Thermal characterisation of a flexible pulsating heat pipe in different gravity conditions

Due to the increased usage of printed, flexible, and wearable electronic devices, the development of flexible thermal management systems is relevant. Pulsating heat pipes (PHPs) are a type of passive two-phase cooling systems that are very promising for various industrial applications, such as battery thermal management, cooling of electronic devices and integration in spatial system. In comparison to other passive heat-exchangers, PHPs have a simple wickless structure leading to a low-cost design, minimal maintenance, high operability, and geometrical versatility. Except for the constraint on costs, flexible PHPs can be an interesting solution also in space applications, such as deployable radiators, booms, astronaut space suit. Understanding and predicting flow pattern transitions in a PHP is a major step for developing reliable design tools and techniques towards applications. The following study concerns an analysis of data obtained during the 77th ESA parabolic flight campaign, focusing on the impact of the bending angle on the performance of novel flexible pulsating heat pipes with different geometries and fluids. The setup consisted of an experimental rack equipped with thermocouples, pressure transducer, a high-speed camera as well as an infrared high-speed camera set at a frame rate of 100 Hz. The tested flexible PHP was a flat polymeric foil made by laser welding of 3 polypropylene sheets. The overall PHP dimension was 260x98x1.5 mm3 with two different geometrical configurations of number of turns and channel width 11 turns / 3mm and 7 turns / 5mm) and two working fluids for a total of 3 experiments: FC72 (7 turns), FC72 (11 turns), ethanol (11 turns). The reported results show the impact of different gravity levels and bending conditions on the fluid dynamics and heat transfer characteristics. The PHP thermal performance and flow dynamics are reported by analysing the experimental thermal data and by post processing of infrared images via a purpose-built Particle Tracking Velocimetry algorithm. A correlation between statistical velocity distribution of tracked bubbles and the thermal data was observed. The microgravity conditions affected the PHP’s thermal performance and bubble motion while showing no significant difference with bending.