Experimental study on transient heat transfer across critical pressure in 2 × 2 rod bundle with wire wraps

An experimental study on transient heat transfer of supercritical water during pressure variation across critical pressure in a 2 × 2 rod bundle has been carried out on the Supercritical WAter MUltiPurpose test loop II (SWAMUP-II). The bundle consists of four heated rods with an Outer Diameter (O.D.) of 10 mm and a pitch-to-diameter ratio of 1.18. The 2 × 2 rod bundle with wire wrap is installed into a square assembly box. The experimental conditions are as follows: pressure ranging from 16 to 26 MPa, mass flux ranging from 850 to 1450 kg/m2 s, heat flux ranging from 250 to 650 kW/m2, inlet fluid temperature ranging from 345 to 365 °C, and depressurization/pressurization rate of 1 MPa/min. The experimental data are obtained and the transient heat transfer characteristics during pressure variation across critical pressure are discussed. The heat transfer near the critical pressure is unstable as the physical properties of water changes fiercely, and the boiling crisis is likely to occur during both pressurization and depressurization, so the pressure range from 21 MPa to critical pressure is defined as a transition pressure section. The thermal-hydraulic parameters affect the transient heat transfer alike during both pressurization and depressurization, and obviously within the transition pressure section. The boiling crisis is more remarkable and the wall temperature increment is bigger with lower mass flux, higher heat flux or higher inlet fluid temperature. In order to avoid boiling crisis within the transition pressure section, the mass flux should be increase to or kept at higher level, the heat flux should be kept at or reduced fast to lower level, and the flow boiling should be kept at subcooled boiling.