A. Sagara

K. Yuki

M. Omae

S. Chiba

S. Toda

National Institute for Fusion Science (NIFS-Repository)

Fig. 2 Heat transfer enhancementby pebble tubesD/3 pebbleFig. 1 TNT loop & pebbleConclusionThe effectiveness of packed bed tubes for the heattransfer enhancement of high Pr fluid like Flibe was verifiedfrom the heat transfer experiment using high temperaturemolten salt HTS. In the future, it is necessary to enhancethe heat transfer more by developing an advanced packedbed tube. Furthermore, it was clarified that the analogy ofheat transfer ina straight smooth tube didn't stand up underthe high temperature conditions. It is considered that fluidcomposition change by thermal decomposition in the vicinityof wall that is a high temperature region seems to be a maincause and that is a very important issue for the future blanketdesign.[1] S. Toda, S. Chiba, K. Yuki, et aI., Experimentalresearch on molten salt thermofluid technology using ahigh-temperature molten salt loop applied for a fusionreactor Flibe blanket, Fusion engineering and design,63-64 (2002), 405-409.method was adopted in order to reproduce the geometricalboundary of circular cylinder with accuracy. Fig. 3 showsthe representative flow field near the wall. As a result ofmaking the cylinder's configuration as the parameter on theheat transfer, it was confirmed that rapid flow formedbetween the cylinder and the wall and wake, whichfluctuates behind the cylinder, strongly affected the heattransfer enhancement. The detailed mechanism of heattransfer enhancement for the packed bed tube and theoptimum design of bed which take the pressure drop intoaccount will become possible by carrying out 3-dimentionalcalculation around the pebble sphere.14 15 16 17 18xJdFig. 3 Flow structure near wallIntroductionTo enable design of a Flibe liquid blanket system inhelical type of nuclear fusion reactor FFHR, it isindispensable to enhance Flibe's low heat transfer abilityoriginated from high Pr number fluid. In the present study,in order to contribute the design of advanced thermal systemin the fusion reactors, heat transfer experiments by use ofhigh-temperature molten salt HTS, which has the equivalentthermofluid characteristics to Flibe, are performed and thefundamental heat removal performance for the high Pr fluidas well as the handling techniques of high temperaturemolten salt is evaluated.Toda, S., Yuki, K., Chiba, S., Omae, M.(Tohoku University)Sagara, A.§ 6. Design Research on the AdvancedThermal Structure System in theNuclear Fusion ReactorHeat transfer experiment with HTS forced-circulation loopIn a molten salt forced-circulation loopTNT(Tohoku-NIFS Thermofluid Loop)[l], it is possible tocirculate the high temperature molten salt such as Flibe,HTS etc. at the maximum fluid temperature of 600C and atthe maximum flow rate of 20L/min. The TNT mainlyconsists of the dump tank, the circulating pump, the testsection, the upper tank, and the air-cooling tower as shownin Fig. 1 and the high temperature molten salt, which ismelted in the dump tank, is filled up into whole the pipingsystems by pressurizing Ar gas inside the dump tank andthen circulated. In order to evaluate the fundamental heattransfer characteristics and develop an advanced heattransfer promoter for the high Pr fluid, several types ofpacked bed tube are adopted as the test section. With thepacked bed tube, the diameter of pebble sphere is D/3(D:inner diameter of pipe) and Cu and SUS spheres areprepared to estimate the effect of thermal conductivity on theheat transfer enhancement. Fig. 2 shows the heat transferperformance of the packed bed tubes. The packed bedtubes indicate much higher heat removal performancecompared with a straight smooth tube especially in a laminarflow region and that is useful finding against the heattransfer with high-speed flow that has a problem on erosion.In addition, it is confirmed that the heat transfer of Cupebble is improved in a higher velocity region and there is apossibility that that results from the synergism of highlyimproved turbulent heat transport near between the wall andthe pebble and the increase of fin effect by high thermalconduction inside the Cu pebble. The heat transferexperiment at the higher flow velocity is desired.Heat transfer enhancement mechanism in packed bed tubeNumerical simulation of high Pr fluid flow aroundgroup of circular cylinder was performed to clarify the heattransfer mechanism in complicated geometry tubes like apacked bed tube etc. and their effectiveness. The Cut-cell248

Design Research on the Advanced Thermal Structure System in the Nuclear Fusion Reactor

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