高穩定度微流道散熱器研究

Development of a Highly Stable Microchannel Heat Sink

盧俊庭(Chun-Ting Lu)

指導教授 ： 潘欽

國立清華大學/原子科學院/工程與系統科學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00149

摘要

近年來，在半導體微小化技術不斷進步之下，相同面積中可容納的電子元件隨之增加，因此，單位面積所產生的熱量會遽增，使得電子元件必然會面臨散熱的難題。微流道沸騰熱傳因具有高傳熱能力與低操作功率而被視為一極具潛力的高移熱能力散熱技術。從文獻中可以發現，微流道內的流動沸騰是一極為不穩定的狀態，且當不穩定發生時，系統的壓力與溫度會發生激烈的震盪。這些不穩定現象對於微流道散熱器的應用而言，可能會造成系統提早乾化進而燒毀電子元件。
本研究主要目的是在發展一高穩定度且高移熱能力的微流道散熱器。微流道散熱器係利用微機電技術(MEMS)與雷射切割來製作三種具有不同人工成核址分佈與數量的多平行漸擴微流道。第一種微流道是不具任何人工成核址的10條平行漸擴微流道；第二種微流道是在流道後半段底部每間隔1 mm製作一人工成核址的10條平行漸擴微流道；第三種微流道是在整條流道(入口至出口)底部每間隔1 mm製作一人工成核址的10條平行漸擴微流道。研究中，藉由改變質量通率與加熱功率來探討人工成核址的分佈與數量對微流道內沸騰熱傳與流動穩定性之影響，並利用高速攝影機觀察微流道內氣泡的成核、成長及脫離後轉變彈狀流的流動現象。
藉由流譜的觀察發現，氣泡半徑的成長與時間會成線性關係而彈狀流長度的變化與時間會成指數關係並會受臨近流道氣泡成長的影響。研究中主要的雙相流譜為彈狀流與環形流。當臨界熱通率發生時，流道出口會有乾化的情形發生，並經常性地有液膜或液柱潤濕此乾化區域，此時雙相流譜主要為氣液界面呈波浪形的環形流。
沸騰熱傳分析結果顯示，熱通量與熱傳遞係會隨質量通率增加而增加。首先，在開始進入雙相沸騰區後，熱通量與熱傳遞係數會有顯著地上升。之後，熱通率隨著壁過熱度的增加而逐漸遞減並趨近一定值。另一方面，熱傳遞係數會隨乾度的增加而減少。這是因為乾度增加時，環形流的液膜有部分乾化，且乾化的面積比列隨乾度的增加而增加之故，藉由這個熱傳遞係數隨乾度減少的趨勢可以推論環形流對流沸騰熱傳可能為主要的熱傳機制。研究結果更顯示，工作流體的溶氧量對於臨界熱通率沒有明顯的影響，而人工成核的分佈與數量對於微流道的沸騰熱傳能力有顯著的提升。本研究亦發展出一可以準確預測本實驗熱傳遞數的經驗公式。
流動沸騰之穩定性研究顯示，在相同的熱通率與質量通率下，具有人工成核址的多平行漸擴微流道具有最高的穩定性，其次為不具人工成核址的多平行漸擴微流道，而多平行矩形微流道是最差的。
綜合本研究結果顯示，整條流道底部皆具有人工成核址的平行漸擴微流道在穩定性與沸騰熱傳能力的表現是最好的，此微流道的設計可被視為一具有高穩定度且高移熱能力的微流道散熱器。

關鍵字

漸擴微流道 ； 沸騰熱傳 ； 微機電 ； 人工成核址 ； 穩定性

並列摘要

With microprocessor performance increasing, the power generation from a microprocessor chip is expected to exceed 180 W/cm2 and the limits of current air-cooling technology will be reached, i.e., forced air heat sinks have become significantly larger with more expensive and noisier. Therefore, there is a need to address the thermal challenge of high-heat-flux for next generation of power electronics. Flow boiling in microchannels, considered as one of the most promising technologies, has the advantages of highest heat fluxes, lowest pumping powers, and the highest efficiency.
This study explores experimentally the flow boiling stability, channel-to-channel interactions and convective boiling heat transfer in 10 parallel diverging microchannels with/without ANS. Three types of diverging microchannel heat sinks (named type-1, type-2, and type-3) were designed. Each microchannel had a mean hydraulic diameter of 120 m. Water and FC-72 was used as the working fluid with different mass fluxes, based on the mean cross section area, ranging from 99 kg/m2s to 999 kg/m2s. Type-1 system did not contain any ANS, whereas type-2 system contained ANS distributed uniformly along the downstream half of the channel and type 3 system contained ANS distributed uniformly along the entire channel. The ANS are laser-etched pits on the bottom wall of the channel and have a mouth diameter of 24 μm, as indicated by the heterogeneous nucleation theory.
Flow visualization shows that slug and annular flow is the dominant two-phase flow pattern. It may imply the dominant heat transfer mechanism may be convective boiling. During CHF, the dryout of annular liquid film appears near the outlet region with frequent rewetting of liquid film with slug bubble or rewetting of liquid column on the dryout surface, while wavy annular flow is the dominant flow pattern.
Moreover, correlations for boiling heat transfer coefficient and the CHF are developed and reviewed, respectively. The proposed correlations for boiling heat transfer coefficient show excellent agreement with the experimental data of the present study. Furthermore, the CHF correlation of Bowers and Mudawar can predict the present CHF data very well with the overall MAE of about 16%. Under boiling condition, a significant improvement in stabilizing the flow boiling, suppressing flow reversals, enhancing heat transfer performance can be obtained by using diverging microchannel heat sinks with ANS. Among three types of microchannels, type-3 system shows the best boiling heat transfer performance. This particular design can be regarded as a highly stable and high-heat-flux microchannel heat sink.

並列關鍵字

diverging microchannel ； flow boiling heat transfer ； MEMS ； artificial nucleation sites ； stability

參考文獻

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