Experimental investigation on transient heat transfer in 2 × 2 bundle during depressurization from supercritical pressure

The experimental investigation on transient heat transfer of supercritical water during depressurization in 2 × 2 bundle with wire wraps has been performed on the SWAMUP-II co-constructed by CGNPC and SJTU. The test section consists of two channels separated by a square steel assembly box with round corners. Water flows downward in the first channel and then turns upward in the second channel to cool the 2 × 2 bundle with wire wraps installed inside the assembly box. The bundle consists of four heater rods of 10 mm in O.D. arranged with pitch-to-diameter ratio of 1.18. Experimental parameter ranges cover pressure from 16 to 26 MPa, mass flux from 850 to 1450 kg/m2 s, heat flux from 250 to 650 kW/m2, inlet fluid temperature from 345 to 365 °C, and depressurization rate of 1 and 2 MPa/min. The experimental data are obtained and heat transfer characteristics during depressurization from supercritical to sub-critical conditions are discussed. The fraction of heat transfer from inner channel to outer channel is lower than 15% under the test conditions. Four different transient heat transfer phenomena during depressurization are observed. The boiling crisis is likely to occur in SCWR during depressurization as the operating condition of SCWR is compared with the test condition. The boiling crisis is less likely to occur with the increase of mass flux, and more likely to occur with the increase of heat flux or inlet fluid temperature. The wall temperature increment during boiling crisis is smaller with higher mass flux, and larger with higher heat flux or inlet fluid temperature. The depressurization rate affects the transient heat transfer itself trivially.