ABSTRACT The depletion of fossil fuel reserves and climate change caused by atmospheric pollution has led the human being to seek alternatives that are less damaging to the environment. The concern and the awareness of the population open space for the study of renewable energies to be deepened, among them the best use of solar energy. The use of alternative materials to replace selective surfaces is a natural trend, since improvements in surface efficiency are usually sought while attempting to reduce costs. Composite substances are already used to obtain some selective surfaces, and, as a result, the search for better processes awakens research on more appropriate and lower cost materials, which represents a great scientific potential in the evolution of these technologies. Thus, the present work consisted in obtaining and studying selective surfaces for applications in low-cost flat plate solar collectors, using residues from the granite industry. Five different surfaces were studied, varying the weight percentage: 100% granite powder, 75% granite powder + 25% chromium oxide, 50% granite powder + 50% chromium oxide, 25% granite powder + 75% chromium oxide and 100% chromium oxide. For the tests, an experimental wooden stand was built, and it was possible to simulate the conditions of a flat plate solar collector. To characterize the surfaces, scanning electron microscope (SEM) techniques, infrared analysis, X-ray diffraction and UV-VIS absorbance determination, as well as graphs with surface temperatures and with radiation, during the tests in the sun, were used. The surfaces` efficiency was determined by the ratio of the absorptivity by the emissivity, as well as the trademark MRTiNOX, a commercial selective surface applied on copper substrate. For this, an efficiency of 23.56 was obtained, while for the 50% granite and 50% chromium surfaces the value of 23.27 (closest to the trade mark) was calculated. Therefore, replacing the traditional components of selective surfaces with granite proved to be a satisfactory solution, contributing to the reduction of costs with work involving solar energy.

Dando continuidade ao estudo referente à utilização de Dinâmica dos Fluidos Computacional (DFC) no ensino de física e engenharia, o presente trabalho visa analisar e comparar, com fins didáticos, soluções obtidas numericamente com soluções analíticas e empíricas, comumente apresentadas em sala de aula e de interesse de laboratórios de graduação, de problemas clássicos envolvendo dinâmica dos fluidos e transferência de calor. Neste estudo o foco maior foi dado a problemas de transferência de calor. Para o cálculo das soluções numéricas foi utilizado um dos softwares representantes do estado da arte de DFC, o ANSYS CFX 12.1. Os problemas escolhidos foram: convecção em placa plana em regime laminar, escoamento laminar em torno de um cilindro aquecido, escoamento interno turbulento em dutos circulares aquecidos e trocador de calor de tubos concêntricos. Estes problemas, aqui apresentados, foram escolhidos e resolvidos por alunos de graduação, avalizando a utilidade da DFC como ferramenta de aprendizado de fenômenos físicos.

Continuing the study on the use of Computational Fluid Dynamics (CFD) in physics and engineering teaching, the present work aims to analyze and compare, for didactic purposes, solutions numerically obtained against known analytical and empirical solutions, commonly presented in the classroom and in undergraduate labs, of classical problems involving fluid dynamics and heat transfer. In this study the main focus was pointed to heat transfer problems. For the numerical solutions computation, one of the Computational Fluid Dynamics (CFD) softwares that represent its state-of-the-art, ANSYS CFX 12.1 was employed. The chosen problems were: laminar heated plate convection, laminar flow around a heated cylinder, turbulent internal flow in heated circular ducts and concentric tubes heat exchanger. These problems were chosen and solved by undergraduate students, demonstrating the usefulness of CFD as a tool for learning physical phenomena.