Start-Up and Operation of a 3D Hybrid Pulsating Heat Pipe on Board a Sounding Rocket

A large tube may still behave, to a certain extent, as a capillary in a micro-gravity environment. This very basic concept is here applied to a two-phase passive heat transfer device to obtain a new family of hybrid wickless heat pipes. Indeed, a Loop Thermosyphon, which usually consists of a large tube, closed end to end in a loop, evacuated and partially filled with a working fluid and intrinsically gravity assisted, may become a capillary tube in space condition and turn its thermo-fluidic behavior into a Pulsating Heat Pipe. This work presents the results obtained on such a hybrid device heated at 200 W both on board a sounding rocket (ESA REXUS 22, microgravity period ~120 s), and on ground in vertical and anti-gravity orientation. Since no steady state occurred in microgravity conditions, the comparison between flight and ground data focuses on the startup phenomenon, whereas the thorough ground test campaign describes the limits and performances of the device working in thermosyphon mode. The expected thermal behavior in microgravity conditions is between that of a purely conductive tube in anti-gravity conditions on ground and that of a gravity assisted thermosyphon. Since a microgravity period of approximately 120 s is not enough to reach a pseudo steady state regime, further investigation on a longer-term weightless condition is mandatory.