圓管奈米流體強制對流之熱傳增益與熵增分析

Investigation of forced convection heat transfer and entropy generation of nanofluid in a circular tube

陳宜鋒(Yi-Feng Chen)

指導教授 ： 洪祖全

國立臺北科技大學/機電學院/製造科技研究所/碩士(2013年)

摘要

本文首先使用計算流體力學方法模擬奈米流體層流及紊流圓管強制對流熱傳增益分析，接著使用熱力學第二定律進行熵增(entropy generation)分析。圓管邊界為固定熱通量(constant heat flux)或固定壁溫條件(constant wall temperature)，流動現象假設為單相流(single phase)。本文探討TiO2奈米流體於層流及紊流對流場中，藉由增加奈米顆粒濃度(volume fraction of nanoparticle)與Re數，分析熱對流係數(heat transfer coefficient)增益現象。結果得到，奈米流體之熱對流係數隨著奈米顆粒濃度與Re數增加而提高，且熱對流係數高於一般水溶液基礎流體(base fluid)。數值分析結果與實驗文獻相當一致，平均誤差在10 %以內。
第二部分則分析Al2O3奈米流體於管流的熵增現象，邊界條件亦使用固定熱通量或固定壁溫條件進行參數分析。造成熵增加有諸多原因，而層流管流熱流場中熵產生主要來自於有限溫差熱傳所引起的熱力學不可逆(Ns)T；但是當雷諾數逐漸提升時，紊流管流熱流場中熵產生則變成黏滯摩擦引起的熱力學不可逆(Ns)P所主導。此部分參數分析包含無因次溫度、無因次長度、Re數、Nu數及奈米顆粒濃度等。整體而言，有限溫差熱傳之熵增現象(Ns)T隨顆粒濃度以及Re數增加而減小；黏滯摩擦之熵增現象(Ns)P隨顆粒濃度以及Re數增加而提升。為了便於觀察熵增趨勢，最後則引入無因次Be數比較有限溫差熱傳(Ns)T或黏滯摩擦(Ns)P兩者的貢獻何項較為顯著。

關鍵字

奈米流體 ； 熵產生 ； CFD ； 層流 ； 紊流 ； 定熱通量 ； 定壁溫

並列摘要

In this study, we aimed at investigating nanofluid laminar and turbulent forced convection heat transfer in a circular tube using computational fluid dynamics, and analyzing entropy generation due to flow and heat transfer in nanofluids employing the second law of thermodynamics. In the first part, boundary condition in a circular tube was subjected to a constant wall heat flux or a constant wall temperature condition. And the flow was assumed to be single-phase. The TiO2 nanofluids in laminar and turbulent flow field were numerically studied. The results showed that forced convection heat transfer coefficient of nanofluids increased with nanoparticles concentration and Re number, and that the forced convection heat transfer coefficient of nanofluid is higher than that of base fluid. It was also found that the numerical results were in good agreement with the experimental data obtained from the literature. The average error was around 10 %.
In the second part, analyses of entropy generation of Al2O3 nanofluids flowing through a circular tube with constant wall temperature or constant heat flux were conducted. There are many factors causing entropy increase. At laminar flow entropy generated mainly from the finite temperature difference heat transfer caused by the irreversible thermodynamics (Ns)T; however, when the Reynolds number is gradually increased, at turbulent flow the dominant factor on entropy generation becomes viscous friction caused by irreversible thermodynamics (Ns)P. Parametric study includes the dimensionless temperature, dimensionless length, Re number, Nu number and nanoparticle concentration. It was seen that entropy generation due to the finite temperature difference heat transfer (Ns)T decreased with the increase of particle concentration and Re number; and that entropy generation caused by the viscous friction (Ns)P increased with particle concentration and Re number. Finally Bejan number was used to compare the contribution of finite temperature difference heat transfer (Ns)T and viscous friction (Ns)P.

並列關鍵字

Nanofluid ； Entropy generation ； Force convection ； Laminar flow ； Turbulent flow ； Constant heat flux ； Constant wall temperature

參考文獻

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