NATURAL-CONVECTION HEAT-TRANSFER IN HORIZONTAL ROD-BUNDLE ENCLOSURES

Natural convection heat transfer in enclosed horizontal N X N arrays ( N = 3, 5, and 7) of electrically heated rods,vith a pitch-to-diameter ratio (P/d) of 1.35 has been experimentally investigated. Each array w as positioned in an isothermal square enclosure with a width-to-diamet er ratio (W/d) of 20.6. Pressurized air or helium was used as the work ing fluid. It was observed that the bottom-row rods were relatively in sensitive to increases in the array size, as they exhibit only slight temperature variations, but the top-row rods demonstrated substantial temperature increases. Natural convection correlations in the form of Nusselt number (Nu(d)) as a function of modified Rayleigh number (Ra-d ()) were obtained for each rod in each array. The correlations cover three flow regimes of conduction, transition, and convection in the ra nge of 6.45 < Ra-d() < 3.08 X 10(5). A generalized enclosure Nusselt number was correlated as a function of enclosure modified Rayleigh num ber and the array size (N). Comparison of the data with previous numer ical prediction showed that this correlation may be readily used to ob tain a conservative estimate of the maximum temperature in the arrays with N = 3, 5, 7, and 9.