AN EXPERIMENTAL INVESTIGATION OF FLAT-PLATE POLYPROPYLENE PULSATING HEAT PIPES

Abstract

A comprehensive analysis of the heat transfer performance and the thermo hydrodynamic behaviour of a flat-plate, flexible polypropylene pulsating heat pipe (PHPs) is presented. In particular, the thermal performance was characterised for pulsating heat pipes with different operational and design parameters, including different numbers of turns of the serpentine channel, different orientations with respect to gravity, different heat transfer fluids, different reciprocal positions of the evaporator and the condenser (configurations), different bending angles (conformations), different gravity levels and with different impermeable coatings as barriers to gas permeation. Flexible polymeric pulsating heat pipes are a passive thermal management technology with important prospective applications in foldable portable electronics, deployable systems (such as cube satellites), soft robotics, and many others, owing to their lightweight and high mechanical flexibility compared to the metallic ones, at the price of a generally much lower thermal conductivity and higher gas permeability. While few flexible polymeric PHPs have been developed in the recent past, to date, there is insufficient information about the dependence of polymeric PHPs’ performance on the above-mentioned operational and design parameters. Several polypropylene PHP prototypes with different channel widths and loop geometries were fabricated using selective transmission laser welding technology. A few prototypes were coated with aluminium oxide (Al2O3) and aluminium layer as barriers to gas permeation using atomic layer deposition (ALD) and thermal evaporation technologies. Water, ethanol and FC-72 were used as heat transfer fluids at a filling ratio of 40−50%. The PHP performance was evaluated through the equivalent thermal resistance of the device, calculated from the surface temperatures of both the evaporator and the condenser during an ascending/descending stepped heat input ramp applied to the evaporator and through the visualisation of the two-phase flow. The experimental findings indicate that the heat transfer fluid is the major influencing parameter that affects the PHP thermal performance followed by the gravity and the number of turns. On the contrary, the conformation and configuration arrangements have no significant effect on the thermal performance.