Mecanica Automotriz

Se presenta el estudio numérico de la transferencia de calor en una cavidad alargada e inclinada calentada diferencialmente desde la parte inferior. Se consideró a la cavidad alargada en dos dimensiones conteniendo en el interior aire. Se consideró que la placa absorbente y la cubierta de vidrio fueran superficies isotérmicas a una temperatura caliente y fría respectivamente, mientras las paredes laterales fueran adiabáticas. Los ángulos de inclinación estudiados varían entre 15° y 35°. Las razones de aspecto consideradas fueron de 8 y 12, en un intervalo de números de Rayleigh de 105 a 106. Las ecuaciones de conservación de masa, momentum y energía fueron resueltas numéricamente mediante la metodología de volúmenes finitos usando el algoritmo SIMPLEC. El modelo numérico fue verificado con resultados de la literatura encontrándose concordancia entre estos. Los resultados mostrados incluyen las distribuciones de las isotermas, isolíneas de corriente y la variación del número de Nusselt con respecto al ángulo para las dos razones de aspecto consideradas. Los resultados indican que la transferencia de calor se incrementa al aumentar el ángulo de inclinación, excepto para el caso de A=8, Ra=105 y =35° donde ocurre una transición en el patrón de flujo, así también, la transferencia de calor se reduce con la razón de aspecto para un número de Rayleigh fijo. Se obtuvieron correlaciones para determinar el número de Nusselt en función del ángulo de inclinación y del número de Rayleigh. Este estudio proporciona correlaciones que podrían ayudar en el diseño de captadores solares planos.

In this paper a numerical investigation of the interaction between two modes of heat transfer, natural convection and surface thermal radiation, in a tilted slender cavity is studied. The bottom and top surfaces of the cavity are heated and cooled at constant temperatures, while its sidewalls remain thermally insulated. The studied parameters are: the Rayleigh number (104  Ra  106), the aspect ratio (8  A  16) and the inclination angle (15◦  λ  35◦ ). The steady state 2-D governing equations have been solved by the finite volume method. All the inner surfaces are assumed to be gray diffuse emitters and reflectors of radiation. The numerical model was reduced and compared to cases reported in the literature finding a good agreement. Streamlines, isotherms and total Nusselt numbers as a function of Rayleigh number for different inclinations are presented. The interaction between the two modes of heat transfer reveals that the decoupling of two mechanisms of heat transfer is not possible; the performance
of the flow patterns, the isotherms and the radiative behavior on the walls was different for the uncoupled and coupled modes of heat transfer in the tilted slender cavity. The steady state results indicated that the radiative surface radiation coupled with natural convection modifies appreciably the
flow patterns and the average heat transfer in the slender cavity. The total heat transfer increases when the inclination angle increases, except when the flow structure changes from the multi-cell to the unit-cell pattern. However, the total heat transfer decreases when the aspect ratio increases. A comprehensive correlation for the total Nusselt number has been proposed.

Double-diffusive natural convection and surface thermal radiation in an inclined cavity that simulates a solar still device is studied numerically. The parameters considered were 103 ≤ RaT (RaM) ≤ 106, 8 ≤ A ≤ 16 and 15º ≤  ≤ 35º. The steady state 2-D governing equations have been solved by the finite volume method. Streamlines, isotherms, isolines of water vapor, mass flow rate of distillate and average Nusselt and Sherwood numbers as a function of Rayleigh number for different inclination angle are presented. The results show that surface thermal radiation modifies the fluid flow from one-cell to multi-cellular pattern due to the surface thermal radiation increases the velocity near the walls, as a consequence the average convective Nusselt number, the total Nusselt number and the Sherwood number were increased about 25%, 175% and 15%, respectively.  The mass flow rate of distillate increases as A, Ra and  increases. The most suitable case for a solar still device is that for RaT = 106 , A =16 and 25º .

The numerical study of double-diffusive natural convection in an inclined rectangular cavity, that simulates a solar still, is carried out. Surface thermal radiation and heat conduction in a glass cover are considered. The study analyzes the effect of the glass cover, the aspect ratio (5≤A≤20) and the tilt angle (15º≤≤35º). The value of A is related to the Rayleigh number. The values analyzed were 9.29x104≤Ra≤ 6.56x106, where the minimum value of Ra correspond to A=20 and the maximum value of Ra correspond to A=5. Streamlines, isotherms, iso-concentration of H2O, condensate of H2O and Nusselt and Sherwood numbers are presented. Results show that the energy transmitted through the glass cover causes asymmetry, increases the velocity near the walls, and modifies the flow pattern, and the heat and mass transfer. When A decreases, the multi-cell pattern decreases, and the convective heat and mass transfer increase up to 41%. As the tilt angle increases, the Sherwood and Nusselt number increase up to 3.8% and the condensate of water increases about 3% when the one-cell pattern prevails; when the multi-cell pattern decreases from =15° to =20° the Sherwood and Nusselt number decrease up to 22% and the condensate of water decreases up to 43%.