Comparison of the thermal performances and flow characteristics between closed-loop and closed-end micro pulsating heat pipes

The thermal performances and flow characteristics of closed-loop micro pulsating heat pipes (CLMPHPs) and closed-end micro pulsating heat pipes (CEMPHPs) are compared through experiments to determine which type of micro pulsating heat pipe (MPHP) performs better. MPHPs that have a meandering rectangular channel engraved on a silicon substrate were fabricated using MEMS techniques. The width and height of the channels are 1 mm and 0.5 mm, respectively. Pyrex glass was used as a cover, enabling the flow visualization in the MPHPs and ethanol was used as the working fluid. A series of experiments were performed at various input powers and inclination angles for the MPHPs with 5, 10, 15, and 20 turns. The flow and thermal characteristics were investigated through high-speed photography and thermometry. In the case of 20 turns, there is little difference in the thermal performances and flow characteristics between the CLMPHP and the CEMPHP, and both MPHPs have orientation-independent performance. However, in the case of 15 and 10 turns, the CEMPHP operates stably regardless of the inclination angle while the thermal performance of the CLMPHP deteriorates significantly as the orientation changes from vertical to horizontal. Specially, in the case of 10 turns, the CEMPHP has up to 1.3 times higher effective thermal conductivity in a vertical orientation and 2.5 times in a horizontal orientation than the CLMPHP. When the number of turns is 5, the thermal performance of the CLMPHP depends on the orientation, while the CEMPHP does not start up at all at any inclination angles. Finally, to ensure orientation-independent performance, the CLMPHP requires as many as 20 turns while only 10 turns is sufficient for the CEMPHP. (C) 2016 Elsevier Ltd. All rights reserved.