Development of CFD analysis to estimate flow behavior of PWR grid spacer

PWR grid spacer has mixing vanes to promote coolant mixing around fuel rods. It is important to estimate flow field at downstream of the grid more precisely because the pressure loss and departure from nucleate boiling (DNB) of the grid are influenced by cross-flow and axial flow generated by mixing vanes. Mitsubishi has developed the Computational Fluid Dynamics (CFD) evaluation method for thermal-hydraulic design improvement of PWR grid spacer. The rod type Laser Doppler Velocimetry (rod LDV) is applied for cross-flow and axial flow measurements at rod gaps and sub-channels to develop the CFD flow field estimation at the downstream of the grid. Test results were compared to the data of Particle Imaging Velocimetry (PIV) technique, which were conducted by Westinghouse. Similarity of both measurements was confirmed qualitatively and some of data were in good agreement quantitatively. Comparison of measured data at the downstream of the grid and estimated values of CFD analyses, which were simulated LDV test conditions, were carried out. As the first step, parametric CFD analyses, which focused on spatial discretisation schemes, were performed by 2-sub channel model. At the second step, to simulate the test conditions and to compare quantitatively CFD and measurement for whole measured positions, flow field around 5x5 grid spacer and 5x5 rod bundle in the test section was modeled and the analysis was performed. It is confirmed that good agreement of CFD and measurements in most of positions but in several positions relatively larger deviation is observed. The causes of large deviation are discussed and future studies are planned. Velocity measurement program was conduced in collaboration with Westinghouse Electric Company. (author)