Thermal characterization of a multi-turn pulsating heat pipe in microgravity conditions: Statistical approach to the local wall-to-fluid heat flux

A Pulsating Heat Pipe (PHP), specifically designed to be hosted on board the Heat Transfer Host of the International Space Station, is tested in microgravity conditions during the 67th Parabolic Flight Campaign promoted by the European Space Agency. The device consists in an aluminium tube (inner/outer diameter = 3/5 mm) closed in a 14 turns loop, half filled with FC-72. The PHP external wall temperature distribution are measured within the adiabatic section by means of a high-speed infrared camera. The resulting thermographic images are used as input data for the solution of the Inverse Heat Conduction Problem (IHCP) in the channels wall to estimate local time-space heat fluxes exchanged at the wall-fluid interface. The adopted post-processing method represents one of the first attempts to estimate the local wall-to-fluid heat flux in PHPs under microgravity conditions. A comprehensive investigation of the wall-to-fluid heat fluxes is performed on the overall device by means of an original statistical approach in order to study the PHP working regimes. The results highlight that such approach is capable of providing similar information regarding the fluid motion inside the PHP to those obtained by the traditional intrusive experimental methods (e.g. direct fluid temperature-pressure measurements) or transparent inserts (e.g. sapphire tube), which perturb the original layout of the PHP and require a complex experimental set-up.