Experimental Analysis of the Fluid Flow in the Flat Plate Pulsating Heat Pipe Under Microgravity Conditions

An experimental study of a flat plate pulsating heat pipe has been performed under various gravity conditions during ESA 69th parabolic flight campaign. A molybdenum plate, with 14 milled rectangular channels with cross-section of 3×3 mm2, was covered with a sapphire window, and tested in vertical position with ethanol as working fluid. If operation in normal and hyper-gravity conditions were characterized by nucleate boiling regime, FP-PHP working like in looped thermosyphon mode, transition into microgravity was accompanied by a flow pattern change into slug/plug regime with thin film evaporation, due to absence of buoyancy forces. Hydrodynamic instabilities, accompanied with short-term periods of emergence of nucleate boiling under microgravity, were observed during several parabolas. Formations of long vapor slugs in the channel lead to thin film evaporation. Combined optical and infrared visualizations showed velocity and amplitude increase of the menisci motions.