Study on effects of micro-post arrays on the operating limit of pulsating heat pipes

Abstract

In this study, effects of micro-post arrays on the operating limit of Pulsating Heat Pipes (PHP) were experimentally investigated. Using MEMs techniques, and anodic bonding, micro-post arrays and channels were etched on a silicon wafer and a glass wafer (#7740 PyrexTM) respectively and bonded to each other to fabricate a PHP with micro-post arrays. The height, diameter, and pitch of micro-post arrays were determined to maximize the supply rate of liquid supplied by the capillarity of micro-post arrays. Also, to investigate effects of arrangements of micro-post arrays, a bare PHP without micro-post arrays (Case 1), PHPs with two different arrangements of micro-post arrays (Case 2, Case3) were fabricated and experimentally investigated. Experimental results showed that Case 3, where micro-post arrays are located in all channels of a PHP did not operate regardless of the input power. On the other hand, Case 2, where micro-post arrays are located in alternating turns of channels, normally operated with the oscillating motion of liquid slugs and vapor plugs. Also, the maximum allowable input power of Case 2 increased by approximately 36% compared to Case 1. When the input power corresponding to the operating limit of Case 1 is applied to Case 1, all liquid films around vapor plugs in the evaporator section dried out. Because there was no liquid film left in the evaporator section, oscillating motion because the phase change of a liquid film was absent. On the other hand, when the same input power was applied to Case 2, liquid films were left around vapor plugs due to the additional liquid supplied by micro-post arrays which are located in alternating turns of channels, were left around vapor plugs. The maximum allowable input power of a PHP increases sustaining the oscillating motion by increased liquid supply rate. Proposed optimization method for dimensions of micro-post arrays and the consideration for the selection of the arrangement of micro-post arrays in this study help to increase the operating limit of a PHP effectively.