Heat transfer augmentation of a circular pipe flow using nano-particle layers

For the advanced fusion reactor FFHR2 (Force Free Helical Reactor) that has been proposed by NIFS, molten salt Flibe (LiF:BeF2=64:36) breeder blanket system is selected because of Flibe's features such as chemical stability, low-pressure operation and low electric conductivity. The Flibe is however high Prandtl number fluid since it has high viscosity and low thermal conductivity. Therefore its heat transfer performance is low compared with liquid Li or Pb-Li. In addition to heat removal of 1MW/m2 on the first wall, electrolysis of molten salt due to MHD effect will take place under high flow rate condition. This indicates that heat transfer enhancement under low flow rate is essential for the Flibe blanket system. In our laboratory, heat transfer characteristics of molten salt HTS (KNO3:NaNO2:NaNO3=53:40:7), have been evaluated, which is used as a simulant fluid of Flibe from the points of view of Be's toxicity and similar Prandtl number. In this paper, we adopt nano-particle layer method to form nano∝micro scale structure on a heating surface using an acid or an alkali includes nano particles. There exist two methods to form nano particle layer. One is NPLS (Nano Particle Layer Structure) method which uses a chemical etching with an acid or an alkali including copper-oxide nano-particles. The other is FP (Fine Particle) method which employs electroless plating with inorganic metal salt solution. At first, immersion experiments of NPLS or FP layers into melted HTS shows that erosion of the FP sample is much less than that of the NPLS sample. Furthermore, a forced-convention heat transfer experiments with a circular tube whose inner surface has the nano-particle layer by the FP method is carried out in a large molten salt circulating loop named as TNT loop. Results show that average Nusselt numbers of the circular tube flow are about 1.3 times higher than that of a bared tube in the range of 3000<Re<13000 and 13<Pr<27.At the same time, immersion experiment of the FP layer under HTS flow is carried out to find that the FP's surface is oxidized by HTS and turned into crystal structure. Secondary, in order to figure out optimal structure of the nano-particle layer, the heat transfer and pressure drop characteristics are evaluated for four tubes each of which has different surface structure. Those tubes were made by changing electroless plating time as a parameter (7, 10, 13, 16 min). The averaged Nusselt numbers of the tube whose plating time is 16 min become the highest while the pressure drops of the tube whose plating time is 7 min are the lowest. Finally, the tube whose plating time is 13 min indicates the best performance from the view point of Nusselt number ratios of the FP's tube to the bared circular tube for equal pumping power. (orig.)