Thermal performance of ammonia-based thin flat loop heat pipe fabricated by additive manufacturing

A 3 mm thick loop heat pipe (LHP) charged with ammonia for the heat dissipation under low temperature environment was designed, fabricated, and evaluated experimentally. A thin and flat thermal path, which keeps the temperature of the heat source lower than -25 degrees C by dissipating 10 W heat load to the heat sink of -35 degrees C, is required for the imaging sensor for a satellite. In this study, a 3 mm thick loop heat pipe (LHP) charged with ammonia was proposed. The monolithic fabrication by additive manufacturing was utilized for the pressure resistant rib structure for high vapor pressure of ammonia. The LHP satisfying the required thermal performance and pressure resistivity was designed based on the simulation. The condenser of the fabricated LHP was connected to the cold plate, and the basic thermal performance was evaluated in the constant temperature chamber ranging the ambient temperature under the horizontal orientation. Under the ambient temperature of -35 degrees C, the evaporator temperature was -26.2 degrees C when 10 W heat load was applied; thus, the requirement was satisfied. The effective thermal conductivity of 6050-7730 W/(m center dot K) was shown when heat load of 5 to 15 W was applied under the ambient temperature ranging from -45 to 20 degrees C. The orientation dependence was also investigated under top heat and bottom heat orientation, and the similar performance as the horizontal orientation was observed. A power cycle test under horizontal orientation demonstrated the fast temperature response and hysteresis-free performance; thus, preferable characteristics as a thermal device was exhibited. The experimental results were compared with the simulation and agreed upon the simulation results.