A STUDY ON THE LOCAL FLOW STRUCTURE AND TURBULENCE QUANTITIES INSIDE A HEATED RECTANGULAR DUCT IN TURBULENT NATURAL AND MIXED CONVECTION HEAT TRANSFER
Reactor Cavity Cooling System (RCCS) is a passive safety system in Very High Temperature gas-cooled Reactor (VHTR). RCCS is comprised of vertical rectangular riser channels, which surround a reactor vessel to remove afterheat emitted from it. The performance of RCCS is determined by the heat removal rate of RCCS riser and accurate prediction of it is important to ensure the integrity of the reactor vessel. In some circumstances, deterioration of heat transfer may occur in buoyancy-dominant conditions where heat transfer mechanism is complicated due to the effect of thermo-physical property variations. For this reason, in this study, to enhance the understandings of heat transfer mechanism of buoyancy-dominant conditions, flow visualization experiment was conducted measuring the local flow structure and turbulence quantities inside a heated rectangular riser duct. Experiment results show steep increase of vertical velocity profiles and their fluctuation distributions due to the buoyancy effect, where Grz/Rez2 ~ 16 and 6, which are included in turbulent natural convection condition and turbulent mixed convection condition, respectively. In both cases, an existence of secondary flows was assured from the horizontal velocity directing the centre of the channel that was observed at the corner of the rectangular test section. Anisotropy of Reynolds stresses of air induced by the buoyancy effect was also confirmed by comparing the distributions of vertical and horizontal velocity fluctuation distributions. Experimental data and outer wall temperature distribution obtained from the experiments would be used to assess prediction capabilities of turbulence models inside a heated rectangular riser duct.