The Effect of Support Grid Design on Azimuthal Variation in Heat Transfer Coefficient for Rod Bundles

Support grids are an integral part of nuclear reactor fuel bundle design. Features, such as split-vane pairs, are located on the downstream edge of support grids to enhance heat transfer and delay departure from nucleate boiling in the fuel bundle. The complex flow fields created by these features cause spatially varying heat transfer conditions on the surfaces of the rods. Azimuthal variations in heat transfer for three specific support grid designs, a standard grid, split-vane pair grid, and disc grid, are measured in the present study using a heated, thin film sensor. Normalized values of the azimuthal variations in Nusselt number are presented for the support grid designs at axial locations ranging from 2.2 to 36.7 $Dh.$ Two Reynolds numbers, $Re=28,000$ and $Re=42,000$ are tested. The peak-to-peak azimuthal variation in normalized Nusselt number is largest just downstream of the support grids and decreases to a minimum value by the end of the grid span. A comparison of the azimuthal heat transfer characteristics between the support grids indicates distinctive results for each type of support grid design tested. The split-vane pair grid exhibits the largest peak-to-peak variation in azimuthal heat transfer of +30% to −15% just downstream of the grid at 2.2 $Dh.$ The disc grid has the most uniform azimuthal heat transfer distribution with a peak-to-peak value of ±4% for all axial locations tested.