多孔性柱狀陣列之池沸騰熱傳增強研究

Pool Boiling Heat Transfer Enhancement with Porous Pin Fin Arrays

施毓倫(Yu-Lun Shih)

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(2009年)

https://doi.org/10.6342/NTU.2009.00440

摘要

隨著高功率元件漸增，散熱需求持續成長，受限於運作溫度，產品的熱管理更顯重要。而燒結多孔毛細結構於發熱面上以增進沸騰熱傳性能的技術，其優異的表現於工業界已獲得肯定，如何進一步改善多孔表面在高瓦數下氣液流阻造成的性能限制，創造在低過熱度下擁有更廣操作範圍的沸騰表面將為重要的議題。
本文旨在使用樹枝狀銅粉於固定之燒結製程下，透過模具燒結方式於多孔表面上製作多孔柱狀陣列，並於R-134a中進行水平測試表面的池沸騰研究，對其熱傳特性進行探討。研究方法藉由改變不同粉末粒徑(d=32~63μm)、底部厚度(t=200~350μm)、柱高(L=400~600μm)與間距(S=350~500μm)，搭配統計方法分析熱測試結果，了解各參數間的影響效應。最後透過和無結構多孔表面比較，了解柱狀陣列的出現對原多孔表面之熱傳行為造成的影響。
統計分析結果顯示，粉末粒徑為影響平均熱傳係數最重要的參數，貢獻百分比為59%，其次是底部厚度與間距，分別為31%與6%，柱高則僅佔0.3% ，影響並不顯著。原因在於粉末粒徑與底部厚部的變化將改變結構之濕表面積以及內部氣液流阻，其中粉末粒徑同時影響孔洞大小及毛細力，故效應最為顯著，建議之提升方向則為縮小粒徑、提高底厚以及縮短間距；柱高與間距則是影響臨界熱通量的主要參數，貢獻百分比分別為58%與23%，反之底部厚度與粒徑僅分別佔 2%與0.7%，影響並不顯著。而在柱高間距(L/S)比大於1.2時，能有效引導氣液分離，使臨界熱通量進一步提升，其中間距的變化則會影響表面上氣液介面的不穩定性，實驗範圍內之臨界熱通量與柱寬及間距之合的平方根(W+S)1/2成反比。
熱測試結果顯示，將底厚與粒徑合併為底厚粒徑比可發現，陣列與無結構多孔表面之平均熱傳係數均隨底厚粒徑比增加而提升，其中表現最佳者同為11，有別以往球粉文獻建議之最佳比例4-6。並於追蹤實驗發現，陣列表面之性能於底厚粒徑比16時則呈現下降趨勢。此外亦發現，間距500μm之陣列其熱傳係數僅與無結構表面相近，當縮小至350μm則呈現明顯提升。性能增強比較方面，在熱通量超過150KW/m2後，本實驗最佳多孔陣列與無結構表面之熱傳係數較光滑平板分別提升7~9倍和5~7倍；臨界熱通量則為光滑平板之2.5倍與1.5倍。

關鍵字

熱傳 ； 池沸騰 ； 毛細結構 ； 熱傳增強表面

並列摘要

With the increasing usage of high-power components, the demand of heat dissipation keeps raising, thermal management of products become more important for the operation temperature limits of those components inside. The excellent performance of the enhanced boiling heat transfer technology by porous structure surface in the industrial community has been sure. How to further improve the performance limitations caused by the vapor-liquid flow resistance in the high-wattage, for the creation of the boiling surface which under low superheat has a wider operating range will be an important topic.
The purpose of this research was to modulate the porous pin-fin arrays on the porous surface sintered with copper dendritic powder in a fixed process of sintering, and the heat transfer performance for pool boiling of saturated R-134a on horizontal finned surface was discussed. The study was conducted following a statistical method with a two-level factorial plan involving four variables: the particle diameter (d=32~63μm), the bottom thickness (t=200~350μm), the fin length (L=400~600μm), the spacing (S=350~500μm). Finally, the performance of the porous surface which with and without porous pin-fin were compared to further understand the influence of porous pin-fin arrays.
Statistical analysis showed that for average heat transfer coefficient, the particle diameter is a primary effect (percent contribution is 59%). Bottom thickness and Spacing are minor effects (percent contribution is 31% and 6%). Fin length has a little effect (percent contribution is 0.3%). This should due to the variation of particle diameter and bottom thickness will change the wetting surface area and internal gas-liquid flow resistance, by the way, the particle diameter impacts pore size and capillary force, and therefore leads to the most significant effect. The better parameters tend to have smaller particle diameter、spacing and higher bottom thickness；for the Critical Heat Flux, the fin length and spacing are main parameters (percent contribution is 58% and 23%). Particle diameter and bottom thickness have little effects (percent contribution is 2% and 0.7%). It’s found that the when the ratio of fin length and spacing (L/S) is greater than 1.2, the vapor-liquid separator could effectively lead to further enhance the CHF. The variation of spacing impacts the instability of vapor-liquid influence thus causing the CHF inversely proportional to the square root of the sum of fin width and spacing (W+S)1/2.
Experimental results showed that it can be found though combining bottom thickness and particle diameter, the average heat transfer coefficient of porous pin-fin surface and porous surface both enhanced with increasing t/d, and have best performance with 11 in the range of parameters studied in present research, was different from the best ratio of 4-6 in previous literature using spherical powder. And follow-up experiment found that as t/d further increased to 16, the performance of porous pin-fin surface showed a downward trend. In addition, the heat transfer coefficient of porous pin-fin surface with S=500μm, was similar to porous surface with the same particle diameter and bottom thickness, but as the spacing shorten to 350μm, it showed a significantly enhancement. In the heat flux over 150kw/m2, the heat transfer enhancement ratios of best porous pin-fin surface and porous surface relatively smooth plane improved by 7~9 and 5~7times; the CHF was about 2.5 and 1.5 times over the smooth plane.

並列關鍵字

Heat transfer ； Pool boiling ； porous surface ； Enhanced surface

參考文獻

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