Heat Transfer to Supercritical Water Flowing in Short Vertical Bare Tubes

Abstract It is well known that water-cooled nuclear-power reactors / Nuclear Power Plants (NPPs) equipped with light- and heavy-water-cooled reactors have relatively low thermal efficiencies within the range of 26–38% compared to those of thermal power plants – combined-cycle up to 62.5% and supercritical-pressure Rankine cycle up to 55%. Therefore, increasing thermal efficiency of next generation NPPs / nuclear reactors is a very important task. For water-cooled reactors / NPPs the conventional approach for increasing thermal efficiency is to move to supercritical pressures, as it was done more than 60 years ago in the thermal-power industry. Currently, together with large-capacity SuperCritical Water-cooled Reactors (SCWRs) SC-Pressure Small Modular Reactors (SCP SMRs) are also being developed. Therefore, solid knowledge of heat-transfer specifics at supercritical pressures is required. Usually, hydraulic-equivalent diameters of modern bundle flow geometries are within 5–10 mm and heated lengths are up to 4 m for SCWRs and can be shorter for SCP SMRs. Also, first SCWRs as test, experimental, or pilot reactors will be with relatively small installed capacities within 100–300 MWel. Due to that it is important to investigate supercritical heat transfer in small diameters tubes as a first and preliminary step instead of bundle geometries and bundle simulators. Usually, heat transfer in bare tubes is considered as the conservative approach compared to bundle tests, because heat transfer in bundle geometries will be higher compared to that in bare tubes due to additional turbulization from various appendages inside sub-channels. The paper presents experimental data obtained in relatively short vertical bare tubes mainly with upward flow of SCW. The experiments have been performed by Dr. V. Razumovskiy1 at the National Technical University of Ukraine. The data were obtained at the pressure of 23.5 MPa, mass fluxes of 2193 kg/m2s (main set of data) and 509 and 248 kg/m2s, and heat fluxes within 104–2660 kW/m2. The experimental data were compared to a well-known subcritical-pressure heat-transfer correlation developed by Dittus and Boelter. The heat-transfer-coefficient data were predicted quite reasonably with this correlation within the normal heat-transfer regime, but were significantly higher those within the deteriorated heat-transfer regime and within the pseudocritical region.