Effects of gravity on the performance of pulsating heat pipes

Pulsating heat pipes made of a thin aluminium plate (250 x 60 x 2.2 mm) with small internal channels and charged with a refrigerant (R-114) have been tested under normal to high gravity (1-2.5 g(0)) and reduced gravity (similar to +/-0.02 g(0)) levels to investigate the effect of gravity on their heat transport characteristics. Reduced gravity experiments were performed aboard Falcon 20 aircraft flying parabolic trajectories, yielding about 20 s of reduced gravity at similar to +/-0.02 g(0). Under normal and hypergravity conditions, both the orientation of the pulsating heat pipe and locations of the heated and cooled sections affected the heat transfer performance. For example, large temperature fluctuations were observed when the heat pipe was oriented vertically and heated at the top. Under reduced gravity, however, the heat pipes showed better operating and heat transport performance than that under normal and hypergravity. These experiments have, for the first time, confirmed that pulsating heat pipes are capable of operating satisfactorily under reduced gravity and, thus, that they should be suitable for deployment in space applications. A theoretical analysis revealed the possibility that this type of a heat pipe with larger channel diameters (up to 5 mm for R114 as a working fluid) could work under microgravity, though they may not work on the ground.