Numerical Analysis Of Mixed Convective Heat Transfer In Vertical Concentric Cylinders 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.Mixed convective heat transfer in the entrance region of vertical concentric cylinders filled with porous media has been studied numerically by using SIMPLE method. Distributions of velocity, local friction coefficient of wall, temperature and Nusselt number are obtained for both buoyancy-aided flow and buoyancy-opposed flow in the vertical annuli of for three cases: (a) inner wall heated at uniform flux (UHF) and outer wall insulated; (b) inner wall heated at uniform wall temperature (UWT) and outer wall insulated; (c) both inner and outer walls have constant temperature. The results obtained are as follows:In the case of inner wall heated at UHF and outer wall insulated for buoyancy-aided flow (Gr>0), the maximum of velocity is shifted toward the inner wall. The velocity of fluid decreased and reverse flow occurred near outer wall. The variation of local friction coefficient is similar to velocity and local minimum value occurred in reverse flow area. The Nusselt number of inner wall(Nu) for Gr>0 is higher than forced flow(Gr=0) and increases as Gr increases. Nu for buoyancy-opposed flow (Gr<0) is lower than Nu for forced flow and Nu for Gr<0 reaches to the minimum value in reversed flow area when Gr is lower. Nu increases with Darcy number(Da) increases.In the case of inner wall heated at UWT and outer wall insulated, the hydrodynamic entrance length and thermal entrance length are greater than that of the first case. The distribution of developed velocity for Gr>0 and Gr<0 is same as the velocity for Gr=0. The local friction coefficient of inner and outer wall inclined to the same value after hitting to the maximum or minimum value. In the buoyancy-opposed flow, the temperature plot intercrossed at the initial entrance of cylinders with lower Gr. Nu decreases with Z increases in the buoyancy-aided flow and local peak value occurred of higher Gr. For Gr<0, Nu decreases to the minimum and then gradually increases to constant value. Nusselt number decreases and the distribution curve of developed velocity approaches the velocity distribution of turbulent flow as Da decreases.In the case of constant temperature on inner and outer wall, the reversed flow occurred in the buoyancy-opposed and buoyancy-opposed flow when is higher and the distribution of velocity keeps invariable with Z increases. The variation of the local friction coefficient of inner and outer wall is similar, and the influence of Gr is contrary. The inner wall Nusselt number(Nui) decreases as Z increases and Gr decreases and local maximum of Nu is appeared when Gr is lower. The outer wall Nusselt number(Nuo) increases as Z increases. Nuo for Gr<0 is higher than Gr>0 within Z<1.83 and it is contrary when Z>1.83. Nui is also increased as Da increases.