水-氧化鋁奈米流體在完全發展層流區域中流動和熱對流特性之數值研究

Numerical Study On Flow And Convective Heat Transfer Characteristics of Al2O3-Water Nanofluids In Fully Developed Laminar Regime

丁獻宏(Hsien-Hung Ting)

指導教授 ： 侯順雄

崑山科技大學/工程學院/機械工程研究所/碩士(2012年)

摘要

本研究係以數值模擬，測量水-氧化鋁奈米單相流體，流經一均勻加熱圓管，且已為充分發展的層流區域中，其壓降和熱對流係數等進行探討，並從古典及現存研究文獻之各項理論，去計算模擬值，進而與參考文獻值比較其間差異性。模擬結果顯示，採用Darcy方程式，對奈米單相流體摩擦係數的分析預測，與參考文獻之實驗數據，已獲良好的印證。然而，在0.3％的體積濃度的奈米流體，相較於純淨水，其熱對流係數提高了8％，其熱對流係數增強結果不能由Shah方程式預測而得。此外，模擬結果亦表明，熱對流係數更是較Shah方程式高出許多。因此，我們也討論了在靜態和動態條件下，對熱對流係數的各種影響，如奈米粒子分散的能量轉移，奈米粒子的遷移歸因於其黏度梯度，非均勻的剪切速率，布朗擴散等有助顯著提高奈米流體的熱對流係數。在尺寸分析和數值解的基礎上，我們可以瞭解到，在第一時間，流速剖面的扁平化，從整體性能，如奈米粒子濃度，因其黏度梯度大，可產生較大的導熱係數和黏度。在這種速度剖面的扁平化情況下，可視為熱對流係數遠超過流體本身的熱導係數的可能機制。

關鍵字

單相流體 ； 完全發展層流 ； 熱傳導係數 ； 熱對流係數 ； 數值模擬 ； 壓降

並列摘要

This numerical simulation study investigated the pressure drop and convective heat transfer coefficient of water–based Al2O3 nanofluids flowing through a uniformly heated circular tube in the fully developed laminar flow regime. From the classical and the existing research literature of the theory to calculate the value of simulation, and then compare the differences of reference values. The numerical simulation results indicate the single-phase flowing nanofluid friction factor and with reference to research literature data from the Dracy’s equation. It shows a good agreement. However, a concentration of 0.3vol% compared with pure water, the convective heat transfer coefficient is up to 8%. That the enhancement cannot be predicted by the Shah equation. Furthermore, the numerical simulation results show that the convective heat transfer coefficient is much higher than the Shah equation. Therefore, We have also discussed both static and dynamic conditions for the different effects of the convective heat transfer coefficient. It helps significantly improve the coefficient of thermal convection of nanofluids such as energy transfer by nanoparticles dispersed, nanoparticle migration caused by the viscosity gradient, non-uniform shear rate, Brownian diffusion. On the basis of dimensional analysis and numerical solution, we can understand the flattening of the velocity profile at the first monent. It was occured by higer nanofluid concentration, higer thermal conductivity and viscosity. Under the flattening velocity profile case, the convective heat transfer coefficient increment is far more than the possible mechanisms of the fluid thermal conductivity coefficient of the increment.

並列關鍵字

Single-phase fluid ； Fully developed laminar flow ； thermal conductivity coefficient ； the convective heat transfer coefficient ； numerical simulation ； pressure drop

參考文獻

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[5] S.E.B Maiga et al., Heat transfer behaviours of nanofluids in a uniformly heated tube, Superlattices and Microstructures 35 (2004) 543-557.

[6] Eastman, J. A., Choi, S. U. S., Li, S., and Thompson, L. J., Enhanced Thermal Conductivity through the Development of Nanofluids, Proc. Symp. on Nanophase and Nanocomposite Materials II, Materials Research Society, Boston, vol. 457, pp. 3–11, 1997.