Operation characteristics of a gravity pulsating heat pipe under different heat inputs

Abstract

Herein, a gravity pulsating heat pipe (GPHP) is proposed to enhance the gravity effect of the working fluid (WF) reflux and increase the heat transfer limit for solving the dry-up issue at the evaporation section of a conventional pulsating heat pipe (PHP) reaching the heat transfer limit under high heat input. The upper and lower parts of the GPHP comprise 10 single parallel-loop open annular channels and cavities without channels, respectively. Using an electrical coolant, HFE-7500, as the WF, the heat transfer characteristics and flow pattern changes of the GPHP under different heat inputs (30–230 W) were experimentally investigated. The results showed that heat input considerably affects the GPHP thermal performance. Under different heat inputs, three heat transfer stages occur, namely low heat-input steady-state operation stage (70–130 W), transition stage (150–170 W) and high heat-input steady-state operation stage (190–230 W). The GPHP starts quickly and abruptly, with a good and stable heat transfer performance under a low-heat input, and the heat exchange is mainly sensible. In the transition stage, because the temperature of the evaporation section exceeds the critical point of nucleate boiling, a large amount of liquid–vapour is formed by gasifying the liquid WF, which fills the cavity structure. Meanwhile, a stable vapour film is formed on the heating wall, which promotes the accumulation of liquid WF at the GPHP's upper part, preventing the heat transfer between the heating wall and WF and deteriorating the heat transfer. Finally, the liquid WF flows back to the evaporation section under high heat input, destroying the vapour distribution in the cavity. Therefore, the heat transfer of the GPHP reaches a stable operation state. At this stage, the proportion of sensible heat transfer decreases and that of the latent heat increases. The cavity structure in the lower part of the GPHP follows a typical pool-boiling process, which has four stages: convective heat transfer, nucleate boiling, transition boiling and film boiling.