Experimental investigation on working fluid selection in a pulsating heat pipe

Abstract

A guideline for the working fluid selection in a micro pulsating heat pipe (MPHP) is experimentally developed. A closed-loop ten-turn MPHP with overall dimensions of $80 mm \times 35 mm \times 1.7 mm^3$ is fabricated using MEMS manufacturing process. Ethanol, FC-72, HFE-7000, R-245fa, and R-134a are selected for the working fluids. Based on room temperature, the filling ratios of used working fluids are identically fixed at 50%. The thermal resistance of the MPHP is evaluated and the flow is visualized with a high-speed camera in a vertical orientation. It is found that the thermal resistance of the MPHP reduces when the condenser temperature rises or the working fluid changes in the following order: ethanol, FC-72, HFE-7000, R-245fa and R-134a. The change in the flow behavior of the working fluid has a significant impact on the thermal resistance of the MPHP: when the flow behavior is an oscillating flow, the thermal resistance of the MPHP is relatively high and it goes down as the flow behavior changes to a circulating flow. A change from the oscillating flow to the circulating flow leads to a rise of the average volumetric fraction of the vapor in the condenser section. The driving pressure for fluid motion and the frictional pressure drop are experimentally shown to be major contributors to the volumetric fraction in the condenser section. Based on these experimental findings, it is postulated that the figure of merit for the selection of working fluid is proportional to the ratio of the frictional pressure drop to the driving pressure for fluid motion. This postulate is consistent with the experimental findings, which demonstrated that the thermal resistance of the MPHP decreases as the pressure ratio decreases. Finally, the figure of merit, which can be used to select a working fluid for the minimum thermal resistance of a PHP as reported in the present study and in other studies, is proposed.