Local flow structure and turbulence quantities inside a heated rectangular riser in turbulent forced and mixed convection heat transfers

Heat transfer deterioration in turbulent mixed convection affects the performance and safety of various pieces of industrial equipment. The reactor cavity cooling system (RCCS), a passive safety system in a very high temperature gas-cooled nuclear reactor (VHTR), incorporates rectangular riser channels to remove residual heat emitted from the reactor vessel, where turbulent mixed convection may occur under some emergency operation conditions. Understanding the heat transfer phenomena in the RCCS riser duct is, therefore, important to ensure the safety of the VHTR. To investigate the heat transfer phenomena inside a heated rectangular duct, a visualization experiment was carried out to obtain the local flow structure and turbulence quantities in turbulent forced and mixed convection heat transfers. Using the particle image velocimetry method, the velocity fields inside the heated test section were captured and time-averaged velocity distributions and root-mean-square velocity fluctuation distributions were obtained under various experimental conditions. Inside a rectangular test section, the flow had a nonaxisymmetric Reynolds shear stress distribution, related to secondary flows near the corner of the test section. According to the overall distributions of the local velocity and turbulence quantities under various experimental conditions, the mechanism of the heat transfer deterioration inside a heated rectangular riser was investigated in relation to the distribution of the Reynolds shear stress. Inside the rectangular test section with the intense buoyancy force, flow laminarization near the corner was preceded by that of the center with decreases in secondary flows and Reynolds shear stress distribution near the corner, which indicate a transition from forced to mixed convection. According to these results of the visualization experiments, it was estimated that the heat transfer deterioration in the rectangular riser would be affected by the aspect ratio of the test section.