擾流器是提升渦輪機葉片內冷卻流道熱傳效率的重要設計之一，其最常見為低阻擋比的肋條和和高阻擋比的折流板。目前對於肋條流場的實驗研究較多，而對折流板的研究卻嚴重不足。因此本文提出一種新型的折流板─百葉窗型擾流器，並以質點影像測速技術(Particle Image Velocimetry，PIV)量測與探討葉片數目(Ns)對正方形雙通道紊流場特性之影響，從而解釋前人相關熱傳數據之趨勢。實驗參數設定包括Ns變化範圍1~4，百葉窗型擾流器間距與葉片角度分別固定為1倍水力直徑(DH)與30度，而以DH為特徵長度與管道截面平均空氣流速為特徵速度(Ub)之雷諾數則為10000。實驗結果以流向平均速度(U/Ub)、垂直平均速度(V/Ub)、流向紊流強度(u’/Ub)、縱向紊流強度(v’/Ub)、紊流動能(k/U_b^2)、合成平均速度(√(U^2+V^2 )/Ub)和雷諾剪應力((uv) ̅/Ub)及其對應壁面紐賽數(Nu/Nu∞)來分析。
結果發現第一與二通道加裝百葉窗型擾流器(Ns=4)能讓近壁面之最大對流與垂直沖擊平均速度大幅提升，而其量值分別高於肋條通道1015%和400%，且平均Nu/Nu∞較肋條通道提高了60%。此外，存在一個臨界葉片數目(Nsc=3)，當Ns>Nsc時葉片後方核心區會出現明顯的不對稱紊性尾流，而當Ns<Nsc時則變為對稱尾流；此一觀察說明了為何在相同條件下，前人熱傳結果之總體Nu/Nu∞與熱性能係數在Ns>Nsc時對葉片數目變化不敏感。未加裝百葉窗型擾流器之彎道內，當Ns>Nsc時會出現反向的狄恩渦旋(Dean Vortices)，反之則恢復成正常狄恩渦漩，此一紊流特殊現象導致內外壁面沖流(Downwash)與升流(Upwash)熱傳機制互換,並提供了文獻中相應壁面位置Nu/Nu∞隨橫向位置變化趨勢反轉之流體動力學解釋。在本文所分析之紊流參數中，發現於第一通道、轉彎區和第二通道熱傳均與垂直平均速度有較高的相關性，此一結論與前人文獻結果一致，而其他參數如u’/Ub、v’/Ub、k/U_b^2、√(U^2+V^2 )/Ub及 (uv) ̅/Ub與熱傳相關性較低。

Turbulators are one of important designs for enhancing heat transfer in internal cooling passages of modern gas turbine. Among all types of turbulators, low blockage ratio ribs and high blockage ratio baffles are most commonly employed. Although there exist a number of studies on flow field measurement in ribbed channels, the experimental research on velocity distribution in baffled channels is limited. In this thesis, a novel baffle-type turbulator called louver is proposed and its slat number (Ns) effects on turbulent fluid flow in a two-pass square channel are experimentally explored by particle image velocimetry (PIV), which aims to explain previous heat transfer data. The examined Ns ranges from 1 to 4 whereas the pitch ratio and slat angle of louver are fixed at 1 times channel hydraulic diameter (DH) and 30-deg, respectively. For all cases above, the Reynolds number based on DH and mean bulk velocity (Ub) is set to 10,000. Experimental results are presented with mean streamwise velocity (U/Ub), mean transverse velocity (V/Ub), streamwise turbulent intensity (u’/Ub), transverse turbulent intensity (v’/Ub), turbulent kinetic energy (k/U_b^2), mean surface velocity(√(U^2+V^2 )/Ub), and Reynolds stress ((uv) ̅/Ub) as well as the corresponding Nusselt number ratios (Nu/Nu∞).
The results show that the louvers raise both maximum near wall tangential and transverse velocity components significantly in the first and second pass of the channel, which are respectively 1015% and 400% higher than those of ribbed channels. Consequently, the corresponding overall Nu/Nu∞ is augmented by 60%. Furthermore, there exists a critical slat number (Nsc=3) beyond which an obviously asymmetric wake region is formed behind the louver and below which the wake region becomes nearly symmetric. Such an observation can explain why the overall Nu/Nu∞ and thermal performance factor are insensitive to the slat number. For the turn region, the curvature induced Dean vortices are reversed as Ns>Nsc and recovered as Ns<Nsc, which reveals the mechanism that upwash and downwash flow exchanges position near inner and outer walls, respectively. This finding provides a physical explanation for reversion of the corresponding transverse Nu/Nu∞ trend in previous literature. Meanwhile, through thermal fluid analysis, it is concluded that the trend of Nu/Nu∞ mainly correlates to V/Ub in both first pass, turn region, and second pass of the channel as previously reported. Other flow parameters, such as u’/Ub, v’/Ub, k/U_b^2, √(U^2+V^2 )/Ub and (uv) ̅/Ub, nonetheless, are in weak correlation with heat transfer.

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