Thermal response of a closed loop pulsating heat pipe under a varying gravity force

A Closed Loop Pulsating Heat Pipe made of a copper tube bent into a 2-D serpentine of 32 parallel channels and filled with FC-72, has been tested on ground and in micro/hyper gravity conditions during the 58th ESA Parabolic Flight Campaign. The device has been investigated both in horizontal and in vertical positions and at different heat loads (from 40 to 100 W). Beyond the standard thermal characterization, dynamic investigations have been performed on ground by changing the device orientation at constant heat input levels. Results show that in the vertical position the PHP thermal behavior is strongly affected by the variation of gravity field both on ground and on flight tests. In particular, during a parabolic flight, the first hypergravity period slightly assists the flow motion, while, during microgravity, the device undergoes a sudden temperature increase in the evaporator zone; the following hypergravity phase is then able to bring the PHP back to the previous thermal regime. The PHP in the horizontal position does not show any sensitive thermal variation during the parabola. A further analysis with a tilting bench in the ground lab proves that microgravity thermal behavior is comparable to the horizontal operation on ground: therefore for capillary closed loop pulsating heat pipes microgravity tests are not strictly necessary for the space application assessment.