Critical heat flux testing on screw cooling tube made of RAFM-steel F82H for divertor application

Critical heat flux (CHF) testing on a cooling tube with a helical triangular fin on its inner surface, screw tube has been carried out to examine its applicability to divertor of DEMO. In DEMO designs, Reduced Activation Ferritic Martensitic (RAFM) steel, such as F82H, is one of the candidate materials for a cooling structure of PFCs instead of Cu-alloy. As thermal conductivity of F82H is about 10 times smaller than that of Cu-alloy, this study is intended to examine heat removal capability of the screw tube made of F82H. The screw tubes are the screw tubes with M10 of 1.5-mm-pitch. The M10 threads are directly shaped in F82H and OFHC-Cu tubes. Incident CHF (ICHF) of the F82H screw tube is reduced to about half of the OFHC-Cu tube. For instance, ICHF of the F82H tube is 13 MW/m2 at the flow velocity of 4 m/s and that of the Cu tube is 25 MW/m2. Numerical analyses indicate that the incident heat flux is highly concentrated for the F82H tube because of its low thermal conductivity. The ratios of the heat flux at the inner surface of the cooling tube to the incident heat flux are around 1.6 for the F82H tube and 1.1 for the Cu tube.