Boiling heat transfer experiment and neutron radiography visualization in small diameter flow channel with plate blockage

Fundamental thermal-hydraulic experiments have been conducted using a tube for flow boiling characteristics study in the Reduced-Moderation Water Reactor (RMWR) core. It is necessary to study geometrical influences of fuel rod spacer on critical power for thermal-hydraulic design development of RMWR core fuel rod assemblies. In the present study, the effect of spacer thickness on high-void fraction two-phase flow and heat transfer is focused on under atmospheric pressure conditions. The neutron radiography (NRG) technique is used to measure void fraction with sufficient spatial resolution. Thermo-couples are implemented at the inside surface of a heated tube. An electrically heated tube is used with plate blockages of various thicknesses. The results show that the boiling heat transfer on the heated tube surface inside the spacer for the thick plate case with 2 mm in thickness was higher than downstream for from the spacer. The forced flow deviation by the chamfer is observed by the NRG flow visualization. The thinner plate with 1.1 mm in thickness brought no significant effect on the local boiling heat transfer, the blockage ratio of which is equal to that of the RMWR spacer. (author)