Experimental study of supercritical water heat transfer deteriorations in different channels

The experimental tests on heat transfer deterioration (HTD) of supercritical water flowing in 4 different kinds of channels, including tube, annular channel, and 2 × 2 bundle, have been carried out on the Supercritical WAter MUltiPurpose test loop (SWAMUP). The tube is made of Inconel alloy with different inner diameters; the annular channel consists of an Inconel alloy heated inner tube and a 304 stainless steel unheated outer tube. Two kinds of bundles were tested. The first kind of bundle consists of 4 Inconel-718 tubes with different Pitch-to-Diameter ratios, and the 2 × 2 bundle with grid spacers is installed into a square assembly box. The other kind of 2 × 2 bundle consists of 4 heater rods, and the rod bundle with wire wraps is also installed into a square assembly box. The heat transfer of supercritical water in simple channel like tube is stronger with larger cross-sectional flow area, but it is stronger with smaller cross-sectional flow area in complex channel like bundle. Two kinds of HTDs are observed in tube and annular channel, only the first kind of HTD is observed in bundle. The first kind of HTD occurs only at high ratio of heat flux to mass flow velocity in different channels, and the second kind of HTD is more likely to occur in simple channel which could be eliminated in bundle by the transverse turbulent flow. The effects of thermal-hydraulic and structural parameters on the second kind of HTD in tube are as follows: the HTD of supercritical water is severer with higher heat flux and the increment of wall temperature is much larger; the HTD occurs earlier and severer with lower mass flow velocity; the HTD delays and the wall temperature rises more slowly when the pressure rises; and the diameter has no evident effect on this kind of HTD. The heat transfer in bundles especially with wire wraps is qualitatively better and more stable than that in other tested channels. In order to avoid the first kind of HTD within the operating condition of SCWR, the low mass flow velocity condition and high heat flux condition should not occur at the same time during operation.