Influence of initial pressure on the flow dynamics and thermal behavior of a single loop pulsating heat pipe

Abstract

Non-condensable gases (NCGs) are a major factor influencing both the thermal performance and the durability of two-phase heat transfer systems. In particular, NCGs can have a substantial impact on the functionality of a pulsating heat pipe (PHP). In this study, the effects of NCGs were investigated at four different initial system pressure levels: 3.172 kPa, 3.741 kPa, 6.263 kPa, and 20.112 kPa. These pressure levels were achieved by varying the degassing levels of the working fluid. A single-loop PHP made of borosilicate glass with a uniform inner diameter of 2.5 mm was used for the experiment. The study was conducted at a 50% filling ratio in a vertical orientation, using water as the working fluid. The results indicate that the presence of NCGs leads to a subcooling effect on the working fluid and hinders the flow inside the PHP. However, the negative effects of NCGs decrease as the heat load increases. It was observed that the average evaporator temperature rises with an increase in the initial pressure. While the thermal performance, in terms of equivalent thermal resistance, was best at the lowest initial pressure, the heat load capacity of the PHP at higher initial pressures increased by nearly 16% compared to those with lower initial pressures.