Design and Verification of High Temperature Sodium Heat Pipes with Annular Wick Structure

Abstract

Heat pipe-cooled nuclear reactors have the advantages of high reliability due to the characteristics of simple system structure, passive heat transfer, single point failure prevention, strong self-adaptive capability and start-up from the frozen state. High-temperature heat pipes are the core components of the heat pipe reactor and are used to transfer heat from the reactor core to the thermoelectric conversion units. In this paper, a 4 m long and 19 mm outer diameter stainless steel sodium heat pipe with annular wick structure and high heat transfer capacity was designed and prepared for the demand of high temperature heat pipes for reactors. The test of heat transfer performance of the pipe was carried out. The heat pipe design was carried out from three aspects, including working fluid selection, structure and material selection, and wick structure design. The calculation of heat pipe heat transfer capacity in this work was based on Cotter and Busse’s heat transfer limits theory. The theoretical heat transfer limit of sodium heat pipe designed in this paper is 6.69-11.47 kW in the working temperature range of 700 ℃ to 820 ℃. According to the design of the heat pipe structure, a heat pipe sample was prepared, and the start-up performance and steady-state heat transfer performance were tested. During the test, the evaporation section, adiabatic section and condensation section of the heat pipe were 1500, 500 and 1 550 mm, respectively. The heat pipes were placed horizontally. High-frequency electromagnetic induction heating device was used to heat the evaporation section of the heat pipe. The heat dissipation power of the condensation section was measured by a water-cooled sleeve with a gas gap. The heat transfer thermal resistance of the gas gap could be adjusted by adjusting the proportion of He-Ar gas mixture in the gap, so as to regulate the working temperature and heat transfer power of the heat pipe. In the start-up test, it was measured that the heat pipe was fully started within 75 min under argon cooling, and the inlet temperature of the adiabatic section reached 705 ℃. Mean-while, the temperature difference between the two ends of the heat pipe start-up section was 151 ℃, and the heat transfer power reached 2.9 kW. At 80 min and 167 min, the temperature of TC6 at the inlet of the adiabatic section reached 732 ℃ and 744 ℃, and the temperature difference between the two ends of the heat pipe working section was 124 ℃ and 77 ℃, respectively. The overall temperature drop of the working section was reduced. In the steady-state heat transfer performance test, the measured steady-state heat transfer power of the heat pipe was 6.93 kW and 10.58 kW respectively at the temperature of 707 ℃ and 746 ℃ at the inlet of adiabatic section. The corresponding axial heat flux density of the heat pipe was 3.92 kW/cm2 and 5.99 kW/cm2, respectively. The measured heat transfer power of the heat pipe reached 92.6% of the theoretical heat transfer limit at the temperature of 746 ℃, and the corresponding heat pipe thermal conductivity was 7 000 times that of pure copper. Compared the heat pipe in this paper with the LANL’s sodium heat pipe, which has similar size and wick structure, the heat transfer capacity of the two heat pipes is comparable at 746 ℃. The heat pipe developed in this paper has been successfully started. The measured maximum steady-state heat transfer power reaches the theoretical limit level, which meets the requirements of the heat pipe reactor for the performance index of the heat pipe. The effectiveness of the heat pipe design method and preparation process were also verified.

Keywords

• sodium heat pipe
• annular wick structure
• start-up experiment
• heat transfer power