A comparison of natural convection heat transfer for a staggered versus analigned array of horizontal spent nuclear fuel rods within a rectangular enclosure

Natural convection heat transfer was experimentally investigated in a stagg ered array of heated cylinders, oriented horizontally within a rectangular isothermal enclosure. The test conditions were characteristic of a spent-fu el assembly during transport or horizontal dry storage. The assembly was co nfigured with a pitch-to-diameter ratio of 1.33 and backfilled with pressur ized helium or nitrogen. The backfill pressure was varied between 1 and 5 a tm, while the assembly power was varied between 1 and 5 W per heater rod. T he resulting data are presented in the form of Nusselt-Rayleigh number corr elations, where the Nusselt number has been corrected fro thermal radiation using a numerical technique. The staggered-array data are compared to perv ious data for a similar-pitch aligned rod array (a simulated boiling water reactor fuel assembly) to determine if convective heat transfer is enhanced or hindered in a staggered configuration. For the overall array, both the staggered and aligned configurations yield Nusselt-Rayleigh curves with a t hree-regime trend, which suggests distinct conduction and convection regime s separated by a transition regime. For lower Rayleigh number (<10(6)), rep resentative of the conduction regime, the aligned-array Nusselt number is 1 0 to 12% higher than the corresponding staggered-array value. However, in t he convection regime at higher Rayleigh numbers, the staggered-array Nussel t number slightly exceeds the aligned-array Nusselt number. This is attribu ted to the fact that the staggered array begins to transition into the conv ection regime at lower Rayleigh number than the aligned array. For both con figurations, the slope of the Nusselt-Rayleigh curve in the convection regi me suggests turbulent flow conditions.