The Influence of Filling Ratio and Number of Turns on the Heat Transfer Performance of Nitrogen Pulsating Heat Pipe

Abstract Pulsating heat pipes (PHPs) are promising high-efficiency heat transfer devices with high cost-effectiveness and flexibility. To investigate the heat transfer performance of cryogenic PHP with a long heat transfer distance, a 0.6 m long nitrogen PHP was tested in this paper. The PHP was made by a stainless-steel capillary tube with an inner diameter of 1 mm and an outer diameter of 1/16 inch in a serpentine arrangement. The lengths of the condenser, adiabatic, and evaporator sections were 50 mm, 500 mm, and 50 mm, respectively. Stepwise heat load tests were conducted at various filling ratios ranging from 15% to 95% to explore the effect of the filling ratio on the heat transfer performance of the PHP. In addition, 6-turn and 12-turn PHP prototypes with identical operating conditions and geometric parameters were tested to investigate the effect of the number of turns on the heat transfer performance. Experimental results showed that the 6-turn PHP exhibited optimal characteristics at filling ratios of 30~50%. In this range, the effective thermal conductivity of the PHP was up to 130 kW/(m·K), which was three orders of magnitude higher than that of oxygen-free high conductivity copper (OFHC) in the same temperature region. Additionally, the temperature difference between the evaporator and condenser was significantly reduced and the maximum thermal conductivity was increased by increasing the number of turns from 6 to 12.