多孔性微流道冷凝熱傳增強研究

Abstract

微流道兩相蒸發器具有高熱傳係數、高均溫性以及低工質需求等優點，被視為極具潛力的散熱技術。近年來電子元件產品的發熱量日益增高，在有限面積下的傳統單相熱交換器無法有效冷卻時，搭配具有相變化的微流道冷凝器被學者認為是有發展潛力的冷凝元件。 本研究內容旨在燒結樹枝狀銅粉以及添加孔洞成型劑碳酸鈉的方式，以金屬粉末燒結技術製造雙孔徑結構表面微流道，藉由改變多孔隙微流道內大小孔的體積比例增強冷凝熱傳。在冷凝發生時，大孔可提供氣體穿過，而小孔可以吸收工質並且降低液膜厚度，導致液膜熱阻下降，進而提升熱傳效果。 首先建立平整表面微流道測試系統做為比較基礎，測試段是以無氧銅表面製作的30條寬、深各為500μm×155μm流道，以水為工質進行熱性能測試。 在平整表面微流道的實驗中，熱傳係數以及壓降都有隨著質量通率上升而上升的趨勢。當質量通率增加，流速加快，使得壁面剪應力上升，造成液膜厚度變薄，熱阻減小，熱傳係數因而增加。實驗結果與傳統流道的熱傳經驗公式比較後，發現明顯低估，顯示已不適用於微流道中。與工質不同之微流道經驗式比較後，結果顯示誤差仍太大，目前的微流道熱傳經驗式仍有改進空間。壓降方面比較近年發展的微流道壓降經驗公式，結果相當吻合，顯示具有一定可靠度。 雙孔徑結構表面實驗結果，參數銅粉粒徑為61~70μm，碳酸鈉體積百分比30%與平整表面微流道進行比較，在壓降增加28.6%下，熱傳性能整體平均提升72.9%，此參數之流道吸排水性佳、熱傳面積大、液膜延展性高，是造成熱傳提升的主要原因。

The microchannel evaporator with two phase heat transfer is considered to be one of the most potential cooling techniques because of its high heat flux, good temperature uniformity and the lesser requirement for coolant flow rate. In recent years, the heat dissipation rate of the high tech products has increased day by day. The traditional single-phase heat exchanger could not efficiently cool down in a limited area, so the microchannel condenser with two phase heat transfer is regarded as a high potential cooling component in the future. The central purpose of the present research is to enhance the condensation heat transfer by utilizing the two pore size distributions of a biporous surface structure. This surface is sintered from the mixture of dendritic copper powders and the pore former, Na2CO3, which formed the different size pores in the microchannel. By changing the volumetric ratio of pore former, it was able to alter the porosity and the numbers of larger pores, further increasing the heat transfer coefficient. During condensation, vapor could go through the larger pores. The smaller pores could absorb the liquid and help to reduce the liquid film thickness. It decreased the heat resistant and increased the heat transfer coefficient. First, a plane surface microchannel system was built as a compared base. The test section of the 30 channels where the width and the depth is 500μm and 155μm, respectively. The test section was made by oxygen-free copper. Water steam is using as working fluid. In the experiment of the plane surface microchannel, the heat transfer coefficient and the pressure drop werw positively related to the increasing mass flux. When increasing the mass flux, the velocity of working fluid becomes faster due to the increasement of wall shear stress. Therefore, it caused the thickness of the liquid film much thinner, decreased the heat resistance and also increased the heat transfer coefficient. Compared with the heat transfer correlation of the conventional channel, the result showed the MAE is quite large. That means there is much room to make progress on the heat transfer correlation of the microchannel. With regard to the pressure drop, comparing with the correlation of microchannel in recent years, it considerably correlated with the results. That shows the result is reliable. For experiment of the biporous surface microchannel, the parameter with copper powder is 61~70 μm diameter and volumetric ratio of Na2CO3 is 30%. Comparing with the plane surface microchannel, the results showed that the heat transfer coefficient is enhanced to 72.9% on average when increased the pressure drop to 28.6% on average. The main reasons of enhancing the heat transfer are high water absorbing capacity and good ability for reducing liquid thickness.