Investigação numérica da convecção natural e da radiação térmica em cavidade aberta preenchida com meio poroso heterogêneo

Abstract

In an attempt to improve the heat exchange in thermal systems, several studies have been conducted on natural convection in heterogeneous porous media. The majority of them have in common the hypothesis of negligible heat transfer by radiation. Recent investigations have shown that surface radiation plays an important role on the overall heat transfer in cavities subject to natural convection. In this study, numerical methods are used to solve natural convection and surface radiation in an open cavity containing several disconnected and conducting solid blocks. The cavity is square with a vertical wall isothermally heated while in the other end it is opened to a fluid reservoir. The horizontal walls are kept adiabatic. The blocks are squared, equally spaced, disconnected, and heat conducting. The fluid properties are assumed constant except for the buoyancy term in which is used the OberbeckBoussinesq approximation. The cavity and block surfaces are grey, opaque and diffuse, with surface emissivity set as unity. The finite volume method is employed with the SIMPLEC algorithm and the effects of surface radiation are added to the solution by the model Surface-to-Surface (S2S). The effects of Rayleigh number, the number of blocks and solid-liquid thermal conductivity are investigated considering the emissivity and the porosity as constants. The average convective and radiative average Nusselt numbers are evaluated, as well as the dimensionless volumetric flow induced into the cavity and the convective and radiative drop phenomena are discussed. To assist in the interpretation of results, streamlines and isotherms are presented. From the results were observed that by considering the thermal radiation heat transfer, the total average Nusselt number increases and in some cases comes to double the values observed for pure natural convection. However, it was observed that the convective Nusselt number decreases. The attenuation of convection heat transfer due the consideration of thermal radiation is called convective drop and shows the duality of natural convection and thermal radiation. The results also show that, despite of the natural convection heat transfer attenuation, the dimensionless volumetric flow induced into the cavity increases.