CFD analysis of the impact of a novel spacer grid with longitudinal vortex generators on the sub-channel flow and heat transfer of a rod bundle

In this study, a novel spacer grid with rectangular wing longitudinal vortex generators (RLVGs) is designed to improve the thermal-hydraulic characteristics of fuel assembly and simplify the spacer grid structures. The impacts of the attack angle (30°, 45°and 60°) and the distribution (two patterns) of RLVGs on the thermal-hydraulic characteristics in the sub-channels of a rod bundle are numerically investigated. Numerical results show that the secondary flow generated by the RLVGs can spread to the downstream region of spacer grid, disturb the boundary layer of the rods and improve the heat transfer of coolant. For the novel design of pattern 1, the spacer grid with RLVGs in attack angle of 45° generates secondary flow around the fuel rods just like a circulating flow in higher velocity, resulting better heat transfer performance and uniform cooling of fuel rods. Based on these results, a novel design of pattern 2 with doubled RLVGs number of pattern 1 is introduced and numerically investigated too. With the same attack angle of 45°, pattern 2 increases the average transverse velocity of the secondary flow and decreases the rod wall temperature in comparison with those of pattern 1 at the same Reynolds number conditions. Maximum increase in Nusselt number in the sub-channels is up to 30% while the increase in the total pressure drop is about 7.6% comparing with those of pattern 1. The present results indicate that the novel design of pattern 2 is an efficient way on improving the heat transfer of the fuel rod bundle, deserving more work to optimize its geometry structure and distribution before practical application.