Study on the Lateral Velocity and Vortex in a Spacer Grid

The spacer grid is one of the most important parts of a fuel assembly for enhancing thermal-hydraulic performance. The mixing vane is a critical component of the spacer grid (or mixing vane grid) to produce lateral flow, turbulence, vortex, and other mixing mechanisms downstream of the grid. The design and arrangement of mixing vanes could influence the flow condition downstream, especially the lateral flow and vortex surrounding the rod bundle, which directly affect not only the hydraulics and heat transfer but also the critical heat flux. Understanding fundamental mixing characteristics is of great importance for fuel assembly design. Instead of using complicated geometries, this paper studies the lateral flow velocity and vortex in a simple 2 x 1 channel with a spacer grid in order to obtain the behavior of a single mixing vane and its effect on the downstream lateral velocity and vortex. Then, the effect of mixing vane arrangements on pressure drop is studied by researching the lateral velocity and vortex in the 2 x 1 channel with two mixing vanes. Several indexes that could reflect the mixing characteristics, such as lateral velocity and vortex, are introduced to assess the performance of the mixing vanes. Because of the presence of walls in both the experiment and the simulation, the lateral flow will reflect upon the impact of the obstruction walls or rods, which would cause vortex rotation. Rotation time and flow pattern are also studied in this paper. Qualitative and quantitative analyses are carried out to obtain the lateral velocity, the vortex angle, and its period of rotation. This work may be applied to the design and optimization of mixing vanes.