Shinya Chiba

Experimental research on heat transfer enhancement for high Prandtl-number fluid

Abstract The experimental research on heat-transfer enhancement for such high Prandtl-number fluid as Flibe has been performed with a large molten salt circulating experimental loop named as “TNT loop” (Tohoku-NIFS Thermofluid loop). Through the experiments, a packed-bed tube is employed as the enhancer for molten salt. It is clarified that the enhancement of packed-bed tube is superior to that of turbulent heat transfer from the viewpoint of the same flow rate. Also, the 1/4-diameter bed is superior to the 1/2-diameter one at the same flow rate. Furthermore, at low flow rate, a little differences of heat transfer performance can be seen between the stainless-steel bed and copper bed. At high flow rate, however, the heat-transfer coefficient ratio strongly depends on the flow rate in the case of the 1/4-diameter copper bed only. As a result, it is considered that the thermal energy is expanded from a heated wall deeply and fast through packed bed at low flow rate. On the contrary, it is also considered that the convective heat transfer in the vicinity of a heated wall is strong at high flow rate. The evaluation from the viewpoint of the pressure drop shows that the turbulent heat transfer is superior to that with packed bed. However, the ratio of heat transfer with bed to turbulent one is steeply improved at low flow rate. Taking account of MHD effect, avoidance of erosion and electrolysis of Flibe, the enhancement under low flow-rate condition can be suitable in a fusion reactor.

Experimental research on molten salt thermofluid technology using a high-temperature molten salt loop applied for a fusion reactor Flibe blanket

Experimental research on molten salt thermofluid technology using a high-temperature molten salt loop (MSL) is described in this paper. The MSL was designed to be able to use Flibe as a coolant, however, a simulant, heat transfer salt (HTS) has to be used alternatively since Flibe is difficult to operate under avoiding a biohazard of Be. Experiment on heat-transfer enhancement, that is required for applying to cool the high heat flux components of fusion reactors, is ongoing. Preliminary experimental results showed that an internal structure of a mixing chamber in the MSL was important to obtain accurate bulk temperatures under severe thermal conditions. For operating the loop, careful handling are needed to proceed how to melt the salt and to circulate it in starting the operation of the MSL. It is concluded that several improvements proposed from the present experiences should be applied for the future Flibe operation.

Heat transfer performance for high prandtl and high temperature molten salt flow in sphere-packed pipes

Abstract Heat transfer performance for high Prandtl number and high temperature molten salt flow in a circular pipe and in sphere-packed pipes are evaluated with modified Tohoku-NIFS Thermofluid Loop (TNT loop) using high-temperature molten salt HTS (KNO3 : NaNO2 : NaNO3 = 53 : 40 : 7), as a stimulant of Flibe (LiF : BeF2 = 66 : 34). The modified TNT loop has much longer entrance region to develop a thermal boundary layer, which enable us to obtain more precise heat transfer data. In the modified TNT loop experiments, the heat transfer characteristics in a circular pipe flow have good agreements with the representative correlations. It is obvious that the analogy for heat and momentum transfer is also valid for high-temperature and high-Prandtl-number molten salt flow. It is also confirmed that the heat transfer performance of sphere-packed pipes increases up to about 4 times higher than that of circular pipe, in case of relatively low flow rate. This can be effective in the Flibe blanket system from the viewpoints of moderating MHD effect and electrolysis.

Oxidation of alcohols with nitroxyl radical under polymer-supported two-phase conditions

The oxidation of alcohols to carbonyl compounds was studied using potassium hexacyanoferrate(III) mediated by nitroxyl radical as the catalyst under polymer-supported organic–aqueous two-phase conditions. Primary alcohols are readily oxidized to the corresponding aldehydes in excellent yield with no over-oxidation to carboxylic acids. Secondary alcohols are converted to the corresponding ketones with a much lower efficiency. The oxidation reactions of primary alcohols in the presence of secondary alcohols is strongly favored. Primary-secondary diols are selectively transformed into hydroxy aldehydes with small amounts of ketoaldehydes, the amount of isomeric keto alcohols remaining is less than 1%.

Asymmetric electrochemical lactonization of diols on a chiral 1-azaspiro[5.5]undecane N-oxyl radical mediator-modified graphite felt electrode.

A graphite felt electrode modified with (6S,7R,10R)-4-amino-2,2,7-trimethyl-10-isopropyl-1-azaspiro[5.5]undecane N-oxyl was prepared for electrocatalytic oxidation of diols; electrolysis of diols on the modified electrode yielded optically active lactones (92.0-96.4%), with an enantiopurity of 82-99% ee.

Effect of Nano-Scale Surface Conditions for Boiling Heat Transfer and Its Enhancement

In this paper, the Leidenfrost phenomena and water mist cooling are described from the viewpoint of surface conditions of heat transfer interfaces. The effect of nano-scale structures on boiling heat transfer phenomena is researched. It is clarified that the Leidenfrost phenomena on a substrate with adhered nanoscale carbons (nano carbons) are different from the one in case of a normal heat transfer interface. The photographs taken by a high-speed camera show that the boiling on a substrate with nano carbons takes the different form in comparison with the one on a normal interface. In case that the surface temperature of a copper substrate is about 140 degree C, a water droplet has a neck of water between itself and the substrate with nano carbons. On the other hand, the nucleate boiling is observed on a normal copper substrate. From the relation between evaporation time and initial surface temperature, heat transfer enhancement can be achieved under the nucleate boiling conditions. Also, the critical heat flux of water mist cooling could be enhanced by adhering nano carbons on heat-transfer interfaces. It is supposed that the wettability between water and copper is improved by the nano carbons.Copyright