An experimental study on characteristics of post-CHF heat transfer in the high subcritical pressure region near to the critical pressure

Supercritical pressurized water cooled reactor (SCWR), which has a once-through water cooled reactor for supplying supercritical pressure steam at high temperature to a turbine system, is one of the next generation reactors for the purpose of improving economic efficiency and safety. In the SCWR system, water pressure passes through the critical pressure during startup, shutdown and in case of loss of coolant accident (LOCA). Similarly, in fossil-fired power plants, once-through boilers of sliding pressure type operate at supercritical pressure during nominal load, but they work near the critical pressure during partial load. In the high subcritical pressure region slightly below the critical pressure, critical heat flux (CHF) phenomenon readily occurs at relatively low heat flux and in a high subcooled region, and then it causes serious damages to fuel rods or boiler tubes due to the abrupt temperature rise in the rod or tube. In this study, experiments on post-CHF heat transfer in such the high subcritical pressure region near the critical pressure (reduced pressure range of 0.924 up to 0.992) in the range of mass flux 400-1000 kg/(m²·s) with a circular tube of 4.4 mm ID in vertical upward flow. HCFC22 and HFC134a were used as the test fluids. Based on the obtained data, characteristics of the post-CHF heat transfer were examined and found to be classified into two distinctive types mainly according to flow rate conditions. The influences of pressure, flow rate and heat flux on the characteristics were also clarified.