A pulsating heat pipe for space applications: Ground and microgravity experiments

A novel concept of a hybrid Thermosyphon/Pulsating Heat Pipe with a diameter bigger than the capillary limit is tested both on ground and in hyper/micro gravity conditions during the 61st ESA Parabolic Flight Campaign. The device is filled with FC-72 and it is made of an aluminum tube (I.D. 3 mm) bent into a planar serpentine with five curves at the evaporator zone, while a transparent section closes the loop, allowing fluid flow visualizations in the condenser zone. Five heaters, mounted alternatively in the branches just above the curves at the evaporator zone, provide an asymmetrical heating thus promoting the fluid flow circulation in a preferential direction. The device has been tested at different positions (vertical and horizontal) and at different heat power input levels (from 10 W to 160 W). Ground tests show that effectively the device works as a thermosyphon when gravity assisted: in vertical position the device can reach an equivalent thermal resistance of 0.1 K/W with heat fluxes up to 17 W/cm2. In horizontal position the fluid motion is absent, thus the device works as a pure thermal conductive medium. The parabolic flight tests point out a PHP working mode: during the micro-gravity period, the sudden absence of buoyancy force activates an oscillating slug/plug flow regime, typical of the PHP operation, allowing the device to work also in the horizontal position. In some cases the hyper-gravity period is able to eliminate partial dry-outs restoring the correct operation until the occurrence of the next microgravity period.