PRELIMINARY MULTI-VARIABLE EXPERIMENTAL ANALYSIS TO DETERMINE THE START UP CRITERIA OF PULSATING HEAT PIPES

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. An innovative experimental apparatus is designed to address the above open issues. 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). The device is used to topologically reproduce a 8 turn PHP with an 2 mm inner diameter tube, filled with pure ethanol (FR = 50% vol.) and tested in horizontal position. The use of transparent heaters allows a complete knowledge of initial fluid distribution and fluid dynamics. The device is tested for 10°C and 20°C condenser temperatures, from 10W to 40W. In the 10 °C condenser temperature test, the device starts up at 10 W and shows an intermittent functioning up to 30W; increasing input power leads to self-sustained oscillations and partial dry-out. In the 20 °C condenser temperature test, the device shows a better performance. No intermittent operation is present, and the average evaporators temperature is ~6°C lower than the one of previous case. Further tests with different configurations will be fundamental for the understanding of the PHP governing phenomena.