Core heat transfer experiment for JRR-3 to be upgraded at 20 MWt, 1

This report describes the experimental results and discussions of the differences in forced convection heat transfer characteristics between upflow and downflow for a reactangular flow channel, which has not been clearly understood in the previous investigations of the heat transfer correlations for the analysis of JRR-3 which is to be upgraded. The JRR-3 is a research reactor of about 20 MWt at a normal condition of operation, whose core is cooled by the downflow at about 6.2 m/s under rather low pressure and temperature. It is of great importance for analyzing the thermal-hydraulics in the downflow and the upflow during the normal operation condition, operational transients and accidents of the JRR-3 to make clear the differences in heat transfer characteristics between upflow and downflow for a rectangular flow channel. Experiments were carried out in which flow velocity was changed from 0.07 to 7 m/s for the flow channel (2.25 mm in water gap) simulating a subchannel in a standard fuel element of the JRR-3. The flow channel is made of two flat plate heater elements which are 750 mm in length, 1 mm in thickness and 40 mm in width. As the results, it was made clear that there are no significant differences in heat transfer characteristics between upflow and downflow for Reynolds Number larger than 2000, but that the heat transfer coefficients in downflow become lower than those in upflow for Reynolds Number less than 700. This is because of buoyant effect. Based on the experimental results described above, forced convection heat transfer correlations were developed for both of upflow and downflow in a rectangular channel, which are applicable for analysis of the JRR-3 to be upgraded at 20 MWt. (author)