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- Three-dimensional natural convection in rooms connected to the outside through large openingsPublication . Ramos, João António Esteves; Martinho, Nuno A. G.; Pitarma, Rui A.; Carvalho, Maria da GraçaThe growing interest in the study of natural convection in geometrically complex enclosures with restricted communication has been stimulated by applications involving energy-efficient passive-solar buildings, natural convection cooling of electronic equipment and room heating and ventilation. Airflow through doorways, windows and other large openings are significant ways in which air, pollutants and thermal energy are transferred from one zone of a building to another or to outside. The present paper describes a numerical method for analysing three-dimensional natural convection in rooms connected to the outside through large openings. The calculations made use of a Computational Fluid Dynamics (CDF) procedure taking into account the effects of buoyancy, heat sources, thermal radiation heat transfer and air flow turbulence. The CDF predictions agree with the experimentally observed features of the flow field, including the vertical air temperature stratification, with a reasonable degree of accuracy.
- O papel da modelação matemática na optimização da climatização de edifíciosPublication . Ramos, João António Esteves; Pitarma, Rui A.; Martinho, Nuno A. G.; Carvalho, Maria da GraçaA crescente necessidade da utilização de meios de cálculo que permitam a previsão e a visualização das características dos escoamentos do ar, da distribuição da temperatura, da distribuição da humidade do ar e do transporte de contaminantes, em espaços de edifícios ventilados, tem incentivado a investigação na modelação física e matemática desses fenómenos, nessas geometrias complexas. Nesta comunicação, apresenta-se uma "ferramenta" de cálculo que permite modelar os fenómenos tridimensionais associados à circulação turbulenta do ar, à transmissão de calor e à transmissão do vapor de água no ar (humidade). O modelo de simulação computacional desenvolvido foi validado para a caracterização do ambiente interior em edifícios nas suas variantes termo-higrométricas, qualidade do ar e conforto térmico. Nesta comunicação apresentam-se previsões do campo da velocidade, da temperatura do ar e da concentração de vapor de água numa sala de escritório com obstáculos e com fontes interiores de calor e de água. Este tipo de avaliação é fundamental para, em cada caso, conhecer, de uma forma detalhada, a influência dos vários parâmetros do ambiente interior nos ocupantes dos edifícios, quer do ponto de vista do conforto térmico, quer da qualidade do ar.
- Evaluation of errors on the CFD computation of air flow and heat transfer around the human bodyPublication . Martinho, Nuno; Lopes, A.; Gameiro da Silva, M.A detailed computer aided design model representing a female human body was created from a 3-D scan of a full scale thermal manikin. The sensible heat losses due to convection and radiation for a person seated in a rectangular room were calculated using CFD simulations, as well as the parameters characterizing the flow around the body including air velocity and temperature. The accuracy of numerical results was evaluated in terms of the physical approximation errors, namely inlet boundary conditions and turbulence models, and spatial discretization errors due to the number of grid elements and grid refinement near solid surfaces. Results were compared to published data from two tests performed in conditions that portrayed the CFD simulations. The use of: a mixed turbulence model between k-e{open}, for zones away from solid surfaces, and k-ω, for zones close to solid surfaces; values of the dimensionless y + parameter inferior to 5 jointly with a low-Reynolds method to model the flow in the near-wall region of the human surface and inlet non-uniform boundary profiles for temperature, velocity and turbulence intensity, leaded to good agreement with experimental results, with a global mean relative error of the order of 7%, with a standard deviation of 4%, in terms of convective and radiative heat flux between the human body and the thermal environment.
- Selecting and Optimizing a Heat Exchanger for Automotive Vehicle Rankine Cycle Waste Heat Recovery SystemsPublication . Santos, Helder; Morgado, Joel; Martinho, Nuno; Pereira, João; Moita, AnaThis paper contributes for the selection and optimization of an adequate compact heat exchanger (HEX) for an automotive vehicle Rankine cycle Waste Heat Recovery Systems (RC-WHRS). Both tube type and plate type HEX were considered. Among the different HEX designs available, a robust cross-ow tube heat exchanger (CFT-HEX), with the working fluid circulating inside the tubes was selected. The selected HEX accomplishes with the specific pressure drops limits on both exhaust gas and working fluid side. The compactness of the CFT-HEX strongly depends on the optimization of the HEX pre-heater section, which mainly depends on the augmentation of the working fluid heat transfer coefficient. The present study data reveals that a transitional flow regime with a working fluid Reynolds number, Re > 3200, allows a significant increase of the Nusselt number (Nu > 20) as compared to laminar flow regime.