénomènes physiques avant de prendre en compte l'ensemble des couplages. Elle porte plus particulièrement sur la convection naturelle au sein des double-façades verticales. Ici, la double-peau est modélisée par un canal vertical dont les deux parois principales sont chauffées sous des conditions de flux imposé. Un banc d'essais a été développé pour étudier la convection naturelle dans un canal vertical en eau. L'eau a été choisie pour se placer dans le cas d'un écoulement de convection pure (pas de rayonnement entre les parois). C'est un canal vertical de 65 cm de haut avec un écartement réglable placé dans une cuve de 1,5 m de haut contenant 160 L d'eau. Les parois sont chauffées à l'aide de 24 chaufferettes indépendantes délivrant un flux de chaleur uniforme, ce qui permet d'appliquer différentes configurations de chauffage. Des mesures de flux et de température sont réalisées au niveau des parois, et un système couplant de la velocimétrie laser Doppler deux composantes à un micro-thermocouple (25 μm) a été développé pour avoir accès aux vitesses verticale et horizontale et à la température dans le canal. Ce banc et son instrumentation sont décrits et les incertitudes de mesure associées ont été caractérisées. La configuration de chauffage uniforme symétrique a été étudiée ici pour différentes puissances injectées. Les profils de vitesse et de température moyennes mettent en évidence la présence d'un changement de régime d'écoulement dans le canal pour un nombre de Rayleigh indéntifié. L'étude approfondie des profils des fluctuations de vitesse et de température a permis de mettre au point une modélisation comportementale de ce changement de régime. De plus, une première approche est développée pour évaluer la pression dans le canal à partir de l'analyse et de l'estimation des différents termes de l'équation de conservation de la quantité de mouvement. / Reducing the building consumption passes through : energy saving, energy efficiency and the use of renewable energy sources for a local production of electricity. For the last point, the building integrated photovoltaic systems represent a promising solution. However, electrical yield and life time of silicon solar cells decrease with the increase of its operating temperature, that is why integrated configurations which limits cell overheating must be developed. The solution considered is the double-skin façade configuration : photovoltaic panels are separated from the building wall(or roof) by an opened air channel. This induces a natural convection flow that cools the rear surface of the photovoltaic panels. This flow can also be used for natural ventilation of buildings in summer or for air preheating in winter. This experimental study is part of a project on photovoltaic double-skin façades. In this project, each physical phenomena are analysed separately first, and the coupling between the different phenomena are studied in a second time. This thesis deals with the natural convection into the vertical double-skin façades. The system is modelled by a vertical channel with isoflux heating at the two mains walls. An experimental apparatus was developed in the laboratory for studying natural convection in water in a vertical channel. Water is used as the working fluid to avoid radiative heat transfer and obtain a pure convective flow. The channel is 65 cm high with an adjustable gap. It is placed in a 1.5 m high glass tank filled with 160 L of distilled water. The main walls are heated through 24 independant electrical heaters maintaining a constant heat flux, which allows differents boundary condition (symmetrical, asymmetrical, uniforme, alternated, etc). Heat flux and temperature measurements are made at the walls. The velocity (horizontal and vertical) and temperature of the flow are measured through a two components Laser Doppler Velocimetry system combined with a micro-thermocouple (25 μm diameter). This apparatus is described in details and the measurement uncertainties were characterized. The symmetrical uniform heating configuration was studied here with the analyse of velocity and temperature profiles in the channel for several input power (46 W to 562 W). These profiles show a change in flow regime in the channel that is analysed with turbulent quantities along the channel. Moreover, the pressure is estimated in the channel from the analysis of all the terms of the momentum conservation equations.

Face à la problématique énergétique, les solutions envisagées dans le domaine du bâtiment s’orientent sur un mix énergétique favorisant la production locale ainsi que l’autoconsommation. Concernant l’électricité, les systèmes photovoltaïques intégrés au bâtiment (BiPV) représentent l’une des rares technologies capables de produire de l’électricité localement et sans émettre de gaz à effet de serre. Cependant, le niveau de température auquel fonctionnent ces composants, influence sensiblement leur efficacité ainsi que leur durée de vie. Ces deux constats mettent en lumière l’importance du refroidissement passif par convection naturelle de ces modules. La configuration privilégiée est une configuration d’intégration au sein d’une enveloppe ventilée qualifiée de double-peau photovoltaïque. La présente étude expérimentale porte sur les transferts de chaleur et les caractéristiques de l’écoulement en convection naturelle dans des canaux chauffés verticaux ou inclinés. Deux bancs d’essais existants ont été complétés afin d’obtenir des données. Ils sont composés de deux plaques planes parallèles séparées par une lame d’air. Les parois sont soumises à des conditions aux limites de type densité de flux imposée. Les températures moyennes à la paroi ont été mesurées par thermocouples. Un système de vélocimétrie par image de particules a permis d’obtenir des profils de vitesse moyenne ainsi que les distributions d’intensité turbulente dans l’écoulement. Les champs de vitesse instantanée ont également été examinés. Trois configurations ont été étudiées avec un nombre de Rayleigh variant entre 3,86 x 105 et 6,22 x 106. La première est un canal vertical avec une des deux parois chauffée uniformément. La seconde est un canal vertical dans lequel les deux parois sont chauffées de façon non-uniforme et alternée. La troisième est de type canal incliné chauffé uniformément sur la paroi supérieure. Le rapport de forme du canal (largeur/hauteur) est de 1/15 pour le deux premières configurations et de 1/16 pour la troisième. Une attention particulière a été portée sur l’identification de la zone de transition laminaire-turbulent. L’étude a permis de mettre en évidence la sensibilité de l’écoulement aux perturbations extérieures. Pour un chauffage uniforme et asymétrique, à partir d’un nombre de Rayleigh Ra* de 3.5 x 106 et pour θ = 60° et 90°, il a été constaté que la propagation de structures cohérentes dans le canal a lieu à partir de la mi-hauteur de ce canal. Ces instabilités favorisent alors les transferts thermiques. Dans le cas d’un chauffage non-uniforme sur les deux parois du canal, l’écoulement est fortement perturbé ce qui conduit à l’augmentation du brassage et de la contrainte de Reynolds sur la majorité de la largeur du canal. Enfin, pour chacune des configurations, des corrélations permettant de quantifier les transferts de chaleur à la paroi et au sein de la lame d’air (nombre de Nusselt moyen en fonction du nombre de Rayleigh) ont été établies. / Among technologies capable to produce electricity locally without contributing to GHG releases, building integrated PV systems (BIPV) could be major contributor. However, when exposed to intense solar radiation, the temperature of PV modules increase significantly, leading to a reduction in efficiency so that only about 14% of the incident radiation is converted into electrical energy. The high temperature also decrease the life of the modules, thereby making passive cooling of the PV components through natural convection a desirable and cost-effective means of overcoming both difficulties. An experimental investigation of heat transfer and fluid flow characteristics of natural convection of air in vertical and inclined open-ended heated channels is therefore undertaken so as provide reliable information for the design of BIPV. Two experimental set ups were developed and used during the present investigations; one located at the CETHIL laboratory in Lyon, the F-device and the other located at the University of New South Wales in Sydney, the R-device. Both channels consisted of two wide parallel plates each of which could be subjected to controlled uniform or non-uniform heat fluxes. The investigation has been conducted by analyzing the mean wall temperatures, measured by thermocouples and mean velocity profiles and turbulent quantity distributions of the flow, measured with a PIV system. Flow patterns close to the heated faces were also investigated. The study is particularly focused on the transition region from laminar to turbulent flow. Three different heating geometric arrangements are examined in the modified Rayleigh number range from 3.86 x 105 to 6.22 x 106. The first is a vertical channel with one wall uniformly heated while the other was unheated, the second was a vertical channel in which both walls were non-uniformly heated and the third is an inclined channel uniformly heated from above. In the vertical configurations the width-to-height channel aspect ratio was fixed at 1:15 and in the inclined ones at 1:16. It is shown that the flow is very sensitivity to disturbances emanating from the ambient conditions. Moreover, the propagation of vortical structures and unsteadiness in the flow channel which are necessary to enhance heat transfer, occurred downstream of the mid-channel section at Ra* = 3.5 x 106 for uniformly and asymmetrically heated channels inclined between 60° and 90° to the horizontal. Indeed, these unsteady flow phenomena appears upstream the location of the inflexion point observed in the temperature excess distribution of the heated wall. In the case of non-uniform heating on both sides of the channel, a stronger ‘disruption mechanism’ exists, which leads to enhanced mixing and increased Reynolds stresses over most of the width of the channel. Empirical correlations of average Nusselt number as a function of modified Rayleigh number were obtained for each configuration.

In this investigation, a single phase fluid is used to study the coupling between natural convection heat transfer within an enclosure and forced convection through computer covering case to cool the electronic chip. Two working fluids are used (water and air) within a rectangular enclosure and the air flow through the computer case is created by an exhaust fan installed at the back of the computer case. The optimum enclosure size configuration that keeps a maximum temperature of the heat source at a safe temperature level (85°C) is determined. The cooling system is tested for varying values of applied power in the range of 15-40W. The study is based on both numerical models and experimental observations. The numerical work was developed using the commercial software (ANSYS-Icepak) to simulate the flow and temperature fields for the desktop computer and the cooling system. The numerical simulation has the same physical geometry as those used in the experimental investigations. The experimental work was aimed to gather the details for temperature field and use them in the validation of the numerical prediction. The results showed that, the cavity size variations influence both the heat transfer process and the maximum temperature. Furthermore, the experimental results ii compared favourably with those obtained numerically, where the maximum deviation in terms of the maximum system temperature, is within 3.5%. Moreover, it is seen that using water as the working fluid within the enclosure is capable of keeping the maximum temperature under 77°C for a heat source of 40W, which is below the recommended electronic chips temperature of not exceeding 85°C. As a result, the noise and vibration level is reduced. In addition, the proposed cooling system saved about 65% of the CPU fan power.

Continued emission of green house gases has energized research activity worldwide to develop efficient ways to harness renewal energy. The availability of large scale energy storage technologies is essential to make renewal energy a reliable source of energy. Redox flow batteries show potential in this direction. These batteries typically need expensive membranes which need replacement be-cause of fouling. The recently proposed soluble lead redox flow battery (SLRFB), in which lead ions deposit on electrodes in charge cycle and dissolve back in discharge cycle, can potentially cut down the cost of energy storage by eliminating membrane. A number of challenges need to be overcome though. Low cycleability, residue formation, and low efficiencies are foremost among these, all of which require an understanding of the underlying mechanisms. A model of laminar flow-through SLRFB is first developed to understand buildup of residue on electrodes with continued cycling. The model accounts for spatially and temporally growing concentration boundary layers on electrodes in a self consistent manner by permitting local deposition/dissolution rates to be controlled by local ion transport and reaction conditions. The model suggests controlling role for charge transfer reaction on electrodes (anode in particular) and movement of ions in the bulk and concentration boundary layers. The non-uniform current density on electrodes emerges as key to formation of bare patches, steep decrease in voltage marking the end of discharge cycle, and residue buildup with continuing cycles. The model captures the experimental observations very well, and points to improved operational efficiency and decreased residue build up with cylindrical electrodes and alternating flow direction of recirculation. The underlying mechanism for more than an order of magnitude increase in cycle life of a beaker cell battery with increase in stirrer speed is unraveled next. Our experiments show that charging with and without stirring occurs identically, which brings up the hitherto unknown but quite strong role of natural convection in SLRFB. The role of stirring is determined to be dislodgement/disintegration of residue building up on electrodes. The depletion of active material from electrolyte due to residue formation is offset by “internal regeneration mechanism”, unraveled in the present work. When the rate of residue formation, rate of dislodging/disintegration from electrode, and rate of regeneration of active material in bulk of the electrolyte becomes equal, perpetual operation of SLRFB is expected. The identification of strong role of free convection in battery is put to use to demonstrate a battery that requires stirring/mixing only intermittently, during open circuit stages between charge and discharge cycles when no current is drawn. Inspired by our experimental finding that the measured currents for apparently diffusion limited situations (no external flow) are far larger than the maxi-mum possible theoretical value, the earlier model is modified to account for natural convection driven by concentration gradient of lead ions in electrolyte. The model reveals the presence of strong natural convection in battery. The induced flow in the vicinity of the electrodes enhances mass transport rates substantially, to the extent that even in the absence of external flow, normal charge/discharge of battery is predicted. The model predicted electrochemical characteristics are verified quantitatively through voltage-time measurements. The formation of flow circulation loops driven by electrode processes is validated qualitatively through PIV measurements. Natural convection is predicted to play a significant role in the presence of external flow as well. The hitherto unexplained finding in the literature on insensitivity of charge-discharge characteristics to electrolyte flow rate is captured by the model when mixed mode of convection is invoked. Flow reversal and wavy flow are predicted when natural convection and forced convection act in opposite directions in the battery. The effect of the presence of non-conducting material (PbO on anode) on the performance of SLRFB is studied using a simplified approach in the model. The study reveals the presence of charge coup de fouet phenomenon in charge cycle. The phenomenon as well as the predicted effect of depth of discharge on the magnitude of charge coup de fouet are confirmed experimentally.

Cílem této práce je analýza teplotního pole vysokonapěťového rozvaděče. Tato analýza je založena na kombinaci experimentálního měření a CFD simulace proudění vzduchu v prostoru rozvaděče. Na základě výsledků pro referenční geometrii jsou předloženy návrhy na úpravu topologie rozvaděče, které mají za cíl snížení teploty kritických komponent rozvaděče během jeho provozu. Druhá část práce je spojena s problematikou oteplení plošných spojů, jakožto hlavních zdrojů tepla v prostoru rozvaděče. Pro popis závislosti oteplení plošného spoje na jeho charakteru byla použita statistická metoda design experimentu. Výsledky z experimentálního měření jsou podpořeny konečně prvkovým modelem kontaktu spoje.

V disertační práci je popsán heliový kryostat s experimentální válcovou konvekční celou pro studium proudění při přirozené konvekci za velmi vysokých Rayleigho čísel Ra (až do řádu 10^15) a Nusseltových čísel Nu (až do řádu 10^4). Pracovní látkou je studené 4He, které umožňuje díky výhodným fyzikálním vlastnostem dosažení velmi vysokých hodnot čísel Ra. Návrh kryostatu je založen na koncepci lázňových NMR kryostatů s nízkým odparem kryokapalin. Ve středu kryostatu je umístěna konvekční cela o průměru 300 mm a výšce 300 mm. Celu tvoří horní a spodní dno a výměnná střední část. Tyto díly jsou spojeny rozebíratelnými přírubami těsněnými indiovým drátem. Výměnná část umožňuje snadnou modifikaci geometrie cely. Hlavní přednost kryostatu spočívá v minimálním vlivu konstrukce cely a použitých materiálů na studovanou konvekci. Cela kryostatu je navržena pro pracovní tlaky do 250 kPa.