微流道冷凝熱傳增強研究

Condensation Heat Transfer Enhancement in Microchannel

凃冠旭(Tu,Kuan-Hsu)

指導教授 ： 陳瑤明

國立臺灣大學/工學院/機械工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.00455

摘要

利用兩相熱傳的微流道蒸發器具有高熱傳係數、高均溫性、單位散熱面積大與低工質需求等優點，被視為極具潛力的散熱技術。科技發展日新月異，各項產品發熱量日益增高，當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。多孔性結構具
大量連通空孔、散熱表面積、高毛細力與高滲透性等優點，預期能提升微流道冷凝器熱傳性能。
本研究於無氧銅表面製作的30條寬、深各為500μm×155μm流道之多孔微流道冷凝器和平板微流道，以水為工質在質量通率範圍65~95 kg/m^2 s進行熱性能測試。探討銅粉粒徑、結構底厚對熱傳性能的影響，再與平板微流道比較流譜、熱傳特性與壓降。
實驗結果與熱傳經驗公式比較後，發現傳統流道熱傳經驗公式明顯低估微流道冷凝器熱傳性能，顯示已不適用於微流道中。與微流道經驗式比較後，結果顯示在誤差範圍內。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。
平板微流道的實驗中，可視化分析可以清楚觀察到冷凝過程中較常見的五種流動形式，包含液滴流(droplet flow)、環狀流(annular flow)、注射流(injection flow)、彈狀流(slug flow)和氣泡流(bubbly flow)。熱傳係數以及壓降都隨著質量通率上升而變大的趨勢。當質量通率增加，流速加快，使得液氣介面剪應力上升，造成液膜厚度變薄，環狀流(annular flow)區域增大而注射流(injection flow)發生位置往流道出口推移，熱傳性能因而提升。平板微流道實驗熱傳係數為23~79kw/m2k。壓降也因工質流速加快及兩相區拉長造成加速度壓降及摩擦壓降增加。
多孔性結構微流道實驗，探討燒結底厚(150~300μm)與銅粉粒徑(1~150μm)等製程參數實驗結果發現，多孔性結構微流道流譜由於多孔性結構表面較為粗糙，流道內部所形成的流譜容易產生擾動，因此與平板微流道流譜有明顯的差異。多孔性結構微流道的吸收延展冷凝水的效果使冷凝液膜厚度變薄，注射流(injection flow)出現的位置與平板微流道相較之下更靠近流道出口而且環狀流(annular flow)區域較平板微流道更大，因此熱傳效果較平板微流道更佳。底厚150μm和粒徑為88μm，微流道熱傳增強幅度最大，微流道有較佳的熱傳性能，實驗熱傳係數為43~161kw/m2k熱傳係數平均較平板微流道提升110%。而在其他厚度以及粒徑組合下亦有顯著提升效果。
在壓降方面，多孔性結構微流道因較多的孔洞造成較高表面粗糙度，整體的壓降較平板微流道大。最大增加15kpa。
總結本文成果，多孔性結構微流道能有效提升冷凝器熱傳性能，對於高功率元件散熱有高度應用潛力。

關鍵字

微流道 ； 冷凝熱傳增強 ； 多孔性結構

並列摘要

The microchannel evaporator with two-phase heat transfer achieves a high heat transfer coefficient, and low working fluid demands; therefore, is considered to have high potential. Thus, the use of microchannel condenser that uses two-phase heat transfer is seen as a cooling component with very high potential. By making it a porous structure that creates a 3D porous network, resulting in advantages such as large evaporation area, high capillary force, and high permeability, the heat transfer performance of the microchannel condenser is expected to increase significantly.
This study used copper to manufacture both the flat-plate microchannel condenser and porous microchannel condenser, with 30 microchannels of width and depth 500μm×155μm; Using water as working fluid, with mass flux range of 65~95 kg/m2 s, for heat transfer performance test. This study first investigates the effect of copper powder size and the structure’s base thickness of a porous microchannel condenser on heat transfer performance, then compares its heat transfer performance, pressure drop, and flow patterns to those of flat-plate microchannel condenser.
Comparing experimental results for heat transfer performance to heat transfer correlation of the conventional channel showed that the MAE is still large. With regard to pressure drop, compare with the correlation of microchannel developed recently, it correlated with our result, indicating a certain degree of reliability.
For flat-plate microchannel condenser, from flow visualizations of flows, 5 most common types of flow in condensation process can be seen clearly: droplet flow, annular flow, injection flow, slug flow, and bubbly flow. Heat transfer coefficient and pressure drop is positive correlative with increasing mass flux. When mass flux increases, the flow velocity increases, and the liquid-vapor interface shear stress increases, resulting in thinning of the liquid film, and the annular flow region increases; the heat transfer performance was thus enhanced accordingly. Heat transfer coefficient is 23~79kw/m2k. The overall pressure drop was also enhanced due to increased flow rate of the working fluid and elongation of the two-phase region.
For porous microchannel condenser, manufacturing parameters such as the base thickness range of 150~300μm and copper powder diameter range of 1~150μm was investigated. Experimental results showed that highest heat transfer coefficient was achieve with base thickness of 150μm and powder diameter of 88μm; heat transfer coefficient is 43~161kw/m2k, on average, the heat transfer coefficient of porous microchannel condenser was increased by 110% compared to that of flat-plate microchannel condenser. The absorption of condensed water build up by a porous microchannel structure allows for the thinning of the condensed liquid film and the annular flow region is more extended, therefore its heat transfer performance is better than that of a regular flat-plate microchannel condenser.
Concerning pressure drop, the overall pressure drop is greater than that for flat-plate microchannel condenser, with a greatest enhancement value of 15kpa.
To summarize this study, porous microchannel effectively enhance the heat transfer performance of condenser, It is highly potential for the high-power thermal management application.

並列關鍵字

microchannel ； condensation heat transfer enhancement ； porous structure

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