Effects of the micro-post array on the maximum allowable input power for a micro pulsating heat pipe

Abstract

This study is performed to experimentally investigate the effects of the micro-post array (MPA) and its arrangements on the maximum allowable input power for a micro pulsating heat pipe (MPHP). The maximum allowable input powers for MPHPs with the MPAs in three different arrangements are measured and compared to that for the conventional MPHP (without the MPA). To do this, ten-turn MPHPs with and without the MPA are fabricated using MEMS techniques. The overall dimensions of the MPHPs are 53 × 38 × 1.5 mm3, and the width and the height of the meandering rectangular channels are 1500 µm and 660 µm, respectively. The MPA in each case is placed on the bottom surfaces of the channels. The dimensions of the MPA are analytically determined in such a way to maximize the rate of the capillarity-induced flow by solving the Brinkman-extended Darcy's equation: the diameter, the pitch and the height are determined to be 33 µm, 97 µm, and 80 µm, respectively. The MPA is placed in three different arrangements: in alternating pairs of channels, in alternating channels, or in all channels. Ethanol is selected as the working fluid. The experiments are conducted in a horizontal orientation, and the flow is visualized with high-speed photography. The experimental results show that the MPA can have either a favorable or an adverse effect depending on its arrangement. If the MPA is placed in alternating channels, the maximum allowable input power is enhanced by 44%, compared to that for the conventional MPHP, and if it is placed in alternating pairs of channels, the maximum allowable input power is enhanced by 36%. On the other hand, the MPHP does not operate at all at any input power if the MPA is placed in all channels. Therefore, in case of the MPHP where a vapor plug oscillates in its own channel, the MPA should be placed in alternating pairs of channels or in alternating channels to enhance the maximum allowable input power for an MPHP.