Experimental heat transfer analysis of a cryogenic nitrogen pulsating heat Pipe at various liquid fill ratios

The experimental results of extensive testing of a cryogenic Pulsating Heat Pipe (PHP) using nitrogen as the working fluid operating between 77 K and 80 K is presented. Fifty-two different test runs were analyzed over a range of different fill ratios, heater input powers, and orientations in order to map thermal performance as a function of the system operating parameters. The temperature difference between the evaporator and condenser section were obtained using platinum resistance thermometers (PRTs), and a pressure transducer was used to record the fluid's pressure oscillations. As opposed to other N2 pulsating heat pipes which use less than 16 parallel tubes, this experimental PHP consists of 40 parallel tubes. The PHP was operated at different liquid fill ratios ranging between 10% and 40% and heat loads between 1 W to 4.5 W. Results show that the PHP's temperature difference between section are small and resulted in high effective thermal conductivities up to 70,000 W/m/K at fill ratios around 20%. Also, it was shown the PHP stopped oscillating at heat loads below 1.5 W but operated successfully above 2 W. Results from experiments conducted in both horizontal and vertical orientations show that gravity has a significant effect on the thermal performance of the PHP even with a high number of turns.