Enhancement of Natural Convection Heat Transfer in Annular Enclosures

Natural convection heat transfer was investigated experimentally in a concentric-cylinders annulus. To enhance the rate of heat transfer, pin fins were installed on the surface of the inner cylinder. Two fin arrangements were employed, namely, staggered and in-line arrangements. The length of the fins varied from 2 to 8 cm and the Rayleigh number Ra changed in the range from 5 x 105  to 5 x 106  The heat transfer medium was air at atmospheric pressure. So, the results reported here are for the Prandtl number Pr  0.7. It was found that, while the presence of fins usually decreases the heat transfer coefficient, the resulting increased surface area due to fins is so significant that the fins of both arrangements increase the rate of heat transfer. with the staggered one being more efficient. The normalized rate of heat transfer Q i / Q 1 was found to be independent of Rayleigh number, and for each fin arrangement, correlations were obtained to express Q i / Q 1 as a function of fins length. In the range of parameters considered in this work, the overall rate of heat transfer increased as much as 300%, corresponding to the longest fin of the staggered arrangement.