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

Nous proposons d'exploiter la convection thermomagnétique, phénomène caractéristique des Ferro fluides, pour améliorer les transferts de chaleur dans les transformateurs. Les équations régissant le système se composent des équations de Navier-Stokes dans l'approximation de Boussinesq, de l'équation de la conservation de l'énergie et des équations de la magnétostatique. Les simulations sont menées avec notre code de recherche parallélisé SFEMaNS (Spectral/Finite Element for Maxwell and Navier-Stokes) pour des géométries axisymétriques, utilisant une décomposition spectrale dans la direction azimutale et des éléments finis de Lagrange dans le plan méridien. Afin de résoudre ce problème spécifique, divers développements sont apportés à SFEMaNS, tels que l'implémentation des forces magnétiques de Kelvin et de Helmholtz. Le code est d'abord appliqué au refroidissement d'un solénoïde dans une cuve cylindrique contenant de l'huile de transformateur ou un ferrofluide à base d'huile de transformateur. Les résultats montrent que l'utilisation du ferrofluide diminue la température maximale du système grâce à la convection thermomagnétique et au changement des propriétés thermophysiques du fluide. L'influence de différents paramètres (fraction volumique de nanoparticules, présence d'un cœur ferromagnétique, propriétés magnétiques des nanoparticules) est étudiée. En particulier, les simulations confirment l'intérêt des nanoparticules magnétiques à faible température de Curie. Nous montrons également sur cet exemple que deux densités de force magnétique égales à un gradient près, telles que les forces de Kelvin et de Helmholtz, donnent le même écoulement. Un bon accord qualitatif est trouvé entre les résultats numériques et expérimentaux utilisant de l'huile de transformateur ou du ferrofluide. Le code est ensuite appliqué au refroidissement d'un système proche d'un transformateur de 40 kVA (20 kV/400 V). Les résultats montrent à nouveau une réduction de la température maximale grâce au ferrofluide. / We propose to make use of thermomagnetic convection, a characteristic phenomenon of ferrofluids, to improve heat transfer in transformers. The governing equations consist in the Navier-Stokes equations under the Boussinesq approximation, the energy conservation equation and the magnetostatics equations. The simulations are performed with the in-house parallel code SFEMaNS (Spectral/Finite Element for Maxwell and Navier-Stokes) for axisymmetric geometries, using a spectral decomposition in the azimuthal direction and Lagrange finite elements in the meridian plane. In order to solve this specific problem, various developments are brought to SFEMaNS, such as the implementation of the Kelvin and Helmholtz magnetic forces. The code is first applied to the cooling of a coil in a cylindrical tank containing either transformer oil or transformer oil-based ferrofluid. The results show that the use of the ferrofluid reduces the maximum temperature in the system due to thermomagnetic convection and the change of the fluid thermophysical properties. The influence of different parameters (volume fraction of nanoparticles, presence of a ferromagnetic core, nanoparticle magnetic properties) is investigated. In particular, the simulations confirm the benefit of magnetic nanoparticles with a low Curie temperature. We also show on this example that two magnetic body forces equal up to a gradient, such as the Kelvin and Helmholtz forces, give the same flow. A good qualitative agreement is found between the numerical and experimental results using transformer oil or ferrofluid. The code is then applied to the cooling of an electromagnetic system close to a 40 kVA (20 kV/400 V) transformer. The results show again a reduction of the maximum temperature when using ferrofluid.