Numerical Analysis Of Natural Convection In Vertical Annulus Filled With A Porous Medium

Heat and mass transfer in porous media can be seen in nature and some processes of industry. These processes relate to water conservancy, geology, chemical engineering and environment. With the rapid development of nuclear industry and electronic industry, the mixed convective heat transfer has been the focus of researchers. Natural convection in a vertical annulus with saturated porous medium was numerically investigated in order to analyze the hydrodynamic and heat and mass transfer characters. The dimensionless equations are obtained by using stream function. For natural convection caused both by inner and outer heat sources, the results show that the anticlockwise swirl can appear near the upper of the inner wall and these zones will move to the bottom as the Rai and A increases. Effected by inner and outer heat sources, two regions of T >1 and T <1 occur in the temperature field at relatively high Rai and A, and the region of T >1 expands with increasing Rai and A . The local Nusselt number on the inner wall increases at first and then decreases along with the Z direction as Rai is small. When Rai big enough (Rai>500), Nui is in the negative value mostly at the same time its absolute value runs up, especially change vapidly in the area of top. In the other hand, the local Nusselt number on the outer wall increases rapidly in the top of annulus. The average Nusselt number on the inner wall decreases linearly as Rai increase. In the case of natural convection with heat and mass transfer, the interaction of heat and mass buoyancies relates to boundary conditions, heat and mass coefficient of expansion of the problems studied. When N >0, two kinds of buoyancies strengthen each other, the velocity of fluid flow and heat and mass transfer rate is accelerated. As N <0 two buoyancies offsets and weakens each other, the convection nearly stops and heat and mass transfer rate drop to minimum. After this point the velocity of flow begins to increase with the buoyancy radio N reduced, heat and mass transfer rate increase again as well. Lewis number represents the comparison of heat diffusion coefficient and mass diffusion coefficient of porous medium, on the terms that studied (N=-5~5), Nu is reduced with increase of Le and Sh increase with increase of Le as N is a constant.