Computational Analysis of Grid Effect of RBHT Experiment Using MARS-KS and FLUENT

Abstract

Understanding of the reflood heat transfer is critical because Peak Cladding Temperature (PCT) may occur during this reflood period. Focused on local thermal-hydraulic phenomena, Rod Bundle Heat Transfer (RBHT) experiment is aimed for the reflood model development and validation. With the RBHT experiment, it was observed that local cladding temperatures near the grids were decreasing, which is opposite to the general increasing trend of the cladding temperature with axial location. On the other hand, MARS-KS code calculated increasing tendency for the cladding temperatures near the grids since a specific grid model is not employed in the current version. In this study, FLUENT simulations were carried out to investigate the validity of the mixing vane grids by considering single-phase steam flow. Calculation results of the MARS-KS code were adopted as boundary conditions because experimental data is limited for CFD calculation. Tendency of the temperature drop was observed near the grid. It is believed that this is due to the heat transfer augment by enhancement of velocity and induced swirl flow. In addition to the mixing vane effect, heat loss effect due to the unheated rod and housing are considered, which was difficult to simulate using the MARS-KS code