Analysis of the turbulent flow in a partially blocked wire-wrapped rod bundle using LES with wall functions

Abstract

One of the most promising nuclear reactor designs is the Liquid-Metal Fast Reactor (LMFR). Its fuel assembly consists of rods containing the fuel material, arranged in a triangular lattice and covered by a hexagonal duct. The rods are separated with wire spacers that are helically wrapped around each fuel pin. Because this geometry is tight and compact, accumulation of debris, cladding deformation or detachment of the wires may generate a partial or total flow blockage of coolant subchannels at different locations. The understanding of the coolant behavior in LMFR fuels under the influence of a partially blocked flow area is required for a thorough safety assessment. A Large Eddy Simulation (LES) with wall functions was executed to simulate the flow on a 61-pin wire-wrapped rod bundle under the presence of a blockage located on the bundle periphery to simulate an abnormal operation condition. The Reynolds number was Re=17000, which is in the turbulent regime. Results of mean velocity and root mean square (rms) fluctuating velocity profiles were compared with PIV experimental data from the Texas A&M Thermal-Hydraulic Research Lab. Both mean and rms velocities were found in reasonable agreement with the experimental data among different locations because the simulation results were within 2σ from the PIV measurements. A pressure form loss coefficient due to the blockage was calculated and found to be Kloss=1.0455. The fluctuating velocities revealed a pair of shear layers that formed on each side of the blockage carrying the highest turbulent intensities. A spectral analysis was conducted to visualize the dominant frequencies on the turbulent flow in the recirculation region. The spatial–temporal two-point cross-correlations based on the fluctuating velocities were calculated at different points along the shear layer and were used to estimate integral lengths scales and convection velocities. The cross-correlations showed larger structures at higher axial elevations from the blockage rear wall. The length scales were found in the range of 3 to 21 mm while the magnitudes of the convection velocities presented values between 0.69U¯in to 1.39U¯in, where U¯in is the inlet velocity. Proper orthogonal decomposition (POD) analysis was utilized to extract the dominant modes in the fluctuating flow. Large flow structures and low frequency time signals were observed for the lowest order modes.