Specifics of heat transfer to supercritical water flowing in a 1-m vertical bare tube with upward flow within the pseudocritical region

SuperCritical Water (SCW) is widely used in modern advanced coal-fired power plants around the world with the main objective to increase thermal efficiency of the Rankine power cycle from about 43% at subcritical pressures up to 55% at SuperCritical Pressures (SCPs). Unfortunately, the vast majority of modern Nuclear Power Plants (NPPs) equipped with subcritical-pressure water-cooled reactors have significantly lower thermal efficiencies (30‒36% (up to 38% for Generation III+ reactors)) compared to those of thermal power plants SuperCritical Water-cooled Reactor (SCWR) is a Generation-IV nuclear-reactor concept. The main advantage of NPPs with SCWRs will be higher thermal efficiencies (45% or even higher). Currently, there are some ideas to develop a Small Modular Reactors (SMRs) at SCPs. Therefore, for safe and reliable operation of future SCWRs more experimental data are required in bundle-flow geometries cooled with SCW. However, such experiments are extremely complicated and expensive. Therefore, as a preliminary, but a universal approach, experimental data obtained in vertical bare tubes (with diameters and heated length) similar to those of fuel bundles cooled with SCW are used. Current paper provides new experimental data obtained in a short (1 m) vertical bare tube (inner diameter 10 mm) cooled with upward flow of SCW. Analysis of this dataset is included. Main emphasis of this research is on SCW behaviour within the pseudocritical region, where all thermophysical properties undergo significant variations. Usually, three heat-transfer (HT) regimes are encountered at these conditions: 1) Normal HT (NHT); 2) Improved HT (IHT); and 3) Deteriorated HT (DHT). Conditions at which these HT regimes appear are also discussed. (author)