Visualization of Flow Patterns in Closed Loop Flat Plate Pulsating Heat Pipe Acting as Hybrid Thermosyphons under Various Gravity Levels

A particular flat plate pulsating heat pipe (FPPHP), filled with FC72, is tested during the 62th and 64th ESA parabolic flight campaigns under vertical orientation. The FPPHP is made of a thin copper plate, in which a curved channel disposed with 11 U-turns is milled and closed on the top face by a transparent borosilicate plate. The particular characteristics is that the equivalent hydraulic diameter of the square channel (2.5 x 2.5 mm²) is above the working fluid capillary diameter on ground, inducing a stratification of the liquid/vapor phases under ground and hypergravity conditions, whatever the orientation. The energy transfer mode in such conditions is represented either by pure pool boiling inside the channels almost completely filled by the liquid phase or by an annular flow pattern inside the channels mostly filled by the refrigerant vapor. Instead, during the microgravity phases, the fluid regime naturally turns into a slug-plug flow pattern. During the transition from 1.8g to 0g a rapid dry-out may occur in some of the channels, followed by a similarly fast reaction of liquid plugs moving towards the evaporator from the condenser zone. Such stop-and-start motion events continue during the whole microgravity period, leading to strong temperature oscillations, but also to a still acceptable thermal performance of the device.