Natural Convection Heat Transfer With Thermal Radiation In A Building Room With Porous Medium Envelope At High Rayleigh Numbers

Natural convection and heat transfer extends all over the natural world and the areas of industrial and agricultural production, in which, the rectangular cavity partially filled with porous medium have a broad application background, such as the design of buildings for thermal comfort, in the field of building energy saving, analyzing the optimum thickness of wall is a typical energy-saving measure, and industrial processes such as industrial drying and catalytic operations, nuclear engineering, grain storage and so on. In view of the composite cavity has such a wide range of practical value, whether it is from the scientific research or from the engineering application, it has the vital significance to study the fluid flow and the energy transfer phenomenon in it.During the practical engineering applications, the space that involves mostly large scale area, so the flow pattern of the fluid is high Rayleigh number. This paper deals with natural convection heat transfer with thermal radiation in a rectangular cavity partially filled with porous medium at high Rayleigh numbers. The governing equations for the momentum and heat transfer in both free fluid and porous medium were solved by the finite element method. Comparisons with experimental and numerical simulation results from the literature have been carried out to check the accuracy of the present numerical method. To explore the effects of Rayleigh numbers (high Rayleigh number and low Rayleigh number) on natural convection and heat transfer in the free fluid and porous medium, especially the high Rayleigh number. Secondly, this paper analyzes the distribution of near wall boundary layer, temperature field and flow field in a variety of surface emissivities. Discussing the influence of surface emissivity. The natural convection heat transfer inside the cavity of the composite porous medium under different thickness was simulated,and discussed the effect of the thickness of the porous medium. Then, analyzing the temperature field and flow field in the composite cavity with conditions of the adiabatic wall and non-adiabatic wall. Finally, simulating the air flow pattern and the temperature distribution in summer and winter, and discussing the difference of winter heating and summer cooling load calculation.It was found that as the same as the fluid in low Rayleigh number (Ra= 104,105,106),the high Rayleigh number (Ra= 108,109,1010) fluid with the increase of Rayleigh number, the natural convection and heat transfer become stronger. Comparison between Ra= 106 and Ra= 1010, temperature of the fluid at Ra=1010 tends to be evenly distributed, and having regular streamlines. The affects of convection and heat transfer are more significant. The boundary layer becomes thinner, the average number of convection Nusselt at the interface is well corroborated this point. The presence of the wall thermal radiation weakened natural convection, making near-wall boundary layer thicker, and the appearance of surface thermal radiation can significantly change the temperature fields even if the given emissivity is quite small. And rises the on the mean convective Nusselt number at the interface with the increase of emissivity. The total Nusselt number and radiative Nusselt number increase with the increase of emissivity. This indicates that surface radiation plays an important part in heat transfer in the cavity. The Nuc at the interface decrease with increase of the thickness of the porous medium d,until d<0.25. And there is no change in Nuc when d is further increased. This means that when the thickness of the porous medium reaches a certain value, continue to increase the thickness, the heat transfer will not be reduced. When the top layer is non-adiabat, the core area of vortex is in the lower part of the cavity. The upper portion of the streamline tends to uneven distribution, the contour of temperature is more distorted in the upper part, the boundary layer becomes thinner, and the temperature of cavity increase significantly and tends to unevenly distributed. In winter and summer, the outdoor environment temperature difference is big, which causes the obvious change of the indoor environment. The high and low temperature distribution on both sides of the winter and summer is just the opposite, so the distribution of the flow field in the cavity is symmetrical. Compared with the summer, the vortex core area is located in the lower part of the fluid region, the heat transfer ability is higher than that of the summer, but the temperature difference is larger in winter. In the two season, there is a large temperature difference between the porous medium and the fluid region.In practical application, the impact of surface radiation on natural convection and heat and mass transfer in the closed cavity is objective existence. And a lot of process is high Rayleigh fluid flow, as for grain storage, construction environment and so on. But there is relatively little research about natural convection heat transfer with thermal radiation in a building room with porous medium envelope at high Rayleigh numbers. So more attentions are needed to the problem as to provide appropriate guidance to engineering practice.