ISE - Teses de Doutoramento
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Browsing ISE - Teses de Doutoramento by Author "Azevedo, Pedro"
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- Estudo da formação e destruição de óxidos de azoto em chamas de hidrogénio e /ou hidrocarbonetos com ou sem presença de compostos azotadosPublication . Azevedo, PedroIn the nineties, the World Health Organization updated their “Air Quality Guidelines for Europe” (WHO, 2000) with the objective to provide detailed information on atmospheric pollutants and their adverse effects on human health. Consequence of exposure to compounds as the nitrogen oxides, volatile organic compounds (VOC), peroxyacetylnitrates (PAN) and aldehydes, upon reacting with solar light, produce the photochemical smog along with the formation the tropospheric ozone. Among other undesirable consequences, the nitrogen oxides act as main harmful precursors for the formation of the photochemical smog (Broekhuizen, 2002). This work aims at studying the formation of nitrogen oxides and their subsequent destruction in hydrogen and/or hydrocarbon flames with or without seeding with nitrogen compounds, hence producing conditions for a reduced kinetic mechanism that can be used in CFD software. This research work was carried out involving both experimental and numerical studies. The experimental work studied the influence of the parameters for hydrocarbons combustion and NOx formation and destruction. The parameters varied were the temperature, equivalence ratio, fuel composition and the presence of nitrogen compounds artificially added. The main reactants used were hydrogen, methane, oxygen and argon and there was seeding of flames with ammonia or nitrogen oxide, varying between 500 and 1000 ppmv. The experimental runs were carried out using a test bench specially built for this work and used laminar flames with equivalence ratios ranging from 0.8 to 1.2. The results were obtained through flue gas monitoring inside the flame and this was achieved by placing a probe with extraction of gases at several axial distances ranging from 10 to 30 mm above the burner. Temperature and velocity of burnt gases were calculated. The concentration of some combustion products, namely oxygen, hydrogen, carbon oxides was determined on a gas chromatograph, the nitrogen oxides were measured using an on line gas analyzer with an electrochemical cell and the levels of both hydrogen cyanide and ammonia were quantified with selective electrodes in aqueous solution. The use of detailed kinetic mechanisms for a 3 D reactive computational fluid dynamics flow as the one representing a flame is very difficult. So a reduced kinetic mechanism was employed and estimation methods were utilised for the calculation of physical properties of chemical species. The reduced kinetic mechanism was compared with three more detailed mechanisms and one reduced kinetic mechanism in one-dimensional chemical kinetic package, Chemkin, employing reaction parameters like flue gas composition distribution, temperature and velocity profiles along burner axial distance and residence time. The results were found to be in good agreement. The reduced kinetic mechanism was then integrated in the computational fluid dynamics package, Fluent, together with physical properties of chemical species for a reactive flow in 3 D. The numerical calculations were then compared with experimental data and there was a good agreement with respect to the overall combustion mechanism of methane. However, the results for nitrogen oxides, though demonstrating the same tendency as experimental work, suggest that some improvements in the model are needed in order to obtain more satisfactory correlation. Based on the validation results, a reduced kinetic mechanism involving 50 reactions integrating nitrogen chemistry was developed that could represent H2 and/or CH4 flames with nitrogen chemistry. The extension of this work to more complex hydrocarbons could be achieved without significantly increasing the chemical reactions or species quantity involved.
- Estudo experimental e numérico de um queimador de gás naturalPublication . Azevedo, PedroNeste trabalho pretendeu-se demonstrar a aplicabilidade da modelação de dinâmica de fluidos através de um código computacional (Fluent/UNS), para a previsão de escoamentos de fluidos e respectiva combustão, numa fornalha horizontal. Foi introduzido o gás natural como combustível, através da utilização de uma mistura com características idênticas à do gás natural. Foram realizados ensaios cromatográficos ao combustível para verificação da composição elementar. Para a construção da malha computacional utilizada no modelo numérico, tornou-se necessária a utilização de dois programas, por forma a permitir a integração dos 37 injectores, respeitantes ao queimador, na face da fornalha. Foi escolhida uma malha com cerca de 78000 células, que garante a indepedência dos resultados. Para modelação do escoamentofoi utilizado o modelo de turbulência k-ε, por se adequar melhor ao escoamento em causa, bem como pelas exigências computacionais. A transferência de calor por radiação no interior da fornalha, é calculada através da utilização do Modelo de Radação de Transferência Discreta (DTRM). Os resultados numéricos foram validados por ensaios experimentais, realizados com um queimador protótipo montado numa fornalha horizontal, cujos resultados foram analisados juntamente com os homólogos numéricos.