介電液於新型微流道熱沉之冷凝熱傳增強研究

Abstract

本研究以實驗的方法探討介電液體HFE-7100在微流道熱沉內之冷凝熱傳特性，並固定出口壓力150 kPa以及冷凝熱傳量100 W，而實驗之參數有質量通率(100 – 200 kg/m2s)、蒸汽乾度(0.1 – 0.9)和不同的擺放方式(-90°垂直往下流動至90°垂直往上流動)。本研究提出結合排水流道以及漸縮流道的原理，測試一種新的微流道熱沉，在微流道冷凝長度一半之處有一階梯狀上升的結構並在角落處加工出一排水流道，希望藉由排水流道的表面張力將液膜吸入並使氣體進入階梯狀上升的結構，一方面使的液膜變薄降低熱阻以提升新型微流道前段的熱傳效果，另一方面密度較低的氣體部分進入後半部分微流道內繼續冷凝。為了證明此微流道熱沉有效，將與一相同水力直徑之矩形微流道熱沉做比較。研究結果顯示，新型微流道熱沉因排水流道提供的表面張力效應會使的冷凝液膜較不易隨著冷凝方向變厚而斷裂造成流譜的轉變，而結合新型微流道前段排水流道的效果以及後段突縮的效應，使得熱傳增益在質量通率100 kg/m2s時高達40%，在較高質量通率200 kg/m2s時熱傳增益會減小至大約20%，但其摩擦壓降也因微流道突縮的影響而有所提升，質量通率200kg/m2s乾度大於0.2時熱傳增益會漸漸被摩擦壓降增加的幅度超越。

This study experimentally investigates the condensation heat transfer characteristics of HFE-7100 within a multiport mini-channel heat sink with a hydraulic diameter of 1.61 mm. Tests are performed at an fixed outlet pressure of 150 kPa while the condensation heat flux is about 27 kW/m2. The test mass fluxes ranges from 100 to 200 kg/m2s with vapor mass quality from 0.1 to 0.9 and subject to inclinations from -90° (vertical downward) to 90° (vertical upward). The present study propose a novel concept to enhance the condensation heat transfer performance by combining the principle of mini-drainage channel and converging channel. The novel mini-channel features a stepped rise structure in the middle of the mini-channel and the hydraulic diameter is 1.51 mm. In addition, a mini-drainage channel in the corner of the stepped channel. Through this design, the condensate will be pulled into the mini-drainage channel to reduce the liquid film thickness, while the gas part of the two phase flow will pass through the elevated mini-channel because its lower density, thereby enhancing the condensation heat transfer performance at the rear part of the mini channel accordingly. For further comparison of the novel design, test results were compared with a rectangular mini-channel which has the same hydraulic diameter as the new one. The results show that the surface tension effect provided by the mini-drainage channel will make the liquid film of the new mini-channel become thinner than the rectangular mini-channel, so the thickness of condensate will be appreciably reduced, the enhancement ratio of heat transfer at a mass flux is 100 kg/m2s can reach up to 40%, but it will be reduced to about 20% as the mass flux is 200 kg/m2,,while the friction pressure drop will also rise, and the increased ratio of friction pressure drop in high vapor quality of 200 kg/m2 will surpass the enhancement ratio of heat transfer.