Study on development and operational characteristics of cryogenic loop heat pipe adopting wick-inserted condenser

Abstract

This study designed and fabricated a cryogenic loop heat pipe (CLHP) operating at 80-120 K. The CLHP used a compact and efficient wick-inserted condenser which enhanced the heat transfer efficiency inside the condenser. This study also verified the comprehensive operational characteristics of the CLHP in detail and proposed the efficient operating strategy of the CLHP. The CLHP needs a specific process to chill down an evaporator before normal operation, which is different from the conventional room-temperature heat pipe. Moreover, the effective thermal conductance, the maximum heat transport capability, and the thermo-hydraulic behavior change by the variation in the governing parameters such as the initial charging pressure of a working fluid, the heat loads on the primary and secondary evaporators, and the cooling capacity of the cryocooler connected to the condenser. The temperatures of the evaporator and the condenser are also sensitively influenced by those governing parameters. Therefore, in order to obtain an accurate thermal performance of the CLHP and operate the CLHP efficiently, the operational characteristics of the CLHP should be investigated comprehensively. Nitrogen was selected as a working fluid considering the target temperature range of 80 ? 120 K. A pressure lower than 2 MPa with a 3-liter gas buffer tank was chosen from the appropriate liquid volume fraction in cryogenic steady state operation. The maximum capillary force was calculated from the pore size, the porosity, and the permeability of the sintered nickel wick, and the effective length from the evaporator to the condenser was decided as 680 mm. The 70-mm long compact condenser adopting the porous annular-cylindrical wick was fabricated for the purpose of raising the heat-exchanging area between the condenser and the working fluid and the condensation heat transfer coefficient. The effect was analyzed and the different condenser were compared. The initial charging pressure of the working fluid, the heat loads on the primary and secondary evaporators, and the cooling capacity of the cryocooler were chosen as the governing parameters because they determine the thermodynamic states of the CLHP. The operating characteristics of the CLHP were investigated by variating those governing parameters. The matching characteristics between the heat loads on the primary and secondary evaporators were also studied. The effective thermal conductivity of the CLHP which is several dozen or hundreds larger than that of the conventional metallic thermal link was obtained. Additionally, the heat switch capability of the CLHP was validated through experiments in various operational conditions. The heat switch function was achieved by controlling the heat load on the secondary evaporator. The switching off process was investigated experimentally and numerically in detail. The CLHP was successfully utilized as a cryogenic heat switch which has significantly large on/off thermal conductance ratios in cryogenic temperature. The on-off thermal conductance ratio was evaluated as 400 to 3900 from the experiments.