COMBINED NATURAL-CONVECTION AND SURFACE RADIATION IN THE ANNULAR REGION BETWEEN A VOLUMETRICALLY HEATED INNER TUBE AND A FINITE CONDUCTING OUTER TUBE

Buoyancy-driven natural-convection heat transfer in enclosures has bee n the subject of considerable research with applications to electronic packaging, solar collectors, and shipping containers for spent nuclea r fuel. A numerical study has been carried out to predict combined nat ural-con vection and radiation heat transfer in the annular region bet ween concentric tubes. The inner tube was volumetrically heated. Both tubes were of finite conductance. The surfaces of the annular region w ere diffuse and gray. The gas in the annulus was assumed to be nonpart icipating. A newly developed hybrid finite element finite difference m ethod was used for the study. This method combines finite element disc retization of geometries with finite difference discretized solution p rocedures for the governing differential equations. This study examine d the effects of surface radiative properties and material conductivit ies on the temperature and velocity fields and on local heat transfer rates. Fluid Rayleigh numbers ranging from 10(3) to 10(7), ratios of s olid to fluid region thermal conductivities ranging from 10 to 10(4), and surface total hemispherical emissivities ranging from 0. 0 to 1. 0 were examined in this study. It was found that the heat transfer acro ss the annulus was dominated by conduction and radiation for the lower Rayleigh number flows. As the fluid Rayleigh number increased, con ve ction became a primary mode of heat transfer. As the surface emissivit y was increased in the annulus, the average Nusselt number on the inne r tube surface decreased. The ratio of thermal conductivity was found to have little effect on the convective and radiative modes of heat tr ansfer, for a fixed value of the fluid Rayleigh number, when the ratio was > 100. When the conductivity ratio was < 100, the inner tube was thermally coupled to the fluid region, and the conductivity ratio affe cted the distribution of convective and radiative flux distributions, resulting in local peaks and valleys in the temperature of the inner t ube.