Experimental Research on Heat Transfer Enhancement for High Prandtl-Number Fluid

The experimental research on heat-transfer enhancement for such high Prandtl-number fluid as Flibe has been performed with a large molten salt circulating experimental loop named as "TNT loop" (Tohoku-NIFS Thermofluid loop). Through the experiments, a packed-bed tube is employed as the enhancer for molten salt. It is clarified that the enhancement of packed-bed tube is superior to that of turbulent heat transfer from the viewpoint of the same flow rate. Also, the 1/4-diameter bed is superior to the 1/2-diameter one at the same flow rate. Furthermore, at low flow rate, a little differences of heat transfer performance can be seen between the stainless-steel bed and copper bed. At high flow rate, however, the heat-transfer coefficient ratio strongly depends on the flow rate in the case of the 1/4-diameter copper bed only. As a result, it is considered that the thermal energy is expanded from a heated wall deeply and fast through packed bed at low flow rate. On the contrary, it is also considered that the convective heat transfer in the vicinity of a heated wall is strong at high flow rate. The evaluation from the viewpoint of the pressure drop shows that the turbulent heat transfer is superior to that with packed bed. However, the ratio of heat transfer with bed to turbulent one is steeply improved at low flow rate. Taking account of MHD effect, avoidance of erosion and electrolysis of Flibe, the enhancement under low flow-rate condition can be suitable in a fusion reactor.