螺旋紐帶藉由V-cut提升熱流特性與改善結垢之研究

Investigation of Thermofluids and Fouling Performance of Twisted Tape Subject to V-cut Configuration

蔡景安(Ching-An Tsai)

指導教授 ： 王啟川

國立交通大學/工學院/機械工程系所/碩士(2020年)

摘要

本研究通過計算流體力學(CFD)詳細研究了不同紐帶之增強機制之比較，並進一步檢查紐帶與管之間的間隙影響、V型切割之數量與分佈之影響，並透過實驗進行趨勢驗證，並以綜合熱性能(Comprehensive thermal performance, CTP)進行比較。最終透過實驗比較自旋紐帶與固定紐帶之除垢效果。由模擬結果表明，當間隙從1.0mm減小到零時，紐森數和摩擦係數將分別增加12.6%和18.1%，並且CTP從1.05-1.07改善至1.12-1.15。同時研究不同角度之V型切割，由結果發現，與傳統等腰V型切割紐帶相比，迎風V型切割紐帶的CTP可以從1.15-1.18增加至1.16-1.21。但背風V型切割會因流經切口區域之紊流方向與主流方向不一致而損害CTP。另一方面，發現每個螺距上切割三個迎風V型切口在CTP上顯示出最高的改善。若再增加，會因阻力的快速增加反而降低CTP。也發現單邊切割V型切口改善效果有限。由熱傳實驗結果表明，在扭曲率為4之螺旋紐帶其CTP可以達到1.08-1.1。但當間隙比從0.043提高至0.135時，Nu和f將同時下降6.7%和11.6%。也發現鈍角V型切口在Nu和f上顯示出更明顯的改善，其CTP為1.18-1.23。另外本實驗初步研究旋轉紐帶除垢之能力，由結垢實驗結果表明，旋轉紐帶能大幅改善污垢之沉積，但在低流量時，推動紐帶旋轉之力減弱，且壁面結垢影響紐帶與管壁之間隙，導致轉速嚴重下降，因此在低流量改善效果並無太顯著。
關鍵字：螺旋紐帶、間隙效應、迎風V-cut、V-cut切割分佈、自旋紐帶、微粒結垢

關鍵字

螺旋紐帶 ； 間隙效應 ； 迎風V-cut ； V-cut切割分佈 ； 自旋紐帶 ； 微粒結垢

並列摘要

In this study, the effects of various twisted tapes (TTs), the gaps between TT and the tube, and V-cut distribution on thermal performance are numerically investigated in detail by using the Fluent software package. Some experiments are also carried out to verify the simulation results. Besides, the descaling ability of self-rotating TTs and a fixed TT is experimentally examined. Furthermore, the influence of different V-cut angles (30°,68° and 120°)on fluid flow and heat transfer characteristics of the tube is also numerically investigated in detail. For all cases in this study, it is found that with the decrease of the gaps from 1 mm to zero, the Nusselt number (Nu), and friction factor (f) are increased by 12.6% and 18.1%, respectively. while the comprehensive thermal performance (CTP) is improved from 1.07 to 1.15. The CTP of windward V-cut TT can be augmented from 1.18 to 1.21 compared to the isosceles V-cut TT. However, the leeward V-cut TT impairs the CTP of the tube due to its inconsistent turbulence direction along with the main helical flow. On the other hand, the results also show that the CTP of cases with three windward V-cuts at each helical pitch yield the most improvement. With the increment of V-cut numbers, the CTP may reversely reduce because of the rapid increase of flow resistance. The CTP of the cases with windward V-cuts on the bilateral edges of the TT is insignificantly improved. The experimental results show that the TT with the twisted ratio of 4 can achieve the highest comprehensive thermal performance (CTP) by 1.10 compared to others. Yet the Nu, and friction factor are simultaneously dropped by 6.7% and 11.6% when raising the gap ratio from 0.043 to 0.135, respectively. It also found that the obtuse V-cut configuration shows more pronounced improvement on Nu and f, and yields the highest CTP of 1.23 within the tested range. The self-rotating twisted tape can significantly improve the deposition of fouling. However, the force that drives the rotation of TT is weakened at a low flow rate. Additionally, fouling on the wall of the tube decreases seriously the speed between the TT and the tube; therefore, the descaling ability of the self-rotational TT is insignificantly improved at low flow rates compared to the fixed TT.
Keywords: Twisted tape; gap ratio; windward V-cut; V-cut distribution; Self-rotating twisted tape; Particulate fouling.

並列關鍵字

Twisted tape ； gap ratio ； windward V-cut ； V-cut distribution ； Self-rotating twisted tape ； Particulate fouling

參考文獻

[1] Wang, Q. W., Zeng, M., Ma, T., Du, X. P., and Yang, J. F., 2014, "Recent development and application of several high-efficiency surface heat exchangers for energy conversion and utilization," Applied Energy, 135, pp. 748-777.

[2] Varun, Garg, M. O., Nautiyal, H., Khurana, S., and Shukla, M. K., 2016, "Heat transfer augmentation using twisted tape inserts: A review," Renew. Sust. Energ. Rev., 63, pp. 193-225.

[3] Smithberg, E., and Landis, F., 1964, "Friction and forced convection heat-transfer characteristics in tubes with twisted tape swirl generators."

[4] Manglik, R. M., and Bergles, A. E., 1993, "Heat transfer and pressure drop correlations for twisted tape inserts in isothermal tubes part 1 laminar flows," Journal of Heat Transfer-Transactions of the Asme, 115(4), pp. 881-889.

[5] Manglik, R. M., and Bergles, A. E., 1993, "Heat transfer and pressure drop correlations for twisted tape inserts in isothermal tubes part 2 transition and turbulent flows," Journal of Heat Transfer-Transactions of the Asme, 115(4), pp. 890-896.