所有的蒸發器及冷凝器螺旋管皆包含彎管或其他組成熱交換器管件的裝置。流體通過彎管時產生擾流，並且也會引起遠大於直管的摩擦壓降。因此，彎管的壓力損失對於冷藏器及其他兩相系統之整體效應而言，是非常重要的。因此空調機熱交換器設計除了具備熱傳遞知識外更需要知道如何計算壓力損失以提供夠用的壓縮機動力。在典型空調機熱交換器常使用U型管。因此，單相流和兩相流在180°彎管內的摩擦性能對於正確的評估空調的熱交換器的效率式非常重要的，目前兩相流壓降的研究大都用在直管，然而關於180°彎管的兩相流研究含數據模式卻非常有限。
本研究之目的乃是以實驗方式在室溫中以空氣和水的單相流乃兩相流流過180°的彎管。本研究共測試了9支180°玻璃彎管，內徑( D )是3.0, 4.9及6.9 mm，曲徑比( 2R/D )為3, 5及7, 其中R為彎管中心線之半徑，此實驗乃是在不同的質通量和乾度下進行，並分別量測對應的體積流率和壓降。水及空氣的單相測試之雷諾數的範圍約400< ReD <10000。兩相測試的混合質通量的範圍為50~1000 kg/m2s.本實驗也以視覺觀察及高速相機擷取個個流動的條件的流譜圖並以D=3, 4.95及6.9mm的數據分別組成三個流譜圖，論文中只列出D=7。此外也紀錄各流譜線之間的轉型數據以在流譜圖上劃出兩個流譜間的轉型線。
在180°彎管中觀察到的流譜數據絕大部分與相同管徑的直管相似，但當波型層流轉變成環狀流時外在。曲徑比較小(2R/D= 3 )時，觀察到流譜短暫地由層流轉成環狀流。並觀察到液體藉由離心力移至管壁外側，在外測液體隨即被移動至管壁上側然後再轉回管壁內側，此現象可能是彎管的二次流所引起。然而此現象並未在較高的氣體乾度流時被發現此原因可能是氣相的慣性力克服二次流的旋轉力，所以在外側的液體未能被捲到管壁的上側及內側。一般而言，通過水平彎管的摩擦壓降包含摩擦力、曲率，以及彎管下游出口處之直管部分之壓力的恢復所產生的效應。
本實驗的兩相流彎管壓降數據已包含彎管下游直管回覆流所引起的額外壓降損，此量測到的彎管壓降會隨著質通量及蒸氣乾度的增加以及曲率半徑與管徑的減少而增加。

All evaporator and condenser coils contain bends or other fittings to compact the heat exchanger size. The design of air-cooled heat exchangers requires the knowledge of heat transfer and frictional loss. For typical air-cooled coils, use of hairpin is very common. As expected, the hairpin that contains many 180° return bends (U bend) will cause higher pressure drop than the corresponding straight tube. As a consequence, the single-phase and two-phase frictional performance of a return bend is very important for accurate estimation of the performance of an air-cooled heat exchanger. While there are a large number of investigations focusing on two-phase pressure drop in a straight tube. However, very limited data, models and correlations are currently available for two-phase flow in 180° return bends.
The objective of this study is to conduct experimental investigations of two-phase flow air-water mixture and single-phase flows of air and water flowing in 180° horizontal return bends at room temperature. A total of nine 180° return bends has been tested for the present study. The test tubes are made of glass tube having inner diameters (D) of 3.0, 4.95, and 6.9 mm with dimensionless tube curvature ratio (2R/D) of 3, 5 and 7, where R is the radius of centerline of bend. Test range of Reynolds numbers for water and air single-phase tests is about 400< ReD <10000. The range of mss flux for two-phase mixture is between 50 ~ 400 kg/m2s.
Two-phase flow patterns were taken by a combination of visual observations and high-speed camera photos to form the flow regime maps for each test tube. The data for the transition between flow patterns were recorded. Most of the observed flow pattern data in 180° return bends are similar to that in straight tubes with the same tube diameter, except for the transition data from wavy stratified to annular. For smaller curvature ratio of 3, one can see the flow pattern recovery region is temporally turned from stratified flow into annular flow. The liquid was observed to switch to the outer tube wall by centrifugal force, the liquid at out wall was soon forced to move toward to the upper side of tube wall and then switch back to the inside of the tube wall by secondary flow. However, this phenomenon is not observed at higher vapor quality due to the lack of liquid and increase of gas-phase inertia.
Two-Phase frictional pressure drop data for 180° return bend including the additional loss in the downstream straight tube were obtained. In general, the frictional pressure drop across a horizontal return bend includes the effects of friction, curvature and pressure recovery at bend exit to the downstream straight tube. The effects of total mass flux, vapor quality, curvature ratio and tube diameter were examined. The resulted two-phase pressure drops were observed to algebraically increase with increasing total mass flux and vapor quality, and decreasing bend radius and tube diameter.

目錄
中文摘要i
英文摘要iii
誌 謝v
目錄vi
表目錄viii
圖目錄ix
符號說明( Nomenclature )xi
第一章 緒 論1
1.2 研究目的4
第二章 文獻回顧5
2.1 直管兩相流摩擦壓降6
2.1.1 基礎理論模式6
2.1.2 經驗式 (Empirical Corrleations)6
2.2彎管兩相流摩擦壓降(Total Bend-Loss Coefficient)9
2.3在彎管的摩擦因子（Friction Factors in Coiled Tubes）：16
2.4 流譜23
2.4.1 流譜型態種類23
2.4.2流譜預測方法26
第三章 實驗系統與分析方法29
3.1 簡 介29
3.2.1 測試段29
3.2.2 照相段30
3.2.3 水循環系統30
3.2.4 空氣循環系統31
3.3 實驗儀器31
3.3.1 溫度量測32
3.3.2 壓力量測32
3.3.3 差壓量測32
3.3.4 水流量量測33
3.3.5 空氣流量量測33
3.3.6 顯微照相機33
3.3.7 照明設備34
3.3.8 資料擷取系統34
3.4 工作流體的熱力物理性質34
3.5 實驗過程35
3.5.1 系統測漏35
3.5.2 實驗操作步驟35
3.6 實驗觀察37
3.6.1 流譜型態分析37
3.6.2 水-空氣流譜實驗數據換算37
第四章 結果與討論39
4.1 摩擦因子：39
4.2 彎管兩相摩擦壓力降41
4.3流譜的比較42
第五章 結論45
參考文獻46
附錄 一 基本量與導出量之不準度77
附錄二 空氣、水進口壓力與溫度關係換算表78
自傳79

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