CFD study of single phase flow in a PWR spacer grid

Spacer grids are an integral part of pressurized water reactor (PWR) fuel assemblies. They provide structural integrity as well as play a vital role in defining the thermal hydraulic behavior of the fuel assembly. The spacer grid design is continuously being improved to achieve better heat transfer characteristics. The studies performed to date include the design optimization and effects of mixing vanes on the thermal hydraulic behavior of fuel assembly. Majority of these studies do not include the effects of springs and dimples of the spacer grid. The present study was performed to analyze the flow behavior and pressure drop across a single coolant channel in the region of spacer grid along with all geometrical details. Detailed analysis of a single coolant channel comprising spacer-grid cell with dimples and spring was done using Ansys-CFX. The mesh independency study was carried out to make a compromise between solution time and accuracy. The problem was solved using K-ε model along with scalable-wall-function option. The Reynolds averaged mass conservation and momentum conservation equations were solved and the pressure and velocity field of flow channel was obtained. The average velocity varies from 2.52m/s to 7.35m/s for Reynolds number variation from 4.0×10⁴ to 11.7×10⁴. The cross flow velocities vary from 0.2 to 0.57m/s for these flow conditions. The Euler numbers calculated for these flow conditions were compared to the documented experimental results and were found to be in very good agreement. The analysis results conclude that single cell approach is an accurate and economical approach for predicting the pressure drop across the spacer-grid. The analysis also confirmed the applicability of k-ε model with scalable-wall-function for the prediction of the single phase hydraulic characteristics of spacer-grids.