甲醇水溶液於漸擴微流道熱沉之對流沸騰研究

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研究生：	沈維晟 Shen, Wei-Cheng
論文名稱：	甲醇水溶液於漸擴微流道熱沉之對流沸騰研究 Convective Boiling of Methanol/Water Mixtures in a Diverging Microchannel Heat Sink
指導教授：	潘欽 Pan, Chin
口試委員:	楊毓民 Yang, Yu-Min 李堅雄 Lee, Chien-Hsiung
學位類別：	碩士 Master
系所名稱：	原子科學院 - 工程與系統科學系 Department of Engineering and System Science
論文出版年：	2011
畢業學年度：	99
語文別：	中文
論文頁數：	92
中文關鍵詞：	沸騰熱傳、雙相流、甲醇水溶液、微流道
外文關鍵詞：	boiling heat transfer, two-phase flow, binary mixture, microchannel
相關次數：	點閱：3 下載：0
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本研究係利用微機電技術製造矽質漸擴微流道熱沉以探討甲醇水溶液在微尺度下流動沸騰的熱傳現象。為了均勻分配流量並降低工作流體在雙相區間時嚴重的回流情形，測試段在工作流體入口端至主要流道間設計樹枝狀分配器並在每一個分配器末端加上突擴結構以求提高流道的上游壓降。研究中將探討溶液濃度和質量通率在流譜的分佈、熱傳分析以及臨界熱通率的影響。
本研究定義了四種流動行態，分別為（１）氣泡長型彈狀流、（２）環形流、（３）液膜破碎、以及（４）乾化。並根據實驗結果分別建立各種流動行態在不同溶液濃度下與壁過熱度及壁熱通率的流譜分佈。其中發現在低及中熱通率條件下，氣泡長型彈狀流以及環形流為主要的流動行態，因此推測在雙相區，甲醇水溶液在微流道中微液膜的沸騰為主要的熱傳機制。實驗結果顯示在這兩個流動形態，熱傳遞係數隨著壁熱通率的增加而增加。相對於在高熱通率條件下，主要的流動行態為液膜破碎。在此流動條件，熱傳遞係數隨著壁熱通率的增加而減少。由於實驗結果顯示相較於其他濃度下最廣的液膜破碎區域存在於甲醇莫耳分率xm = 0.3，因此本研究推測表面張力在氣液介面的震盪為其中一項影響液膜破碎的主因。在熱傳遞係數方面，甲醇水溶液的熱傳遞係數隨著壁過熱度的增加幾乎維持在一個定值。顯示甲醇水溶液在雙相區間的移熱能力上較純水及純甲醇穩定。本實驗的臨界熱通率在xm = 0.3時達到最大值，之後隨著甲醇濃度的增加造成質傳上的阻力提高而導致臨界熱通率明顯的迅速下降。在質量通率方面，隨著質量通率的增加，單相及雙相熱傳都有較好的表現。

This study conducts an experimental investigation and visualization for convective flow boiling heat transfer of methanol/water mixtures in parallel diverging multichannels with a hydraulic diameter of 468.5μm. In order to reduce flow reversal in the two-phase region, a tree-like inlet distributor with a sudden expansion structure in front of each channel is designed. The MEMS technology is applied to fabricate the microchannel. The effect of molar fractions and mass fluxes on flow patterns, heat transfer, and critical heat flux are studied.
Four distinct two-phase flow patterns can be identified, namely, (a) bubbly-elongated slug flow, (b) annular flow, (c) film breakup, and (d) dryout. Moreover, flow pattern maps on the planes of wall superheat or wall heat flux versus molar fractions, separately, are established based on the data of this study. For low to medium heat fluxes, the dominant flow patterns are elongated slug flow and annular flow, which indicates that the evaporation of a thin liquid film is the major mechanism of heat transfer for convective flow boiling of methanol/water mixtures in a microchannel. For these two-phase flow patterns, the heat transfer coefficient generally increases with an increase in heat flux.
The liquid film breakup is the dominant flow pattern for high heat fluxes. For these particular flow pattern, the heat transfer coefficient decreases with an increase in heat flux. Moreover, the temperature span of film breakup region has a widest range for xm = 0.3, suggesting that the most important factor of the liquid film breakup be the agitation of the surface tension on the liquid-vapor surface. A molar fraction of 0.3 presents the highest CHF in the present study.
The present study also reveals that an increase in the mass flux may result in the better performance for heat transfer in both single and two phase regions.

摘要    i
Abstract    ii
Acknowledgements    iv
Table of Contents    v
List of Tables    ix
List of Figures    x
Nomenclature    xiii

Chapter 1
Introduction    1
1.1.    Background    1
1.2.    Motivation and Objective    5
1.3.    Thesis Scope    7
Chapter 2
Literature Review    8
2.1.    Flow Boiling of Methanol/Water Mixtures    8
2.2.    The Marangoni Effect during the Phase Change for Mixtures    13
2.3.    Flow Boiling Instability in Microchannels    18
2.4.    Effects of Inlet Configurations on Flow Boiling Instability in Multichannels    20

Chapter 3
Experiment Details and the Fabrication of Test Section    23
3.1.    The Experiment Setup    23
3.1.1.    Degassing Module    24
3.1.2.    Test Loop    26
3.2.    Instrumentation and Measurement    30
3.3.    The uncertainty of the measurements    31
3.4.    The Design Parameters of the Test Section    32
3.4.1.    Main Microchannel    32
3.4.2.    Fractal Tree Structure Distributor    33
3.4.3.    Sudden Expansion Inlet    36
3.5.    The Fabrication of the Test Section    37
3.5.1.    The Designing Goal    37
3.5.2.    The Principle of Fabrication Processes    37
3.5.3.    The Fabrication Processes of Test Section    39
3.5.4.    Determination of the Practical Hydraulic Diameter    41
3.6.    Experimental Procedures    45
3.7.    Data Reduction    48
3.7.1.    Energy Conservation and Heat Loss Estimation    48
3.7.2.    Determination of the Heat Flux    50
3.7.3.    Determination of the Heat Transfer Coefficient    52
Chapter 4
Results and Discussion    53
4.1.    Uniform Flow Distribution by the Inlet Distributor with a Sudden Expansion Structure    53
4.2.    Two-phase Flow Patterns    56
4.2.1.    Identification of Boiling Region by Flow Pattern Visualization    57
4.2.2.    Flow Region Maps    62
4.3.    Boiling Heat Transfer Characteristics    65
4.3.1.    Boiling Curve    65
4.3.2.    Heat Transfer Coefficient    68
4.3.3.    Critical Heat Flux    73
Chapter 5
Conclusions and Future Work    76
5.1.    Conclusions    76
5.2.    Future Works    78
Appendix A
The Experiment Datum    85

                                

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