Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/3285
標題: 黏度隨溫度變化對平板暫態強制對流熱傳之影響
作者: 郭俊聰
tsung, kuo chun
關鍵字: Viscosity independent temperature
黏度隨溫度變化
Transient forced convection
Flat plate
Arrehnius's law
暫態強制對流
平板
Arrehnius定律
出版社: 化學工程學系
摘要: The subject of this thesis is to establish a rigorous and precise method to solve the unsteady forced convection heat transfer with a viscosity dependent temperature. Based on Arrehnius's law, the variable viscosity is respectively proposed as (1) the viscosity is decreased with temperature (VDT) and (2) the viscosity is increased with temperature (VIT) due to a step changing uniform wall temperature (UWT) on a semi-infinite flat plate. In order to make a comparison, in a system of the constant viscosity, the simultaneous transient momentum and heat transfer over a flat plate with uniform heat flux (UHF) and case of UWT is also analyzed. At first, a thickness of developing thermal layer, which simultaneously cover the limiting cases of the transient heat conduction and a steady forced convection, is introduced by means of the scale analysis from the unsteady heat transfer equation to define the transformed variables such as a dimensionless time and coordinates. Finally, the method of line (MOL) with the central finite difference and Newton's iteration is applied to solve the non-similar transformed momentum and energy equations under cases of the variable and constant viscosity, respectively. As shown from the results, some notable conclusions are summarized as follows: (1)For a system of the variable viscosity, the time response, from transient molecule diffusion to a steady momentum transfer, is increased with values of Pr that base on a specific temperature. (2)In middle of the transient forced convection heat transfer, the slope of Nu/Re1/2 varying with time parameter t is gradually enlarged with value of Pr, and it is clearer for the case of VDT in contrast to VIT case. (3)The unsteady heat transfer will be early dominated by the mechanism of a steady forced convection when value of Pr is reduced in both cases of VDT and VIT. Additionally, for the application on practical engineering, the correlation of unsteady friction and convective heat transfer coefficient are respectively developed by the correlation of transient molecule diffusion and a steady momentum transfer and a exact solution of transient heat conduction as well as the correlation of a steady forced convection heat transfer in this content. Comparing with numerical solution, the deviations of the correlation are less than 4%.
本文主要建立一套暫態動量與能量傳遞藕合之非線性方程式解析方法,探討以Arrehnius定律表示黏度隨溫度變化效應,在等溫壁情況下,對於半無限長平板之暫態強制對流熱傳系統的流場,溫度場,及摩擦阻力與熱傳速率之影響。其中黏度隨溫度變化效應分別為(1)黏度隨溫度上升而遞減(VDT)及(2)黏度隨溫度上升而遞減(VIT)。另一方面,文中也解析常黏度於等溫壁及等熱通量壁情況下之暫態強制對流熱傳以用來和變黏度系統做比較。 首先依據系統為暫態分子擴散傳遞而後漸漸發展出穩態強制對流傳遞之特性,引進可同時包含初期與終極兩極端效應的熱邊界層成長厚度量階,作為系統之特性長度,以得到一組適當轉換變數。之後利用MOL方法配合Runge-Kutter-Gill積分法與牛頓法迭代程序,求解暫態動量與能量傳遞之無因次方程式,進而分別獲得在變黏度效應及常黏度系統下從暫態熱擴散至穩態強制對流之整個暫態熱傳程序的數值解。 對於等溫壁與等熱通量壁兩種瞬間加熱方式,平板的暫態強制對流熱傳隨著以參考溫度T¥為基準之普蘭特數與不同黏度變化情形,由解析之結果顯示: (1)Pr值較小的流體,動量傳遞由暫態分子擴散機構至穩態強制對流機 構所需要的時間較長,而反之則相反。 (2)對於變黏度流體系統,在暫態傳遞程序的中間階段其Nu/Re1/2值隨 時間t的下降幅度,隨著Pr值變大而擴大,此一現象以VDT情況較VIT 情況來得明顯。 (3)當Pr值變小,則系統愈早脫離暫態熱擴散區域而轉入穩態對流熱傳 區域,此現象在變黏度與常黏度系統皆可看到。 另外為方便實際工程上之應用,本文亦利用動量之暫態分子擴散與穩態流力及能量之暫態熱傳導與穩態熱對流之相關式,提出相當準確之暫態摩擦阻力及暫態強制對流熱傳相關式,其與數值解最大偏差不會超過4﹪。
URI: http://hdl.handle.net/11455/3285
Appears in Collections:化學工程學系所

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