Thermal performance and flexibility evaluation of metallic micro oscillating heat pipe for thermal strap

Abstract

• Micro oscillating heat pipe with metal pipes and check valves is fabricated. • Thermal performance is tested in atmospheric and thermal vacuum environment. • Maximum thermal conductance of 0.8 W/K or more is achieved under both environments. • Conductance is decreased along with condenser temperature under thermal vacuum. • Dynamic stiffness of the oscillating heat pipe is less than the graphite strap. Achieving a high level of pointing accuracy in spacecraft requires that vibration and thermal distortions not be relayed to the structure. Therefore, a flexible thermal strap is essential to dissipate heat from heat-and-vibration-generating components, such as cryocoolers. This study developed a flexible and highly conductive thermal strap for space application with a micro-Oscillating Heat Pipes (OHP). So far, few studies have focused on the flexibility of a micro-OHP, particularly that of the tube type. This paper presents the results of testing strap’s thermal performance and dynamic stiffness, which is a key parameter of flexibility but has been rarely evaluated for heat pipes, including OHP. To achieve the triaxial flexibility and resistance against space environment, the OHP consists of a 10-turn micro-OHP composed of metal tubes with an inner diameter of 0.4 mm or less. It is filled with HFC-134a as the working fluid. Ten electroformed check valves are mounted in the OHP. The liquid slug is induced to flow from the condenser to the evaporator, and the OHP operates stably even in a horizontal position. Thermal performance tests have shown the maximum thermal conductance of the strap to be 0.8 W/K and the heat transfer rate to be at least 7 W. The thermal vacuum test (TVT) showed that, below 20 ℃, the thermal conductance decreases with the condenser temperature and that the OHP does not operate below −15 °C. Dynamic stiffness tests were performed on the OHP to determine the dynamic stiffness in the two directions perpendicular to the flow (along the Y and Z axes). The stiffness parallel to the flow was obtained by a finite element methods analysis using a structural model of a bent OHP based on earlier test results. The dynamic stiffness is less than 0.2 N/mm in Y and Z axis, which is less than that of the graphite thermal strap used for space application. During and after the stiffness test, the OHP continued to operate stably without any change in the temperatures of the evaporator and condenser.