Numerical investigation on thermal-hydraulic performance in 7×7 rod bundle with spacer grid and guide tubes

In this paper, the thermal-hydraulic performance in the downstream of the spacer grid in a 7 × 7 rod bundle is numerically studied. The guide tubes effects are revealed by the comparisons of the flow and heat transfer behavior in two types of rod bundles, one without guide tubes and the other with guide tubes. It is found that the cross flow diagonals are fully developed at Z = 2 Dh and unable to sustain at Z = 8 Dh downstream of the spacer grid. The guide tubes discontinue the cross flow diagonals and would decrease the secondary flow intensity. The downstream temperature distribution is influenced by the evolution of the secondary flow as well as the guide tubes. The non-uniformity of the temperature distribution has the lowest value at Z = 2 Dh and the guide tubes would make the temperature distribution more uneven. Furthermore, the normalized Nusselt number in both fuel rod bundles reaches the peak at Z = 2 Dh and then decreases consistently. However, the heat transfer can be enhanced by 15% at least because the secondary flow still exists. The normalized Nusselt numbers downstream of the spacer grid in the rod bundle with guide tubes are smaller than those without guide tubes. Overall, the guide tubes would inhibit the secondary flow and weaken the heat transfer capability downstream of the spacer grid.