Experimental investigation on working fluid selection in a micro pulsating heat pipe

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 x 35 x 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. The filling ratio of the working fluids is fixed to be 50% at room temperature. 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 as the condenser temperature rises or the working fluid changes in the following order: ethanol, FC-72, HFE-7000, R-245fa and R-134a. The decrease of the thermal resistance is due to the enhanced latent heat transfer resulting from the increase of the average volumetric fraction in the condenser section. Based on these experimental findings, the average volumetric fraction in the condenser section is assumed to be related to the driving pressure for the fluid motion and the frictional pressure drop. Then, it is postulated that the figure of merit for the working fluid selection is proportional to the ratio of the driving pressure for the fluid motion to the frictional pressure drop. This postulate is consistent with the experimental findings, which demonstrated that the thermal resistance of the MPHP decreases as the inverse of 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.