Numerical investigation of grid spacer effect on heat transfer of supercritical water flows in a tight rod bundle

The numerical investigation was carried out for the effect of grid spacer on the local heat transfer performance of supercritical water flows within the inner sub-channel of a tight, hexagon rod bundle using commercial CFD code STAR CCM+ 6.04. The standard two layer k-epsilon model by Wolfstein with two layer all y⁺ wall treatment was used as turbulence model. The main objective has focused on the special thermal hydraulic characteristics caused by the standard grid spacer and the grid spacer with split-vanes. The circumferential heterogeneity of heat transfer performance and corresponding mechanism, as well as the effect of heat conduction within the stainless steel cladding, were analyzed in detail. It was found that the local heat transfer within the grid strap is greatly enhanced due to the reduced flow area. In the downstream region of the standard grid spacer, the enlarged circumferential temperature difference and decreased heat transfer near the narrow gap are caused by the aggravated flow choking effect. In the downstream region of the grid spacer with split-vanes, the circumferential distribution of heat transfer performance is greatly changed. The local heat transfer is enhanced just downstream of the grid spacer. However, big circumferential temperature difference and a decreased heat transfer region are also caused by the swirling flow, indicating that the swirling flow has adversely affected the local heat transfer. The improved heat transfer performance downstream of the grid spacer with split vanes is more pronounced for the higher Reynolds number case. The circumferential distribution of cladding temperature is more uniform and the enhanced heat transfer caused by split-vanes is more pronounced when the heat conduction is considered.