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Alves, Thiago Antonini - One of the best experts on this subject based on the ideXlab platform.
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Aplicação da transformada integral e da transformação conforme na solução de uma classe de problemas difusivo-convectivos em domínios de geometrias não-convencionais
Universidade Estadual Paulista (UNESP), 2006Co-Authors: Alves, Thiago AntoniniAbstract:O presente trabalho trata da solução de uma classe de problemas difusivo-convectivos, tanto de natureza elíptica como parabólica, em domínios de geometrias não-convencionais, através da aplicação da Transformada Integral. Para facilitar o tratamento analítico e a aplicação das condições de contorno, antes da aplicação da Técnica da Transformada Integral Generalizada - TTIG sobre a equação governante do problema estudado, emprega-se uma Transformação Conforme - TC visando efetuar uma mudança de coordenadas adequada. Analisa-se inicialmente o problema hidrodinâmico do escoamento laminar completamente desenvolvido de fluidos Newtonianos no interior de dutos. Para a obtenção do campo de velocidades do escoamento aplica-se a TTIG sobre a equação da quantidade de movimento. Os parâmetros hidrodinâmicos de interesse, tais como: velocidades média e máxima, fator de atrito de Fanning, fator de Hagenbach, número de Poiseuille, comprimento de entrada hidrodinâmico e queda de pressão são calculados para as diversas geometrias. Feito isso, efetua-se o estudo dos problemas difusivo-convectivos relacionados à transferência de calor do escoamento laminar hidrodinamicamente desenvolvido e termicamente em desenvolvimento de fluidos Newtonianos com perfil de temperatura de entrada uniforme em dutos submetidos a condições de contorno de Dirichlet. Para a obtenção do campo de temperatura aplica-se a TTIG sobre a equação da energia e então, calculam-se os parâmetros térmicos de interesse: temperatura média de mistura, números de Nusselt local e médio e comprimento de entrada térmica. Realiza-se, quando possível, a comparação dos resultados obtidos para os parâmetros termos-hidráulicos com os disponíveis na literatura.The present work describes the solution of a class of elliptical-parabolic diffusiveconvective problems, on unconventional geometries, employing the Generalized Integral Transform Technique (GITT). In order to facilitate the analytical treatment and the application of the boundary conditions, a Conformal Transform (CT) is used to change the domain into a more suitable coordinate system, just before GITT is to be applied. First of all, using this procedure, the Hydrodynamic problem of fully developed Newtonian laminar flow inside ducts is analyzed. In order to obtain the velocity field, GITT is applied on the momentum equation. Interesting Hydrodynamic parameters, such as: maximum and minimum velocity values, Fanning friction and Hagenbach factors, Poiseuille number, Hydrodynamic Entry Length, as well as pressure loss, are evaluated for several geometries. Following that, diffusive-convective problems are studied in relationship to the heat transfer in Hydrodynamically fully developed and thermally non-developed Newtonian laminar flow inside ducts under Dirichlet boundary conditions, considering uniform temperature entrance profile. In order to obtain the temperature field, GITT is applied on the energy equation, evaluating the relevant parameters: bulk mean temperature, average and local Nusselt numbers and thermal Entry Length. The results are compared, as much as possible, with the parameter values available in the literature.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
Alves, Thiago Antonini [unesp] - One of the best experts on this subject based on the ideXlab platform.
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Aplicação da transformada integral e da transformação conforme na solução de uma classe de problemas difusivo-convectivos em domínios de geometrias não-convencionais
Universidade Estadual Paulista (UNESP), 2006Co-Authors: Alves, Thiago Antonini [unesp]Abstract:O presente trabalho trata da solução de uma classe de problemas difusivo-convectivos, tanto de natureza elíptica como parabólica, em domínios de geometrias não-convencionais, através da aplicação da Transformada Integral. Para facilitar o tratamento analítico e a aplicação das condições de contorno, antes da aplicação da Técnica da Transformada Integral Generalizada - TTIG sobre a equação governante do problema estudado, emprega-se uma Transformação Conforme - TC visando efetuar uma mudança de coordenadas adequada. Analisa-se inicialmente o problema hidrodinâmico do escoamento laminar completamente desenvolvido de fluidos Newtonianos no interior de dutos. Para a obtenção do campo de velocidades do escoamento aplica-se a TTIG sobre a equação da quantidade de movimento. Os parâmetros hidrodinâmicos de interesse, tais como: velocidades média e máxima, fator de atrito de Fanning, fator de Hagenbach, número de Poiseuille, comprimento de entrada hidrodinâmico e queda de pressão são calculados para as diversas geometrias. Feito isso, efetua-se o estudo dos problemas difusivo-convectivos relacionados à transferência de calor do escoamento laminar hidrodinamicamente desenvolvido e termicamente em desenvolvimento de fluidos Newtonianos com perfil de temperatura de entrada uniforme em dutos submetidos a condições de contorno de Dirichlet. Para a obtenção do campo de temperatura aplica-se a TTIG sobre a equação da energia e então, calculam-se os parâmetros térmicos de interesse: temperatura média de mistura, números de Nusselt local e médio e comprimento de entrada térmica. Realiza-se, quando possível, a comparação dos resultados obtidos para os parâmetros termos-hidráulicos com os disponíveis na literatura.The present work describes the solution of a class of elliptical-parabolic diffusiveconvective problems, on unconventional geometries, employing the Generalized Integral Transform Technique (GITT). In order to facilitate the analytical treatment and the application of the boundary conditions, a Conformal Transform (CT) is used to change the domain into a more suitable coordinate system, just before GITT is to be applied. First of all, using this procedure, the Hydrodynamic problem of fully developed Newtonian laminar flow inside ducts is analyzed. In order to obtain the velocity field, GITT is applied on the momentum equation. Interesting Hydrodynamic parameters, such as: maximum and minimum velocity values, Fanning friction and Hagenbach factors, Poiseuille number, Hydrodynamic Entry Length, as well as pressure loss, are evaluated for several geometries. Following that, diffusive-convective problems are studied in relationship to the heat transfer in Hydrodynamically fully developed and thermally non-developed Newtonian laminar flow inside ducts under Dirichlet boundary conditions, considering uniform temperature entrance profile. In order to obtain the temperature field, GITT is applied on the energy equation, evaluating the relevant parameters: bulk mean temperature, average and local Nusselt numbers and thermal Entry Length. The results are compared, as much as possible, with the parameter values available in the literature
Amirah M Sahar - One of the best experts on this subject based on the ideXlab platform.
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effect of hydraulic diameter and aspect ratio on single phase flow and heat transfer in a rectangular microchannel
Applied Thermal Engineering, 2017Co-Authors: Amirah M Sahar, J G Wissink, Mohamed M Mahmoud, T G Karayiannis, Mohamad Ashrul S IshakAbstract:Abstract The effect of aspect ratio and hydraulic diameter on single phase flow and heat transfer in a single microchannel was investigated numerically and the results are presented in this paper. Previously, many studies in literature investigating the effect of geometrical parameters reached contradictory conclusions leaving some phenomena unexplained. Additionally, most researchers studied the effect of channel geometry by varying the channel height for a constant channel width or varying the width for a constant height. This means that the hydraulic diameter and aspect ratio vary simultaneously, which makes it difficult to identify the relative importance of the aspect ratio and the hydraulic diameter. In the present study, the effect of hydraulic diameter was studied by varying the channel width and depth while keeping the aspect ratio constant. The range of hydraulic diameters was 0.1–1 mm and the aspect ratio was fixed at 1. In the second set of simulations, the aspect ratio ranged from 0.39 to 10 while the hydraulic diameter was kept constant at 0.56 mm. The simulations were performed using the CFD software package ANSYS Fluent 14.5. The geometry investigated in this study includes symmetrical cylindrical inlet and outlet plenums and a microchannel. The fluid entered and left the channel vertically from the top in a direction normal to the channel axis. The dimensions of the inlet/outlet plenums (diameter and height measured from the channel bottom surface) were kept constant while the width and depth of the channel were varied. The simulations were conducted for a range of Reynolds numbers (Re = 100–2000) and water was used as the working fluid. A three dimensional thin wall model was used to avoid conjugate heat transfer effects. A constant heat-flux boundary condition was applied at the bottom and vertical side walls of the channel, while the upper wall was considered adiabatic. The friction factor was found to decrease slightly with aspect ratio up to AR ≈ 2 after which it increased with increasing aspect ratio. The results demonstrated that the slope of the velocity profile at the channel wall changes significantly with aspect ratio for AR > 2. The effect of the aspect ratio and hydraulic diameter on the dimensionless Hydrodynamic Entry Length is not significant. Also, the aspect ratio does not affect the heat transfer coefficient while the dimensionless Nusselt number increases with increasing hydraulic diameter. The friction factor was found to increase with increasing hydraulic diameter.
Michael W. Patterson - One of the best experts on this subject based on the ideXlab platform.
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thermal hydraulic performance testing of printed circuit heat exchangers in a high temperature helium test facility
Applied Thermal Engineering, 2014Co-Authors: Sai K Mylavarapu, Richard E Glosup, R N Christensen, Michael W. PattersonAbstract:Abstract In high-temperature gas-cooled reactors, such as a very high temperature reactor (VHTR), an intermediate heat exchanger (IHX) is required to efficiently transfer the core thermal output to a secondary fluid for electricity generation with an indirect power cycle and/or process heat applications. Currently, there is no proven high-temperature (750–800 °C or higher) compact heat exchanger technology for high-temperature reactor design concepts. In this study, printed circuit heat exchanger (PCHE), a potential IHX concept for high-temperature applications, has been investigated for their heat transfer and pressure drop characteristics under high operating temperatures and pressures. Two PCHEs, each having 10 hot and 10 cold plates with 12 channels (semicircular cross-section) in each plate are fabricated using Alloy 617 plates and tested for their performance in a high-temperature helium test facility (HTHF). The PCHE inlet temperature and pressure were varied from 85 to 390 °C/1.0–2.7 MPa for the cold side and 208–790 °C/1.0–2.7 MPa for the hot side, respectively, while the mass flow rate of helium was varied from 15 to 49 kg/h. This range of mass flow rates corresponds to PCHE channel Reynolds numbers of 950 to 4100 for the cold side and 900 to 3900 for the hot side (corresponding to the laminar and laminar-to-turbulent transition flow regimes). The obtained experimental data have been analyzed for the pressure drop and heat transfer characteristics of the heat transfer surface of the PCHEs and compared with the available models and correlations in the literature. In addition, a numerical treatment of Hydrodynamically developing and Hydrodynamically fully-developed laminar flow through a semicircular duct is presented. Relations developed for determining the Hydrodynamic entrance Length in a semicircular duct and the friction factor (or pressure drop) in the Hydrodynamic Entry Length region for laminar flow through a semicircular duct are given. Various Hydrodynamic entrance region parameters, such as incremental pressure drop number, apparent Fanning friction factor, and Hydrodynamic entrance Length in a semicircular duct have been numerically estimated.
Manoel Máximo Milaré - One of the best experts on this subject based on the ideXlab platform.
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Study of the Hydrodynamic boundary layer in the performance of a vertical cold water thermal storage tank during the discharging cycle.
Universidade de São Paulo, 2008Co-Authors: Manoel Máximo MilaréAbstract:Quando água quente é introduzida em um tanque de termoacumulação vertical, durante o ciclo de descarga, ocorre uma mistura parcial dessa água com a água fria armazenada. A intensidade dessa mistura depende de parâmetros geométricos do difusor de entrada e da dinâmica do escoamento no início do processo. Inúmeros estudos têm sido desenvolvidos no intuito de se projetar adequadamente o difusor de entrada, de forma a minimizar essa mistura, que corresponde a uma perda de capacidade do tanque. Porém, há uma outra forma de perda de capacidade do tanque, devida aos efeitos viscosos do escoamento. Quando um fluido escoa dentro de um tubo, há a formação de uma camada limite hidrodinâmica, devida à viscosidade do mesmo. Na região do comprimento hidrodinâmico de entrada, em cada seção transversal do tubo e dentro da camada limite, os efeitos viscosos geram um gradiente (radial) de velocidade. Fora dessa camada (no núcleo), esses efeitos são desprezíveis e o fluido escoa com velocidade uniforme. Porém, ao longo desse comprimento, o movimento do fluido no núcleo é acelerado. O escoamento em um tanque vertical de termoacumulação ocorre na região do comprimento hidrodinâmico de entrada. Considerando que não haja mistura das massas de água quando se inicia a introdução da água quente, irá se forma uma superfície de contato entre as mesmas. Essa superfície, por possuir velocidade no núcleo maior que a velocidade média de descarga, atingirá o difusor de saída mais rapidamente. Assim, para que seja garantida descarga de água na temperatura da água armazenada durante todo o ciclo de descarga, a altura do tanque deverá ser maior que a altura determinada utilizando-se a velocidade média do escoamento, sem considerar os efeitos viscosos (modelo ideal). A diferença entre essas alturas constitui, por si só, uma perda de capacidade do tanque. O estudo dos efeitos do escoamento viscoso no desempenho do tanque mostra que dois números adimensionais surgem naturalmente: o número de Reynolds relativo ao diâmetro, ReØ, necessário para se determinar o comprimento hidrodinâmico de entrada, e o número f, que relaciona a posição da superfície de contato das massas de água com o comprimento hidrodinâmico de entrada. Os resultados mostram que a perda devida aos efeitos do escoamento viscoso pode não ser desprezível e que a escolha do diâmetro do tanque é de fundamental importância para minimizar esses efeitos. Também, a análise qualitativa do escoamento sugere que as perdas de capacidade do tanque nos ciclos de descarga e carga não são iguais, pois a diferença de densidade das massas de água age de maneira diversa em cada caso.When warm water is introduced into a vertical thermal storage tank, during the discharging cycle, a partial mixing between it and the cold water stored will occur. The extension of that mixing depends upon diffuser geometric parameters and the dynamics of the flow at the beginning of the process. So many works have been developed, trying to find the better diffuser design so that undesired mixing can be minimized. The mixing is accounted as a tank loss of capacity. But, there is another kind of tank loss of capacity due to viscous effects of the flow. When a fluid flows inside a tube, it forms a boundary layer due to fluid viscosity. At the Hydrodynamic Entry Length region, in each cross section of the tube and inside the boundary layer, the viscous effects create a (radial) velocity gradient. Outside of the boundary layer (inside the core), the viscous effects are negligible and the fluid flows with a uniform velocity. However, along the Entry Length that velocity gets higher at each section. In a vertical thermal storage tank, water flows at the Hydrodynamic Entry Length region. Considering that no mixing occurs at the beginning of the warm water introduction at the tank top, it will form a contact surface between the warm water and the cold one already in the tank. That contact surface has the velocity inside the core higher than the mean discharging flow velocity and it will reach the outlet diffuser at the tank bottom more quickly. Then, the tank height must be greater than that determined using the mean velocity flow alone without any viscous effect (ideal model), so that the discharging flow has the same stored water temperature during all the discharging cycle. The difference between those heights constitutes itself a tank loss of capacity. The study of the viscous effects on the tank performance shows that two non-dimensional parameters appear naturally: the Reynolds number related to the tank diameter, ReØ, used for determining the Hydrodynamic Entry Length, and the f number that relates the position of the contact surface of the water masses to the Hydrodynamic Entry Length. Results show that the loss due to viscous effects, may not be negligible and the choice of the tank diameter is essential to minimizes those effects. Also, there is a difference between tank losses of capacity during the charging and discharging cycles, since the effect of the density difference between the warm and cold water works in a different way in each one of those two situations