近年來隨著自然資源的枯竭，提升蛇形熱交換器的效率已經是迫在眉睫的課題，一般而言，蛇形通道熱傳增益的技術可分為兩類：被動式和主動式。對於被動式的熱傳增益，安裝擾流器是提升紐賽數比(("Nu" ) ̅/〖"Nu" 〗_"o" )最有效的方法之一，而擾流器根據阻擋比可區分成三大類：肋條(ribs)、折流板(baffles) 以及介於二者之間的中庸設計(in-betweens)。儘管國際上多數研究團隊已充分探討肋條或折流板，但對於中庸設計的研究卻較為匱乏，因此本研究提出一種新型的中庸翼形擾流器，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雙通道之主動與被動式熱傳增益。
對於被動式熱傳增益之熱流實驗，測試端入口流量維持穩定，翼形擾流器以多對並排之方式安裝在雙通道的側壁，其攻角(α)、相對厚度比(t/C)、截距比(Pi/DH)與雷諾數的變化範圍分別為10°至30°、0.08至0.23、0.6至∞以及5,000至20,000。從PIV實驗中發現，本研究之翼形擾流器可導引出兩對擁有藤原效應(Fujiwhara effect)的同向渦旋，並提升整體紐賽數比(("Nu" ) ̅/〖"Nu" 〗_"o" )至5.4，其較先前文獻提出之百葉窗型擾流器的("Nu" ) ̅/〖"Nu" 〗_"o" 約高出25%。此外，翼形擾流器還消除了常見於肋條與折流板後方的迴旋死區，進而使整體摩擦係數比("f" ̅/"f" _"o" )相較於百葉窗型擾流器有61.5%至66.8%的降幅。進一步就熱流相關性分析後，發現截面的無因次平均流向渦度和側向平均垂直速度(((|"V" |/"U" _"b" ) ) ̅_"sp" )與側向平均的紐賽數比((("Nu" ⁄〖"Nu" 〗_"o" ) ) ̅_"sp" )呈現高度相關(皮爾遜相關性係數R ≥ 0.7)。在定泵功率的條件下，熱性能係數(TPF)在α = 20°、t/C = 0.20與Pi/DH = 0.7時達到最大值1.68，且於15 ≤ "f" ̅/"f" _"o" ≤ 80區間內，高於先前文獻報導之最佳值1.41。
基於上述優化後的擾流器參數(α = 20°、t/C = 0.20與Pi/DH = 0.7)，藉由將入口條件切換為脈動流來進行主動式熱傳增益熱流實驗。脈動流速度波型為仿三角波，史卓赫數(Strouhal number，簡稱St)的變化範圍為0到0.67。實驗結果發現脈動流不僅增強擁有藤原效應之同向渦旋，也一併消除在轉彎區的角落渦旋(corner vortices)，因此其("Nu" ) ̅/〖"Nu" 〗_"o" 相較於穩態流有13%的提升。此外，脈動流之無因次側向平均的側向速度紊動取代穩態流之((|"V" |/"U" _"b" ) ) ̅_"sp" 作為與(("Nu" ⁄〖"Nu" 〗_"o" ) ) ̅_"sp" 的第二高度相關的流力參數。隨著St的增加，("Nu" ) ̅/〖"Nu" 〗_"o" 和"f" ̅/"f" _"o" 呈現類似波浪號的變化趨勢，並在St = 0.67時分別達到最高值5.2及第三低值50.8，而TPF則取得最大值1.39。最後針對本研究之新型翼形擾流器提出("Nu" ) ̅/〖"Nu" 〗_"o" 及"f" ̅/"f" _"o" 對應Re、α、t/C、Pi/DH和St之經驗公式。

With the depletion of fossil fuels nowadays, it has become an urgent issue to improve the efficiency of serpentine heat exhangers. In general, the techniques for augmenting the thermal performance of serpentine channels can be classified into two categories: passive and active. For passive heat transfer enhancement, turbulators are one of the most effective ways to enhance the Nusselt number (("Nu" ) ̅/〖"Nu" 〗_"o" ). According to the blockage ratio (BR), there exist three types of turbulators: ribs (low BR), baffles (high BR), and in-betweens (moderate BR). Although many researchers have paid attantion to ribs or baffles, the studies of in-betweens are limited. In this study, novel wing-shaped turbulators (in-betweens) are proposed and their effects on the passive and active heat transfer enhancement in a two-pass square channel are experimentally investigated by Particle Image Velocimetry (PIV), Infrared Thermography (IRT), and pressure sensors.
For the passive heat transfer experiments, the inlet of the test section is maintained at a steady-state flow condition. The wing-shaped turbulators are mounted in-line on two sidewalls of the channel with the ratios of turbulator clearance and truncation gap to the channel hydraulic diameter of 45.5 mm respectively fixed at 0.25 and 0.06. Varied parameters include turbulator attack angle (α = 10°, 15°, 20°, and 30°), maximum thickness to chord line ratio (t/C = 0.08, 0.13, 0.16, 0.2, and 0.23), pitch ratio (Pi/DH = 0.6, 0.7, 0.8, 1, and ∞), and Reynolds number (Re = 5,000-20,000). It is newly observed that there are wing-induced Fujiwhara co-rotating vortices. This unique flow feature elevates ("Nu" ) ̅/〖"Nu" 〗_"o" up to 5.4 higher than the highest value of 4.4 reported previously for the louvered channel. In addition, the elimination of wake vortices by the present wing-shaped turbulators results in the 61.5% to 66.8% reduction of friction factor ratio ("f" ̅/"f" _"o" ) compared with the louvered channel. From the thermal-fluidic regression analysis, the dimensionless cross sectionally averaged streamwise vorticity magnitude and spanwise-averaged mean transverse velocity (((|"V" |/"U" _"b" ) ) ̅_"sp" ) have the high correlation coefficient (R ≥ 0.7) with the spanwise-averaged local Nusselt number ratio ((("Nu" ⁄〖"Nu" 〗_"o" ) ) ̅_"sp" ) in both the first and second pass. In terms of thermal performance factor (TPF), the wing design at α = 20°, t/C = 0.20, and Pi/DH = 0.7 is preferable to other values of α, t/C, and Pi/DH since it provides the TPF value of 1.68 higher than the previous reported value of 1.41 for 15 ≤ "f" ̅/"f" _"o" ≤ 80.
Based on the preferable turbulator parameters (α = 20°, t/C = 0.20, and Pi/DH = 0.7) aforementioned, the active heat transfer experiments are further conducted by switching the inlet condition to a pulsatile flow profile. The pulsating velocity is in a triangular-like waveform with Strouhal number (St) varied from 0 to 0.67. Relative to the steady-state case, the flow pulsation not only strengthens the Fujiwhara co-rotating vortices but also eliminates the corner vortex streams, leading to the ("Nu" ) ̅/〖"Nu" 〗_"o" enhancement by 13%. Moreover, the spanwise-averaged dimensionless periodically averaged spanwise velocity fluctuation replaces ((|"V" |/"U" _"b" ) ) ̅_"sp" as the second strong correlation flow parameter with (("Nu" ⁄〖"Nu" 〗_"o" ) ) ̅_"sp" . Focusing on the overall thermal performances, both the ("Nu" ) ̅/〖"Nu" 〗_"o" and "f" ̅/"f" _"o" reveal the tilde-like trends with St. The highest ("Nu" ) ̅/〖"Nu" 〗_"o" of 5.2 and third lowest "f" ̅/"f" _"o" of 50.8 occur at St = 0.67, resulting in the highest value of TPF up to 1.39. Finally, to facilitate the engineering applications of the present wing-shaped turbulators, empirical correlations of ("Nu" ) ̅/〖"Nu" 〗_"o" and "f" ̅/"f" _"o" versus Re, α, t/C, Pi/DH, and St are proposed.

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