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' with a pitch-to-diameter ratio (P/d) of 1.35 hasbeen experimentally investigated. Each array was positioned in an isothermal squareenclosure with a width-to-diameter ratio (W/d) of 20.6. Pressurized air or heliumwas used as the working fluid. It was observed that the bottom-tow rods were relativelyinsensitive to increases in the array size, as they exhibit only slight temperaturevariations, 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. Thecorrelations cover three flow regimes of conduction, transition, and convection inthe range of 6.45 < R^*_d < 3.08 × 10⁵. A generalized enclosure Nusselt numberwas correlated as a function of enclosure modified Rayleigh number and the arraysize (N). Comparison of the data with previous numerical prediction showed thatthis correlation may be readily used to obtain a conservative estimate of the maximumtemperature in the arrays with N = 3, 5, 7, and 9.