Large eddy simulation of turbulent flow in the rod bundle with different spacer grids

Large eddy simulation (LES) of the swirling flow in the rod bundle subchannels with spacer grids are presented. According to the rod bundle flow benchmark experiment, the 5×5 rod bundle with a split-type spacer grid is simulated by this transient simulation with a fine hybrid mesh. Mean velocity components, RMS (root mean square) velocity components and turbulent kinetic energy profiles at different locations downstream the mixing vanes are compared between the experiment and simulation results. LES results can reproduce the mixing turbulent flow velocity field well. We also have studied the influence of the mixing vanes' bending degree on the subchannel flow structures by using LES to simulate the 3×3 rod bundle with the typical PWR spacer grid. The mixing vanes' bending degree is modified to 35deg and 40deg based on the original spacer grid, whose mixing vanes' bending degree is 30deg. The larger bending degree produces a stronger mean coolant mixing effect, while larger pressure loss and fluctuation come together with this effect. The increased pressure loss would generate additional flow resistance, and larger fluctuation of coolant flow would make the mixing effect unstable, as well as would cause flow-induced vibration and grid-to-rod fretting phenomena. The LES method is a viable tool to optimize the spacer grid design and provide transient flow field data for mechanical analysis. (author)