An Experimental Study of Convection Heat Transfer Within Enclosed Horizontal Rod-Bundles

Natural convection heat transfer in enclosed N x N arrays of electrically heated rods has been experimentally investigated. Three arrays (N = 3, 5, and 7) were constructed with a rod diameter of 0.942 cm, length of 61.0 cm, and a pitch-to-diameter ratio (P/d) of 1.35. Each array was positioned in an isothermal square enclosure with a width-to-diameter ratio (W/d) of 20.6. Steady-state surface temperatures were measured on all half-symmetry rods while the arrays were subjected to working fluids of helium and air, containment pressures of 6.05 x 10−5 to 7 atm, and input rod powers up to 905 W/m2. The results showed that the rods located in the bottom rows of a given array have the lowest surface temperatures, while the top rods have the highest surface temperatures. Furthermore, it is observed that the bottom rods are relatively insensitive to increases in array size, as they exhibit only slight temperature variations, but the top rods demonstrate substantial temperature increases. Natural convection correlations in the form of Nusselt number (Nud) as a function of modified Rayleigh number (Ra*d) were obtained for each rod in each array. The correlations are subdivided into the three flow regimes of conduction, transition, and convection, which span a modified Rayleigh number range of 6.45 < Ra*d < 3.08 x 105. The reported correlations can be used to obtain conservative temperature predictions in N=3, 5, or 7 arrays, or they may be combined with a radiation model to calculate accurate temperature predictions. A generalized enclosure Nusselt number was correlated as a function of enclosure modified Rayleigh number and the array size (N). This correlation may be readily used to estimate the maximum temperature in the N=3 to 7 arrays.