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 the aircraft during the 60th ESA parabolic flight campaign, reproducing hyper- and microgravity conditions. The tested FPPHP is made of two brazed copper plates, where on one internal side a unique, continuous rectangular channel (1.6 x 1.7 mm²) with 12 bends is machined. The channel is filled with FC-72 as a working fluid with a volumetric filling ratio of around 50%. Tests have been made both with the PHP positioned horizontally and vertically (bottom heated). Experimental results on ground show that the thermal device is able to transfer more than 180 W in both inclinations, while the horizontal operation is characterized by many stop-and-start phases and a lower thermal performance. The tests during the parabolic flights showed that the FPPHP in vertical inclination is influenced by the variation of gravity field, even if, due to the new geometry with insertion of grooves between the channels, it continues to operate also under microgravity, being very little affected by the transient hyper-gravity phases. Actually, with the novel FPPHP, there was no observable parameter change during the variations of the gravity field in horizontal inclination. Finally a Flat Plate Pulsating Heat Pipe able to operate under microgravity conditions and with a transient gravity force has been built, with a global thermal resistance which is 50% and 25% of a void only-conduction plate, respectively in horizontal and vertical orientation. The temperature homogeneity is below 10 K in the evaporator section and below 3 K in the condenser section with a thermal power transfer from 30 to 180 W. A minimum thermal resistance of 0.12 KW-1 or about 16kWm-1K-1 of equivalent thermal conductivity was reached and it is further improving with increasing heating powers.