가스-용융염 열교환에 관한 실험적 연구

Abstract

The Very High Temperature Reactor (VHTR), one of the most challenging next generation nuclear reactors, has recently drawn an international attention due to its higher efficiency and the operating conditions adequate for supplying process heat to the hydrogen production facilities. To make the design of VHTR complete and plausible, the designs of the Intermediate Heat Transport Loop (IHTL) as well as the Intermediate Heat Exchanger (IHX) are known to be one of the difficult engineering tasks due to its high temperature operating condition (up to 950°C). A type of compact heat exchangers such as Printed Circuit Heat Exchanger (PCHE) has been recommended for the IHX in the technical and economical respects. In this study, the Flinak molten salt, an eutectic mixture of LiF, NaF and KF (46.5:11.5:42.0 mole %) is considered as the heat transporting fluid.

A double-pipe type heat exchanger was constructed using small diameter tubes to investigate the pressure drop and heat transfer characteristics of molten salt flow in minichannels. The outer and the inner diameters of the inner tube are 3.18 mm and 1.40 mm and of the outer tube are 6.35 mm and 4.57 mm, respectively. The length of test section is 500 mm. The molten salt flows through the inside of the inner tube and the gas flows through the annulus of 0.7 mm gap.

For laminar flow of the molten Flinak in the 1.4 mm-inner diameter circular tube, the measured friction factors were close to the analytical result of 64/Re within the difference of -23 ~ +7%. The uncertainty in Flinak viscosity was considered to be the cause of this difference. The Flinak viscosity was calculated using the measured pressure drop and a new correlation of Flinak viscosity was proposed. The heat transfer data were successfully obtained for the counterflow of the double-pipe heat exchanger. Despite the sizable heat loss of the gas flow to the surroundings, the heat transfer coefficients of the Flinak flow in small channel were calculated using a modified LMTD method which accounted for the heat loss. The resulting Nu numbers were generally in the range of 3.66 and 4.36, which are the analytical values for laminar flow in circular tubes.

A model heat exchanger was designed and fabricated to investigate the thermal performance of the plate-fin heat exchanger with minichannels. Minichannels were chemically etched on one side of a metal sheet and two of these sheets were diffusion-bonded to form a plate. Six plates and five off-set fin layers were then brazed to form the heat exchanger. The plates and fins were made of Inconel 600. To evaluate the pressure drop and heat transfer performance of the model heat exchanger, a series of tests were carried out using air and water. The water flows through the inside of the minichannels and the air flows through the fins. For laminar flow of the water in channels, the measured friction factors were close to 64/Re within the difference of ±8%. The heat transfer measurement was satisfactory such that the difference between the heat loss of water and the heat gain of air was -3.2 ~ +4.8%.