U形通道常用於大型渦輪發動機葉片散熱與大型熱交換器當中。隨著近年來能源議題受到重視，不論是渦輪發動機或熱交換器都希望能提升其能量轉換效率。在U形通道中安裝擾流器是提升紐賽數比(("Nu" ) ̅/〖"Nu" 〗_∞)熱傳增益最有效的方法之一，而擾流器大致上可區分成三大類：肋條(ribs)、折流板(baffles)以及百葉窗(louver type)。其中肋條類型因為其低阻擋比與易於製造的特性，是最常見並且適用範圍較廣的擾流器。儘管非常多研究團隊已針對肋條式擾流器進行探討，但對於U型管道中轉彎區的研究卻較為匱乏，因此本研究提出一種離壁式曲面肋條，並使用質點影像測速儀(Particle Image Velocimetry，簡稱PIV)、紅外線熱像儀(Infrared Thermography，簡稱IRT)和壓差感測器探討其於正方形U型雙通道之熱傳與流場。
在熱傳增益之熱流實驗中，測試端入口流量維持穩定，離壁式曲面肋條以多對並排之方式安裝在雙通道中以形成上下對稱之通道，其漸縮角(α)、漸擴角(β)、離壁間距(C/H)與雷諾數(Re) 的變化範圍分別為25°至45°、30°至50°、0.25至1以及5,000至20,000。從流場實驗中發現，離壁式曲面肋條可對加熱壁面導引出衝擊冷卻流和中心加速主流。中心加速主流能夠有效加強轉彎區所形成之狄恩渦旋，得轉彎區紐賽數比(("Nu" ) ̅/〖"Nu" 〗_∞)相較於文獻中最高有10%~80%的提升。進一步就區域熱流相關性分析後，發現在第一與第二通到轉彎連接區紐賽數比("Nu"/〖"Nu" 〗_∞)與主流速度|"U" |/"U" _"b" 呈現高度相關(皮爾遜相關性係數R ≥ 0.8)，而在轉彎區中心處發現紐賽數比(("Nu" ) ̅/〖"Nu" 〗_∞)與側向速度|"W" |/"U" _"b" 呈現高度相關(皮爾遜相關性係數R ≥ 0.8)。在C/H = 0.38條件下，(Nu) ̅/〖Nu〗_∞ 和 "f" ̅/"f" _∞會隨著α 和 β提升而提升。而在在α=25˚ 和 β = 40° 條件下，(Nu) ̅/〖Nu〗_∞ 和 "f" ̅/"f" _∞會隨著C/H上升而先上升再下降，並且最高熱性能係數(TPF)1.4 出現在C/H = 0.38與 "f" ̅/"f" _∞= 10-25範圍內文獻最高相近。最後，本研究提出適用於離壁式曲面肋條之經驗公式，("Nu" ) ̅/〖"Nu" 〗_∞ 和 "f" ̅/"f" _∞之平均誤差分別為2.24% 和 4.95%。

U-shaped channels are widely used in heat exchangers as well as cooling of gas turbine blade. Among all types of heat transfer enhancement techniques, mounting turbulators in the U-shaped channels is one of the most simple and effective ways. Based on blockage ratio (BR), turbulators can be roughly classified into three major types: ribs (low BR), baffles (high BR), and in-betweens (moderate BR). The rib is the most commonly used turbulator type because of their low friction factor and simple manufacturing process. Although many studies have focused on ribs, few have investigated their influence on the turn region. In this study, novel detached curved ribs are proposed, and their influence on thermal fluid flow in a two-pass square channel is experimentally investigated using particle image velocimetry (PIV), infrared thermography (IRT), and pressure sensors.
The inlet of the two-pass test section is maintained at a fully developed flow condition. Detached curved ribs were mounted in line to create a top–down symmetrical channel with a channel hydraulic diameter of 45.5 mm. Various parameters were investigated, including the convergence angle (α = 25°, 35°, and 45°), divergence angle (β = 30°, 40°, and 50°), clearance ratio (C/H = 0.25, 0.38, 0.56, 0.75, and 1), and Reynolds number (Re = 5,000–20,000). The experimental results show that the detached curved ribs direct flow towards the heat wall and accelerate near wall flow, resulting in impingement and strong convective cooling, respectively. Additionally, the highly accelerated core flow aids the Dean vortices in the turn region, leading to 10%–80% enhancement in Nusselt number ratio ( ("Nu" ) ̅/〖Nu〗_∞ ) compared with previous highest results. Further Pearson correlation analysis revealed that the Nu/〖Nu〗_∞ at the first and second halves of the turn is highly correlated (Pearson correlation coefficient R ≥ 0.8) with the mean streamwise velocity |"U" |/"U" _"b" , and the midturn Nu/〖Nu〗_∞ is highly correlated (R ≥ 0.8) with the spanwise velocity |"W" |/"U" _"b" . For fixed C/H=0.38, both (Nu) ̅/〖Nu〗_∞ and "f" ̅/"f" _∞ increase with increasing α and β. For fixed α=25˚and β = 40°, (Nu) ̅/〖Nu〗_∞ and "f" ̅/"f" _∞ values first increase and then decrease with increasing C/H, with the maximum TPF value of 1.4 occurring at C/H = 0.38 which is comparable to previous highest data with "f" ̅/"f" _∞= 10-25. Finally, empirical correlations of ("Nu" ) ̅/〖"Nu" 〗_∞ and "f" ̅/"f" _∞ with Re, α, C/H, and β for the present design are proposed with an average difference of 2.24% and 4.95%, respectively

Abstract ii
List of Tables vii
List of Figures viii
Nomenclature xii
Chapter 1 Introduction 16
1-1 Opening Remarks 16
1-2 Literature Survey 18
1-2-1 Rib Shape 18
1-2-2 Rib Arrangement 22
1-2-2-1 Clearance Ratio 22
1-2-2-2 Pitch Ratio 23
1-2-2-3 Rib Angle 25
1-2-3 Turn Region 27
1-3 Objectives 29
Chapter 2 Experimental Apparatus and Conditions 49
2-1 Model Configuration 49
2-2 Experimental Apparatus and Measurement System 51
2-2-1 Coolant Flow and Control system 51
2-2-2 Flow Field Measurement 52
2-2-2-1 Particle Image Velocimetry Principle 52
2-2-2-2 Illumination System 52
2-2-2-3 Particle Image Shooting System 53
2-2-2-4 Particle Seeding System 53
2-2-3 Heat Transfer Measurement 54
2-2-3-1 Infrared Thermography Principle 54
2-2-3-2 Heating and Data Acquisition System 55
2-2-4 Pressure Drop Measurement 56
2-3 Test Condition and Data Processing 56
2-3-1 Heat transfer and Pressure Drop Measurement 56
2-3-2 Flow Field Measurement 58
2-4 Validation of Measurement Techniques and Uncertainty Analysis 59
Chapter 3 Results and Discussion 74
3-1 Local Flow Features and Heat Transfer 74
3-2 Relationship between Nusselt Number and Flow Parameters 87
3-3 Effect of the Clearance Ratio 89
3-4 Effect of Curved Convergence Angle 92
3-5 Effect of Curved Divergence Angle 94
3-6 Overall Thermal Performance 96
Chapter 4 Conclusions and Recommendations for Future Research 125
4-1 Conclusions 125
4-2 Future Recommendations 126
Appendix-A Validation of Measurement Techniques 127
Appendix-B Relationship between BR & Nu 130
References 157

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