圓形旋轉物體在不同形狀的密閉容器內的熱傳與熵增現象之研究

在許多工程結構中常常可看見許多與容器有關的結構物，其應用實例如下：馬達、風機與大型通風系統的軸承、核能發電廠反應爐的隔熱、熱交換器、與攪拌器等，如何增加熱傳能力與降低能源耗損已成為近年來熱傳領域的研究主題之一。本文利用計算流體力學軟體FLUENT針對不同密閉容器(包含圓形、正方形、三角形)正中央放入圓形旋轉物體，進行模擬來觀察物體旋轉對容器內流場及溫度場的影響並分析熵的產生量。本文假設容器內流體的普朗特數等於2×104，考慮流體布林克曼數所影響之黏滯摩擦熱，並假設流體流動屬於層流狀態。模擬過程的邊界條件，為等溫容器與等溫旋轉體，旋轉體表面為高溫、容器壁面為低溫，旋轉體旋轉時固定角速度，並忽略自然對流。根據模擬結果，本文對於不同容器與圓形旋轉物體所造成的流場與容器角落形成的渦流之間交互作用的情形以及溫度場的相似性有詳細的探討，並分析其紐賽數與熵增值造成之影響。本研究發現雷諾數愈低，正方形及三角形的旋轉體表面與壁面之紐賽數變動的現象越大，圓形案例中則未發現此情形。在高雷諾數的案例中，容器壁面的熱傳性能與容器的形狀無關，然而在較低雷諾數情況下，三角形與圓形容器熱傳性能分別為最好與最差，而系統熵增中三角形容器不可逆性是三種容器中最高的，其所耗損的功是最高的，因此三角形與圓形的系統熵增分別是最高與最低。

關鍵字

熵增；計算流體力學；泰勒不穩定度；紐賽數；黏滯耗散效應

並列摘要

In many engineering applications, there are a lot of equipments composed of the cavity, including motor, fan bearings with large ventilation systems, insulator in nuclear reactor thermal power plants, heat exchangers and mixers, etc. In recent years, the improvement in heat transfer capability and the reduction in energy consumption have become the research focus in the field of heat transfer. In this study, the CFD software-FLUENT was adopted to simulate the flow and temperature fields, and the entropy generation of rotating circular object within the cavity with various shapes (including round, square and triangle). Several assumptions were made in the simulation model such as the Prandtl number of the fluid within the cavity was set to be 2×104, the Brinkman number of fluid affected by viscous friction heat was considered and the flow was assumed to be laminar. The boundary conditions for isothermal cavity and isothermal rotating object were set where the surface temperature of rotating object was determined at higher temperature and the contacting surface of cavity was set at lower temperature. The motionless object was set in rotation with a constant angular velocity. The effect of natural convection was neglected. According to the simulation results, the evolving flow field and the interaction of the rotating objects with the recirculating vortices at the corners were elucidated. Moreover, similarity of the flow and thermal fields for various rotating objects was discussed and the entropy generation was analyzed. The study found that the variations of average Nusselt numbers on the surface of rotating object and both square and triangle cavity walls become more obvious as the Reynolds number decreases. However, such behavior does not appear in the cases of the round shape cavity. For the cases of higher Reynolds number, the time-integrated average Nusselt number of the isothermal cavity and rotating objects are independent of shape of the object. However, at lower Reynolds number, the triangle object clearly exhibits superior heat exchange capability followed by the square and circle objects. As for system entropy generation, the triangle-shape-system causes the highest value of irreversibility and it consumes the most useful energy. Therefore, the triangle-shape-system and round-shape-system exhibit the best and worst performance of heat transfer, respectively.

並列關鍵字

Entropy generation ； Taylor instabilities ； Computational Fluid Dynamics (CFD) ； Nusselt number ； Viscous dissipation

參考文獻

[22] 翁國軒，在密閉容器中旋轉物體所造成流場與熱傳振盪暫態現象之研究，碩士論文，國立台北科技大學能源與冷凍空調工程系，台北，2006。

[1] S. M. Saeidi and J. M. Khodadadi, "Forced Convection in a Square Cavity with Inlet and Outlet Ports," Int. J. Heat Mass Transfer, 49, 2006, pp. 1869-1906.

[2] X. Shi and J. M. Khodadadi, "Laminar Fluid Flow and Heat Transfer in a Lid-Driven Cavity Due to a Thin Fin," J. Heat Transfer, 124, 2002, pp. 1056-1063.

[3] X. Shi and J. M. Khodadadi, "Laminar Natural Convection Heat Transfer in a DiffeRentially Heated SquaRe Cavity Due to a Thin Fin on the Hot Wall," J. Heat Transfer, 125, 2003, pp. 612-623.

[4] S. Ostrach, "Natural Convection in Enclosures," Advances in Heat Transfer, Hartnett, J.P., and Irvine (eds.), 8, Academic, New York, 1972.

國際替代計量

圓形旋轉物體在不同形狀的密閉容器內的熱傳與熵增現象之研究

全文下載

主題瀏覽