Analysis of grid spacer effects on the flow and heat transfer of supercritical water flow in an inner sub-channel of a SCWR based on the second law of thermodynamics

In this paper, a numerical investigation was performed to understand the effects of grid spacers on the thermal hydraulic performance of supercritical water flows in the inner sub-channel of a tight, hexagon rod bundle. The Speziale-Sarkar-Gatski Reynolds stress model was verified, compared with experimental data and then used to simulate the heat transfer of supercritical water flows in a sub-channel with grid spacers, including a standard grid spacer and a split-vane type grid spacer. The entropy generation analysis method was used to analyze the effects of the angle of the split-vanes downstream of the grid spacer. It was found that the weakened heat transfer in the narrow gap region was observed downstream of the standard grid spacer, whereas it did not appear downstream of the grid spacer with split-vanes, indicating that the split-vanes have significant capacity to improve heat transfer. A comparison of various angles of split-vanes showed that small angles have lasting and stronger intensity of secondary flow than larger angles at the cost of a moderate pressure drop. The second law analysis illustrates the entropy generation behavior downstream of the grid spacer with split-vanes of different angles. Entropy generation analysis under various conditions was conducted to gain further insight. Based on these results, the optimal angle of split-vanes can be achieved.