Effects of oscillation amplitudes on heat transfer mechanisms of pulsating heat pipes

Abstract

The effects of oscillation amplitudes on heat transfer mechanisms of pulsating heat pipes (PHPs) are experimentally investigated. For this, a five-turn closed-loop micro PHP (MPHP) with overall dimensions of 28×67.5×1.6mm3 is made of glass top and silicon bottom. A meandering micro-channel with hydraulic diameter of 0.835 mm is engraved on the glass. Ethanol is selected as the working fluid and the filling ratio is fixed at 45%. Experiments are performed at various input powers in a vertical orientation with bottom-heating mode. The oscillation amplitude, as a representative parameter of the flow characteristics, is obtained from a high-speed imaging technique. The heat flux distribution at the interface between the fluid and the wall is simultaneously obtained from a high-speed infrared imaging technique. From the synchronization of the images of the flow visualization and heat flux distribution, the transfer of heat occurring at the interface between the fluid and the wall is divided between sensible heat and latent heat. Then, the heat transfer mechanisms of the MPHP are investigated by quantitatively analyzing the proportion of latent heat to overall heat. As the input power increases from 7 W to 16 W, the oscillation amplitude increases from 3.4 mm to 8.3 mm. The proportion of latent heat continues to increase from 54.8% to 81.9%. Due to the increased proportion of latent heat, the thermal resistance of the MPHP decreases by 13.5%. From these experimental results, it is confirmed that the proportion of latent heat increases with the oscillation amplitude, resulting in a decrease of the thermal resistance of the MPHP. These experimental findings can give a clue to improving the thermal performance of PHPs: the thermal performance of PHPs can be increased by increasing the oscillation amplitude or the proportion of latent heat in either the evaporator section or the condenser section.