Experimental study of a closed loop flat plate pulsating heat pipe under a varying gravity force

This paper reports on an experimental study of a closed loop Flat Plate Pulsating Heat Pipe (FPPHP) tested on ground and on board of an aircraft during the 60th ESA parabolic flight campaign, during which hyper- and microgravity conditions were reproduced. The tested FPPHP consists in two brazed copper plates, at which location a continuous rectangular channel (1.6 x 1.7 mm²) with 12 bends in the evaporator is machined. The channel is filled with FC-72 as working fluid with a volumetric filling ratio of 50%. Tests have been conducted with the FPPHP positioned both horizontally and vertically (bottom-heated). The FPPHP presents an innovative design, involving the milling of grooves between the channels. Experimental results on the ground show that the thermal device can transfer more than 180 W in both inclinations, and that the horizontal operation is characterized by repeated stop-and-start phases and lower thermal performance. The FPPHP can operate under microgravity conditions and with a transient gravity force, with global thermal resistance reaching 50% and 25% of a void only-conduction plate, in horizontal and vertical orientation respectively. The temperature homogeneity remains within 10K in the evaporator section and 3K in the condenser section with thermal power transfer up to 180 W. Minimum thermal resistance of 0.12 KW-1 was recorded, with its value rising as heating grew more powerful. A parabolic flight test demonstrated that the FPPHP in vertical inclination is rapidly influenced by variation of gravity field, even if, due to the novel geometry, it continues to operate under microgravity. In horizontal inclination, on the other hand, there was no observable parameter change during gravity field variations.