Preliminary Multi-Variable Experimental Analysis To Determine The Startup 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. An innovative experimental apparatus is designed to systematically investigate the above phenomena. It consists in a square loop made of four borosilicate transparent glass tubes joined at corners by means of brass connectors. The external tube surface is coated with several transparent Indium Tin Oxide heaters. The device is used to topologically reproduce 5, 7, and 11 turns (i.e., heated sections in the evaporator) PHPs with an 2 mm inner diameter tube, filled with pure ethanol and tested in horizontal position. The condenser temperature is varied from 10°C to 40°C; the input power goes from 10 W to 40W. It is observed that the startup occurrence does not depend only on the number of heating sections but also on the condenser temperature. Increasing condenser temperature lowers the critical number of heated sections. At the same time, with the increase of the condenser temperature the startup time increases. Moreover, thanks to the direct fluid visualization, the increase of condenser temperature is linked to the formation of long liquid plugs that are found to be detrimental in terms of startup time.