Development of a heat transfer coefficient correlation for buoyancy-aided turbulent mixed convection of air inside a vertical channel

Highlights: • Experiments for heated upward flow of air inside a vertical channel were performed. • A heat transfer coefficient correlation for turbulent mixed convection was proposed. • Prolonged flow development length was considered in the correlation. • The correlation predicted more accurate Nu by 27.3% than the extant correlation. -- Abstract: Passive cooling systems in many nuclear power plants utilize natural air circulation for cooling. When this cooling process occurs in a vertical duct, it concurrently has the characteristics of natural convection and forced convection; this state is referred to as mixed convection. The heat transfer rate for mixed convection of buoyancy-aided turbulent flow is less than that for forced convection at a corresponding Reynolds number. Previous studies on the mixed convection have focused on supercritical fluids; however, studies on heat transfer correlations for atmospheric air are limited. Therefore, in this study, experiments were conducted on the atmospheric air inside a vertical rectangular duct under the turbulent mixed convection condition, to determine the heat transfer correlations. The existing correlations for turbulent mixed convection were evaluated based on the experimental results. In addition, a new correlation was developed that yields the smallest discrepancy with the experimental results. The proposed correlation includes a modified buoyancy number and considers changes to the flow structure, according to the traveling distance of the flow. Finally, the proposed correlation was validated using existing data from experiments in which air was the working fluid.