To achieve the massive production of hydrogen, a thermo-chemical cycle coupled to a High Temperature Gas-cooled nuclear Reactor (HTGR) is considered as one of the systems exhibiting high application potential. The Reactor Cavity Cooling System is a key safety system that ensures the integrity of the HTGR during accident conditions; therefore, it is necessary to verify its performance for the safety of the HTGR as well as for the reliability of the coupled hydrogen production system. The RCCS performance depends on the heat transfer rate of riser ducts; however, the mixed convection that is likely to occur in the riser ducts can complicate the verification process. In this study, an experimental facility was constructed to investigate heat transfer experiment inside a riser duct and experiments with various heat flux and flow rate conditions were carried out. The experimental results demonstrated that the mixed convection occurred under certain experimental conditions in the riser duct, which resulted in heat transfer deterioration. The evaluated heat transfer coefficients from the experimental results were not consistent with those predicted by extant mixed convective heat transfer correlations which were derived from data obtained for different test section configurations. Therefore, a modified correlation was proposed to fit the experimental data for the RCCS riser duct with an average error of 6.06%. The correlation will contribute to the verification of RCCS performance and the credibility of the HTGR-coupled hydrogen production system. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.