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研究生(外文):Kuo-Hsuan Weng
論文名稱(外文):Transient Leading to Periodic Fluid Flow and Heat Transfer in a Cavity Due to a Rotating Object
外文關鍵詞:sliding meshTaylor instabilitiesComputational Fluid Dynamics (CFD)Nusselt number
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在許多工程結構中常常可看見與容器有關的結構物,包括了混合腔體、電子產品冷卻、建築通風、太陽能集熱器及儲熱設備等等之設計,其內部複雜的流場及熱傳特性一直為工業界及學者所探討。本文利用計算流體力學軟體FLUENT針對密閉方形容器正中央放入不同形狀(包括圓形、正方形及正三角形)及不同尺寸的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。本文假設容器內流體的普朗特常數等於5,並假設流體流動屬於層流狀態,模擬過程中考慮兩種不同的邊界條件,一為等溫旋轉體,另一為絕熱旋轉體;在不同的邊界條件下,物體於時間t = 0時開始旋轉同時固定角速度,並忽略自然對流。根據模擬結果,本文對於旋轉物體所造成的流場與容器四個角落形成的渦流之間交互作用的情形以及對於不同形狀物體所形成流場與溫度場的相似性有詳細的探討。在兩種邊界條件中,本研究發現雷諾數愈低,正方形及正三角形旋轉體造成的旋轉體表面及容器壁之暫態平均紐賽數出現週期振盪的現象愈明顯,然而圓形案例中則未發現週期振盪的情形。在等溫旋轉體的所有案例中對暫態平均紐賽數取時間平均計算後得知,在高雷諾數的案例中,容器壁的熱傳性能與旋轉體的形狀無關,然而在較低雷諾數情況下,三角形和圓形旋轉體之性能分別為最好與最差,而在絕熱旋轉體的所有案例中也呈現出相同的現象。
In many engineering applications, there are a lot of equipments composed of the cavity, including mixing chamber, electronic cooling products, ventilation of buildings, collection of solar energy and heat storage system, etc. The complex flow field and heat transfer characteristics within the cavity are always attractive to the industry and researchers. In this study, the CFD software FLUENT is adopted to simulate the flow and temperature fields of various rotating objects (circle, square and equilateral triangle) with different sizes placed in the middle of a square cavity. The Prandtl number of the fluid within the cavity is set to be five and the flow is assumed to be laminar. Two kinds of thermal boundary conditions are considered in this study. One is the isothermal rotating object and the other is the adiabatic rotating object. The motionless object is set in rotation at time t=0 with a constant angular velocity. The effect of natural convection is neglected. According to the simulation results, the evolving flow field and the interaction of the rotating objects with the recirculating vortices at the four corners are elucidated. Moreover, similarity of the flow and thermal fields for various shapes is discussed. In the cases of both boundary conditions, the periodic oscillation of transient variations of the average Nusselt numbers on the surface of the rotating object and cavity walls becomes obvious as the Reynolds number decreases. However, periodic behavior does not appear in the cases of the circle object. For the cases of isothermal rotating objects, time-integrated average Nusselt number of the cavity is independent of shape of the object at higher Re. However, at lower Re, the triangle object clearly exhibits superior heat exchange capability followed by the square and circle objects. The phenomena found in the cases of isothermal objects also appear in the cases of adiabatic objects.
摘 要 i
誌 謝 v
目 錄 vi
表目錄 ix
圖目錄 x
第一章 緒論 1
1.1 前言 1
1.2 文獻回顧 3
1.3 研究目的 5
第二章 統御方程式 6
2.1 幾何外形 6
2.2 統御方程式 8
第三章 數值模式 12
3.1 數值方法 12
3.1.1 對流–擴散方程式的差分形式 12
3.1.2 壓力–速度耦合關係的處理 13
3.2滑動網格(Sliding Mesh) 16
3.2.1 滑動網格技術 16
3.3 收斂條件 16
第四章 等溫旋轉體數值模擬 17
4.1圓形 17
4.1.1 構型介紹 17
4.1.2 網格建立及邊界條件設定 18
4.1.3 格點獨立分析 19
4.1.4 結果與討論 22
4.2正方形 33
4.2.1 構型介紹 33
4.2.2 網格建立及邊界條件設定 34
4.2.3 格點獨立分析 36
4.2.4 結果與討論 39
4.3三角形 57
4.3.1 構型介紹 57
4.3.2 網格建立 58
4.3.3 格點獨立分析 60
4.3.4 結果與討論 63
第五章 絕熱旋轉體數值模擬 77
5.1 圓形 77
5.1.1 構型介紹 77
5.1.2 結果與討論 78
5.2 正方形 84
5.2.1 構型介紹 84
5.2.2 結果與討論 85
5.3 三角形 99
5.3.1 構型介紹 99
5.3.2 結果與討論 100
第六章 時間平均紐賽數之探討 111
6.1 等溫旋轉體數值模擬之探討 111
6.2 絕熱旋轉體數值模擬之探討 114
第七章 結果與討論 116
7.1 等溫旋轉體數值模擬之結論 116
7.2 絕熱旋轉體數值模擬之結論 117
參考文獻 118
符號彙整 121
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