Large Diameter Pulsating Heat Pipes On Board The Esa Rexus 18 Sounding Rocket

The Pulsating Heat Pipe (PHP) is a promising two-phase passive heat transfer device for the thermal management in space applications. A recent parabolic flight experiment has already shown that a capillary tube PHP has the same thermal performance either on low gravity environment or on ground with the evaporator and condenser placed horizontally with respect to the gravity vector. An increase of the inner tube diameter theoretically may lead to an increase of the thermal performance but it is only achievable in the presence of low gravity conditions. In order to verify such concept, two large diameter PHPs have been tested on board REXUS 18 rocket. The tested PHPs consist of two closed loops made of aluminium tubes with fourteen curves arranged on two planes. Both are filled with the refrigerant FC-72 and have an inner tube diameter larger than the critical diameter (3 mm) and around the critical diameter (1.6 mm), respectively. The heat input is supplied by two heating cables wrapped around the tube and placed asymmetrically with respect to the curves, so as to promote the fluid circulation in a preferential direction. The heat is transferred through the devices and, at the end, released in a phase change material via latent heat of fusion without any appreciable temperature buildup for the entire duration of each experimental run. In the occurrence of the milli-gravity conditions experienced on board the rocket, surface tension prevails over buoyancy and the flow pattern inside the devices switches to the slug and plug PHPs typical operational regime. The temperature and pressure trends reveal such a regime transition and provide further information on the usability of large diameter PHPs in microgravity environments.