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 N-2 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. (C) 2017 Elsevier Ltd. All rights reserved.