機翼形擾流器於正方形雙通道紊流流場與熱傳增益相關性之實驗研究

Experimental Study of Airfoil-Shaped Turbulators on Turbulent Thermal Fluid Relationships in a Twin-Pass Square Channel

劉明翰(Min-Han Liu)

指導教授 ： 劉通敏 ； 王春生

國立清華大學/工學院/動力機械工程學系/碩士(2021年)

摘要

近年來隨著自然資源的日漸枯竭，能源議題變得格外重要，而提升熱交換器的效率能夠減少能源的損耗，其中又以安裝擾流器以提升熱交換器的熱性能表現(Thermal Performance Factor,簡稱TPF)為最有效率方法之一。本研究提出新型的機翼形擾流器 (Airfoil-Shaped turbulator) 來提升正方形雙通道的TPF，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)和壓力傳感器來量測其在通道內的流場結構，局部溫度分佈和壓力損失的影響。
　　機翼形擾流器採用3D列印的技術進行製造，研究的參數包括外形(A、B)、攻角 (α = 15˚、20˚、25˚)、相對厚度 (t/C = 0.15、0.20、0.25、0.30)與離壁間距 (g/t = 0.18、0.27、0.36、0.54)。本研究於雷諾數範圍(Re = 10000)進行流場實驗，5000 ≤ Re ≤ 20000進行熱傳實驗。從PIV實驗中可以看出，機翼形擾流器在近壁面流場形成側向速度，導引流體至側壁，進一步在二次流流場產生一對渦漩結構，把核心流體帶往上下加熱壁面，再經過多個擾流器後，提升渦漩的強度與擴大渦漩的範圍，形成熱傳增益。透過IRT實驗可以看出，在外型A、t/C=0.20、g/t=0.36條件下，紐賽數比和摩擦係數比隨著α提升而提升。在外型A、α = 20˚、g/t=0.36條件下，紐賽數比隨著t/C的提升而有明顯的上升或下降。在外型A、α = 20˚、t/C=0.20條件下，紐賽數比隨著g/t先上升後下降。透過觀察及變化趨勢，外型B、α = 20˚、t/C = 0.20、g/t = 0.27為本研究的最優化設計，於5000 ≤ Re ≤ 20000分別為5.25-3.87與28.2-26.9。與前人最佳擾流器相比，在彎道區域內紐賽數比平均提升5.5%，摩擦係數比降低13.5-10.2%，因此在7 ≤ 摩擦係數比 ≤ 80，TPF優於先前其他文獻，即TPF = 1.72。進一步探討流力因子與側向平均紐賽數比的分析後，發現側向速度，縱向速度，渦度與側向平均紐賽數比具有中高度相關，相關係數分別為R = 0.63、0.85與0.77。最後，本研究彙整前人文獻，包括平滑管道、百葉窗形擾流器、機翼形擾流器，與本研究數據，提出熱傳經驗公式能夠廣泛應用於雙通道內。

關鍵字

機翼形擾流器 ； 正方形雙通道 ； 熱流相關性 ； 質點影像測速儀 ； 紅外線熱像儀

並列摘要

In recent years, with the depletion of natural resources, energy issues have become extremely important. Improving the efficiency of heat exchangers can reduce energy loss. Specifically, installing turbulators in heat exchangers is one of the most efficient methods to improve their Thermal Performance Factor (TPF). This research proposes a new type of airfoil-shaped turbulators to enhance the TPF, and uses Particle Image Velocimetry (PIV), Infrared Thermography (IRT), and pressure sensors to measure the flow field structures, local temperature distributions, and pressure losses in the two-pass square channel mounted with these new turbulators.
　　The airfoil-shaped turbulators are fabricated by 3D printing technology. Varied parameters include type (A, B), attack angle (α = 15˚, 20˚, 25˚), relative thickness (t/C = 0.15, 0.20, 0.25, 0.30), and clearance ratio (g/t = 0.18, 0.27, 0.36, 0.54). In this study, the PIV experiments are conducted at Reynolds number (Re) = 10000, and the IRT experiments are conducted at 5000 ≤ Re ≤ 20000. From the PIV experiment, it is observed that the airfoil-shaped turbulators induce a spanwise velocity near the top and bottom wall, which leads fluids from the innerwall to outerwall, and further generates a pair of vortex structures in the secondary plane, thus bringing the core cold fluids towards the top and bottom heating walls. With the fluid flow passing through multiple consecutive turbulators, the strength and size of the vortex pair are enhanced, resulting in augmented heat transfer performance. Through the IRT experiment, for type A at t/C = 0.20 and g/t = 0.27, both Nusselt number and friction factor increase with increasing α . For fixed type A at α=20˚, and g/t = 0.27,Nusselt number is insensitive to t/C. For type B at t/C = 0.20 and g/t = 0.27, Nusselt number first increases and then decreases with increasing g/t. Under constant pumping power, type B with α = 20˚, t/C = 0.20, and g/t = 0.27 is the optimal design among all examined cases. The corresponding Nusselt number and friction factor are 5.25-3.87 and 28.2-26.9 at 5000 ≤ Re ≤ 20000, respectively. Compared with the previous best design for 7 ≤ friction factor ≤ 80, the turn averaged Nusselt number increases by 5.5% whereas the overall friction factor values decrease by 13.5-10.2%, leading to the best TPF = 1.72. Further correlation analyses between the mean flow factor and the spanwise-averaged local Nusselt number reveal that the spanwise velocity, transverse velocity, vorticity have the medium and high correlation with the spanwise-averaged local Nusselt number, with the correlation coefficients R = 0.63, 0.85, and 0.77, respectively. Finally, by combining the thermal-fluidic data of previous designs, including smooth channels, louverd-shaped turbulators, wing-shaped turbulators, and present new turbulators, general empirical heat transfer formula for the two-pass square channel are proposed.

並列關鍵字

Airfoil-shaped Turbulators ； Two-Pass Square Channel ； Thermal Fluid Relationships ； PIV ； IRT

參考文獻

[1] Dewan A, Mahanta P, Raju KS, Kumar PS. Review of passive heat transfer augmentation techniques. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy. 2004,218(7):509–527.

[2] Chen CC. Experimental study of passive and active heat transfer enhancement in a two-pass square channel with novel-shaped turbulator. 2020, National Tsing-Hua University.

[3] Han JC, Dutta S, Ekkad S. Gas turbine heat transfer and cooling technology, second edition. CRC Press; 2012.

[4] Liou TM, Chang Y, Hwang DW. Experimental and computational study of turbulent flows in a channel with two pairs of turbulence promoters in tandem. Journal of Fluids Engineering. 1990;112(3):302–310.

[5] Liou TM, Chen SH. Turbulent heat and fluid flow in a passage disturbed by detached perforated ribs of different heights. Journal of Fluids Engineering. 1998;41(12):1795–1806.