Development of Innovative modular experimental apparatus for the investigation of start-up and dryout processes in Pulsating Heat Pipe

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the start-up and the thermal crisis phases, that are crucial to define their operational limits. This is due to the complex phenomena involving the thin liquid film evaporation at the triple line (tube wall/liquid/vapor) during the start-up and dry-out process. The design of an innovative single loop modular experimental PHP, especially designed to address the above open issues, is described here. It consists in a square loop made of four borosilicate transparent glass tubes (2mm inner diameter; 5 external diameter) joined at corners by means of brass connectors. The external tube surface is coated by means of series of transparent Indium Tin Oxide (ITO) heaters (16 patches of 30 mm length). This solution allows to directly visualize and analyze the liquid film dynamics inside the tube by means of a high-speed grey-scale camera and consequently to characterize the start-up and dry-out processes. Cooling is provided by removable condensers consisting in two aluminum spreaders sandwiched on tube and cooled down by a peltier cells/cold-plates system. Each brass connector is equipped with a fluid thermocouple, a pressure transducer and an optic fiber phase detector to determine the local thermodynamic state. The modular design of evaporators and condensers allows to change their position and length to reproduce a large number of PHP configurations. A large test case matrix comprising all the varying parameters of interest (e.g. flow patterns, dominant frequencies, local heat transfer coefficients, overall thermal resistance) is proposed. The obtained data will be fundamental for the understanding of the PHP governing phenomena and for models validation