Hybrid Pulsating Heat Pipe for Space Applications with Non Uniform Heating Patterns: Ground and Microgravity Experiments

A hybrid Loop Thermosyphon/Pulsating Heat Pipe named Space Pulsating Heat Pipe (SPHP) is tested both on ground and in hyper/micro-gravity conditions during the 63rd ESA Parabolic Flight Campaign. The device, partially filled up with FC-72 (50% Filling Ratio), is made of an aluminum tube (Inner/Outer Diameter 3 mm/5 mm) bent into a planar serpentine with five curves at the evaporator zone. A transparent section closes the loop in the condenser zone, permitting the fluid flow visualization. Each of the five heaters, mounted alternatively on the branches, just above the curves, is controlled independently, in order to test different heating distributions. The device is tested at different total heat inputs (50 W, 70 W and 90 W), both on ground and in hyper/micro gravity conditions. Data are collected by recording the heat power provided to each heater, the tube wall temperatures, the fluid pressure and fast speed images up to 200 fps are also recorded in the transparent section. An image processing software is developed in order to calculate the bubble flow velocity. On ground, where the device acts like a thermosyphon, the non-uniform heating promotes the fluid net circulation in a preferential direction, stabilizing the operation of the device and thus increasing the thermal performance with respect to the homogeneous heating. The parabolic flight tests point out a working mode in microgravity for such SPHP: the sudden absence of buoyancy force, activating an oscillating slug/plug flow regime, allows the device to work also without gravity assistance. It was found that particular heating distributions can shorten the stop-over periods observed when the device is uniformly heated up, stabilizing a pulsating twophase flow motion when the gravity field is absent.