Flow visualization experiment in a two-side wall heated rectangular duct for turbulence model assessment in natural convection heat transfer

Highlights: • Flow visualization methodology and wall temperature measurement strategy were established. • Velocity distribution and turbulence quantities were measured in a heated rectangular duct. • Secondary flows were observed at the corner of the rectangular test section. • RANS turbulence models have limitations in predicting the flow behaviors. - Abstract: Reactor Cavity Cooling System (RCCS) is one of the passive safety systems in Very High Temperature gas-cooled Reactor (VHTR). RCCS incorporates riser channels connected with a chimney to remove the residual heat emitted from the reactor vessel using natural circulation, and therefore, heat transfer phenomena in the RCCS riser duct is of great importance to ensure the safety of the VHTR. To enhance the understanding of heat transfer mechanism in a heated riser duct with rectangular shape, visualization of the local flow structure was conducted with an experimental facility for the natural convection heat transfer applying the Particle Image Velocimetry (PIV) method. With measured thermal boundary conditions by IR thermometry, CFD analysis was performed for the test facility using two different turbulence models. By comparing local flow structure from the experimental data and CFD calculation results, turbulence model assessment was conducted to confirm the prediction capability of the models. Experimental data show the magnitudes and locations of maximum velocity peak which is induced by buoyancy force and the anisotropic behaviors of velocity fluctuations near wall region. CFD calculations overestimate the maximum velocity near the wall compared with experimental data, and they seem to have limitations in reproducing the secondary flow observed at the corner of the test section in the experiment. The established methodology for flow visualization and the measurement strategy for thermal boundary conditions could contribute to obtain extensive experimental data and turbulence model assessment results, which would improve the understanding of the heat transfer mechanism to enhance the performance prediction capability for the passive heat removal system of the VHTR.