A Study on the local Flow Structure in a Strongly Heated Rectangular Riser Duct with a Flow Visualization Experiment and CFD Analysis

Reactor Cavity Cooling System (RCCS) is a passive safety system of Very High Temperature gas-cooled Reactor (VHTR) being developed at Korea Atomic Energy Research Institute (KAERI). RCCS incorporates rectangular riser channels to remove the afterheat emitted from the reactor vessel. Because an accurate prediction of heat removal rate through the RCCS riser is important to ensure the safety of the reactor, understanding of the heat transfer phenomena in the RCCS riser is needed. In the RCCS riser channel, mixed convection heat transfer may occur in some circumstances whose heat transfer mechanism becomes complicated with the effect of thermo-physical properties variation. In the mixed convection, due to the buoyancy force induced by temperature and density differences, local flow structure and heat transfer mode near the heated wall have significantly dissimilar characteristics from both forced convection and natural convection. To predict accurate heat removal rate in RCCS riser duct, the local flow structure and turbulence quantities should be investigated in the rectangular riser duct. CFD analysis was conducted to validate the prediction capability of turbulence models with V2F and realizable k- ε turbulence model. The peak velocity near the wall region was over-estimated at the mid-plane by both of the turbulence models. Further investigations will be conducted for the local flow structure and turbulence quantities in mixed convection condition as well as natural convection condition.