Computational fluid dynamics analysis of buoyancy-aided turbulent mixed convection inside a heated vertical rectangular duct

Highlights: • CFD analysis was conducted for turbulent mixed convection in a heated rectangular riser. • CFD analysis results with RANS turbulence models were compared with experimental data. • The reason for the differences in heat removal rate depending on the model was discussed. • Heat transfer deterioration inside a heated rectangular duct was investigated. Heat transfer in buoyancy-aided turbulent mixed convection inside a vertical rectangular duct with air was investigated through steady-state computational fluid dynamics (CFD) analysis with Reynolds-averaged Navier–Stokes turbulence models. Heat transfer can occur in the passive safety system of a nuclear reactor, which uses natural circulation as an initial driving force owing to its relatively low flow rate and strong heat flux condition. One example is the reactor cavity cooling system in a very high temperature gas-cooled nuclear reactor, whose high operating temperature enables high-efficiency power generation and hydrogen production. In buoyancy-aided turbulent mixed convection, heat transfer can deteriorate under specific conditions. Therefore, heat transfer must be analyzed because its deterioration can affect the performance of the safety system, which has an influence on the safety of the nuclear reactor. In this study, the results of a CFD analysis were compared with experimental results obtained from the riser heat transfer experimental facility, which is an experimental heat transfer facility with a 4.0 vertical rectangular duct test section. The CFD analyses were performed using a realizable k–ε model and a v²–f model, and the results were compared with the experimental results. It was confirmed that the results of the v²–f model exhibited good agreement with the experimental results. Based on this fact, four additional idealized cases were calculated using the v²–f model to investigate heat transfer deterioration in the turbulent mixed-convection regime. The calculation results revealed that the gradient of the velocity profile from the wall toward the center was reduced outside the near-wall region owing to buoyancy, which led to a reduction in the shear stress, turbulence generation, and heat transfer. In addition, the velocity gradient beyond the near-wall region reached a negative value above a certain buoyancy number, and turbulence generation and heat transfer started to recover. This analysis of heat transfer deterioration can provide insight into the construction of a heat-transfer coefficient correlation for buoyancy-aided turbulent mixed convection, and it will contribute to the development of heat transfer predictions and the evaluation of safety systems in nuclear reactors.