Numerical Investigation On The Flow And Heat Transfer Characteristics In Fuel Assemblies With Guide Tube

In modern Pressurized Water Reactor(PWR),17×17 fuel assembly is often used,in which there is 24 guide tubes.Guide tubes not only provide channels for control rods to move,but also support the fuel assembly.Guide tubes do not generate heat,so the flow and heat transfer in the guide tube sub-channels is different from that in the fuel rod sub-channels.However,the influence of the guide tubes on the thermal-hydraulic performance in the fuel assembly has not been discussed in most of the studies.Therefore,the thermal-hydraulic performance in a 7×7 rod bundle with guide tubes is studied in this paper.First,for both the single-phase heating condition and the two-phase adiabatic condition,the effects of flow behavior after the spacer grid on the temperature and phase distributions in a 7×7 rod bundle without guide tubes are respectively studied.The results indicate that swirl flow forms at the top of vanes and develops into the diagonally cross flow at 2D_h.Then it disappears at 8 D_h where the cross flow among sub-channels adjacent formed and the new swirl flow occurs at the center of sub-channels of the rod bundle at the exterior regions and extends until 18 D_h.The secondary flow enhances the heat transfer with low temperatures and high void fraction in the center of the sub-channels at the exterior regions.But the mass mixing between sub-channels is weak.Secondly,the thermal-hydraulic performance in the bundle with guide tubes is analyzed b the comparisons of the results for the the bundles without guide tubes.The results indicate that a single swirl forms at the top of mixing vane in the guide tube sub-channels due to the single vane at the top of the spacer grids.The guide tubes segregate the diagonally flow,which causes mass mixing with each other among several sub-channels.The influence distance of swirl is limited.For the flow field in the rod bundle with guide tubes,the downstream at 8 D_h is the transition location where the flow structure changes.The more complex flow field at the downstream of the spacer grid in the rod bundle with guide tube than that without guide tube due to interactions of secondary vortices.The rod bundle without guide tube has a high safety coefficient with the secondary vortices located at the center of sub-channel,thus reducing the boiling crisis risk.While the secondary vortices in the rod bundle with guide tubes are near the fuel rod wall adjacent guide tube and so the boiling crisis risk is high.Finally,the comparisons of standard Nu number for three types of rod bundles are performed in the sub-channel approach and the heat transfer capacity for the flow at the downstream of the spacer grid is analyzed.,The phase distributions and the mixing factor of swirl flow are quantitatively evaluated by the analysis of the non-uniformity coefficient of void fractionΔα.The results indicates that the mixing ability in sub-channels adjacent to the guide tubes is weak,while the mixing capacity in sub-channels far away from guide tubes is enhanced when the decay rate of secondary flow has slowed down.The secondary vortices formed at the downstream of grid can slow the decay of mixing capacity effectively in the sub-channels adjacent to the guide tubes.The secondary vortices mainly forms at external domain of the rod bundle,so the influence of secondary vortices on the thermal-hydraulic performance of the guide tube sub-channels in central domain of the fuel bundle is insignificant.