請用此 Handle URI 來引用此文件:
http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/33940
標題: | 多孔微流道蒸發器熱傳增強研究 Heat Transfer Enhancement in Porous Microchannel Evaporator |
作者: | Bing-Han Liu 劉秉翰 |
指導教授: | 陳瑤明(Yau-Ming Chen) |
關鍵字: | 微流道,沸騰熱傳增強,多孔性結構, microchannel,boiling heat transfer enhancement,porous structure, |
出版年 : | 2011 |
學位: | 博士 |
摘要: | 微流道蒸發器具有高熱傳係數、高均溫性與低工質需求量等優點,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,熱傳增強之微流道更具其應用價值。多孔結構具有大量成核址與連通孔洞,廣泛應用熱傳增強表面,預期對於微尺寸熱傳有顯著之提升。本研究設計製作多孔表面微流道蒸發器並探討銅粉粒徑、結構底厚與孔徑分佈對熱傳性能的影響,最後與平整表面微流道比較熱傳特性、壓降、壓力不穩定性與熱傳增強效果。
測試段分別於1平方英吋無氧銅表面製作62條寬深為225μm×660μm流道的多孔表面微流道蒸發器和相同尺寸之平整表面微流道蒸發器,以R-134a為工質進行熱測試。 平整表面實驗發現核沸騰與強制對流沸騰機制皆出現於微流道蒸發器,熱傳係數在乾度小於0.4前,主要隨熱通量上升而增加,不隨質量流率、乾度變化,應屬於核沸騰機制。而乾度大於0.4後,熱傳係數隨質量通率上升而上升,此模式與強制對流沸騰較為相近。如考慮表面張力之熱傳經驗式,與實驗結果相近。臨界熱通量在微流道主要隨流量上升而增加;對於考慮表面張力之經驗式,預測實驗結果相當接近。流動壓降則隨流量與熱通量上升而增加,均質流模型對微流道壓降較不適用,而考慮表面張力之分離流模型有良好預測結果。壓降震盪顯示平整表面微流道具有流道之間不穩定性,其振盪幅度在接近起始沸騰與臨界熱通量時最大。 多孔性結構微流道蒸發器的熱傳結果與平整表面明顯不同,熱傳係數在低熱通量即達到峰值,隨乾度上升而下降,質量流率主導熱傳係數變化,為強制對流沸騰,熱傳係數較平整表面平均提升5倍。多孔微流道臨界熱通量主要隨質量通率增加而增加;相較於平整表面臨界熱通量提升幅度並不顯著。多孔性結構微流道蒸發器壓降與平整表面趨勢相近,整體較平整表面高,但增加最大不超過50%。多孔表面微流道能有效抑制流道不穩定性,特別在高熱通量不會受到大量汽泡影響而無法操作,振盪可減少約1/6;沸騰起始時壓降最大振幅亦較平整表面微流道低1/2,顯示其在相變化操作較為穩定。 探討燒結底厚(150~375μm)、銅粉粒徑(1~100μm)與其比值(2~20)等製程參數實驗結果發現,底厚375μm和粒徑為32μm較適合微流道熱傳增強,而厚度對粒徑的比值對熱傳性能有很大影響,該比值為8~12時在微流道有較佳的熱傳性能。此外實驗亦發現孔徑為影響熱傳最主要的多孔結構參數,較小孔徑有較好的熱傳性能。而雙孔徑分佈較單一孔徑更能提高多孔微流道熱傳結果,最高可達單一孔徑的兩倍。 總結本研究成果,多孔性結構微流道蒸發器在熱傳有明顯提升,可提高二相操作之穩定性,在有限的壓降增幅下極具工業應用潛力。 The microchannels evaporator, which possesses the advantages of high heat transfer coefficient, good temperature uniformity, and small requirement for coolant flow rates, is considered as a potential cooling technology. In recent years, the raising of heat dissipation rate in electrical products becomes an important issue. The heat-transfer enhanced microchannels are suitable for the applications. The porous structure with a large number of nuclear sites as well as the re-entrance cavities is expected to enhance the heat transfer performance in a microstructure. In the present study, porous microchannel evaporators are designed and manufactured. The effects of powder size, thickness of structure, and pore size distribution upon the heat transfer performance are investigated. The comparisons of heat transfer characteristics, pressure drop, pressure instability, and heat transfer enhanced effects between the plane and the porous microchannel evaporator are made. The flow boiling experiments were conducted with a plane and a porous microchannel evaporator using R-134a as coolant. Both microchannels had 62 channels (225μm in width; and 660μm in depth) on copper substrates with one square inch in area. For the plane microchannel evaporators, the results showed that the nucleation boiling and the force convection boiling mechanisms both appeared in microchannels. When the quality in the microchannels was smaller than 0.4, the heat transfer coefficient mainly increased with increasing heat flux and did not vary with the mass flow rate or the quality. This region (quality was under 0.4) was dominated by the nucleation boiling mechanism. On the other hand, when the quality was larger than 0.4, the heat transfer coefficient increased with a increasing mass flux. This region (quality was over 0.4) was dominated by the force convection boiling. The experiment results were substituted into the correlations in which the surface tension force was taken into consideration. The predictions showed a good agreement with experimental data. The critical heat flux (CHF) increased with increasing flow rates. A CHF correlation that incorporates the surface tension force showed an excellent accuracy for the experimental data. Pressure drop were raised by increasing flow rates and heat fluxes. The separation model incorporating surface tension force had a good agreement. The pressure drop oscillation suggested that the presence of instability inside the plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation (ONB). For the porous microchannels evaporator, the experimental results depicted that the heat transfer coefficient reached a peak value at low quality and decreased with a increasing quality. However, the heat transfer coefficient did not vary with the mass flow rate. This was apparently different from the plane microchannels. The heat-transfer behavior dominated by the mass fluxes belongs to the force convection boiling mechanism. In contrast of the plane microchannel evaporator, the heat transfer coefficient in the porous microchannels evaporator had an enhancement of 5 times in average. The CHF in porous microchannel evaporator increased with increasing mass fluxes and did not enhanced significantly. Furthermore, the trend of pressure drop in porous microchannel was similar in the plane microchannels. The pressure drop was higher than plane microchannels; however, the maximum pressure drop was not over 50%. The amplitude of average pressure drop oscillation near the high heat flux as well as ONB was 1/6 and 1/2 smaller than in the plane microchannels. This result presented that the porous microchannels evaporators provided a stable boiling behavior when the nucleation began. The porous microchannel evaporators were sintered under the following parameters: the powder diameter dp ranged from 1~100μm, thickness of porous structure δ ranged from 150~375μm, and δ/dp ranged from 2~20, respectively. The investigation on the effect of particle size dp as well as thickness δ indicated that the ratio of the thickness to the particle size δ/dp had a significance in the heat transfer performance. This ratio must be properly chosen in order to reach a better heat transfer performance. The better ratio of δ/dp was between 8~12 in our work. Moreever, the pore size distribution dominated the heat transfer behavior. Smaller pore size with a higher heat transfer capacity. The bi-porous structure was better than the mono-porous structure in about 2 times. To conclude the present study, the porous microchannel evaporator is highly potential for the industrial applications. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/33940 |
全文授權: | 有償授權 |
顯示於系所單位: | 機械工程學系 |
文件中的檔案:
檔案 | 大小 | 格式 | |
---|---|---|---|
ntu-100-1.pdf 目前未授權公開取用 | 4.44 MB | Adobe PDF |
系統中的文件,除了特別指名其著作權條款之外,均受到著作權保護,並且保留所有的權利。