Experimental study of the flat plate pulsating heat pipe operation during dry-out and flow re-activation periods under microgravity conditions

•This research work focuses on the flow behavior of the flat plate pulsating heat pipe (FPPHP).•The FPPHP is tested at microgravity conditions during parabolic flights.•The FPPHP is filled with ethanol with a volumetric filling ratio of 40%.•Velocities and accelerations of the liquid slugs’ oscillations inside PHPs are obtained thanks to high-speed visualization.•Flow transition identification. An investigation of thermal performances of the flat plate pulsating heat pipe (FPPHP) and liquid-vapor interfaces oscillation influence on the flow pattern inside FPPHP for microgravity conditions have been performed. Tested pulsating heat pipe represents a seven U-turns closed-looped FPPHP made of molybdenum base (200 × 80 × 3 mm3) and sapphire cover plate (200 × 80 × 5 mm3) and designed for 69th ESA Parabolic Flight Campaign. Serpentine channel with depth of 2.5 mm and width of 3 mm is engraved in molybdenum base that, with following gluing of sapphire plate on the base, gives 3 × 3 mm2 square channel (0.5 mm of glue thickness). Ethanol is chosen as working fluid with volumetric filling ratio of 40%. Experimental tests are performed for heat loads range from 100 W to 150 W. Microgravity periods are mostly characterized by the significant temperature augmentation in the evaporator zone (temperature rise up to 36 K for 150 W of applied power). This temperature rise is a result of fluid accumulation in the condenser and dry-out of evaporator – heat transfer only by the FPPHP wall conduction. Flow re-activation, accompanied by the flow pattern change from slug-plug to semi-annular and temperature drop, was observed only for few cases during microgravity phases. Video post-processing was performed to define flow parameters influencing flow transition for microgravity conditions, as well as to determine liquid plugs velocities and accelerations for both phases. Classical criteria used for slug-plug flow pattern definition are validated for this case: obtained average Weber number value is almost the same as the reference value (Weref = 4), but average Garimella number is more than ten times higher than reference (Gaexp = 1980, Garef = 160). Finally, the magnitude of calculated driving pressures for the liquid plugs reactivation motion is in the range of 50-630 Pa with average value of 360 Pa.