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

Transient Leading to Periodic Fluid Flow and Heat Transfer in a Cavity Due to a Rotating Object

翁國軒(Kuo-Hsuan Weng)

指導教授 ： 施陽正

國立臺北科技大學/機電學院/冷凍空調工程系所/碩士(2006年)

https://doi.org/10.6841/NTUT.2006.00396

摘要

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。

關鍵字

滑動網格 ； 計算流體力學 ； 泰勒不穩定度 ； 紐賽數

並列摘要

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.

並列關鍵字

sliding mesh ； Taylor instabilities ； Computational Fluid Dynamics (CFD) ； Nusselt number

參考文獻

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