利用多孔性介質增強振動渠道之混合對流熱傳效率

Abstract

本研究主要目的在於透過數值方法探討於頂部高溫面加裝一多孔性介質對於往復振動渠道在強制進口流場、渠道往復運動以及高溫浮力效應混合作用下，渠道內部流場和溫度場的變化並討論冷卻流體對高溫壁面的熱傳增益效果。 對於移動邊界問題部分採ALE（Arbitrary Lagrangian-Eulerian）的方法，用以描述流場和溫度場的變化。首先利用葛拉金有限元素法（Galerkin Finite Element Method）搭配ALE座標系統，計算穩態在有強制進口流影響的狀況下，加入多孔性介質對於系統內部流場及溫度場的影響；第二部份則為計算加以一固定振幅及頻率的擾動，觀察其穩定周期內的流場、溫度場變化；第三部分則利用上述的分析結果，計算管道內流體之熱傳係數，比較多孔性介質對高溫面熱傳效率、平均時間熱傳率的影響，並搭配實驗方法探討具有頂部加熱面的彎曲渠道於往復運動時混合性對流對壁面熱傳效率之影響情形，來輔助確認數值模擬結果，以增加對於高溫工作面熱傳分析研究的完整性。而實驗設備大致上可以分為三部份︰冷卻渠道、往復運動機構、溫度控制系統，工作流體為空氣。 由數值模擬往復振動渠道在頂部高溫面加裝多孔性介質後，可以改善頂部高溫壁面的熱傳效果。在本文中，多孔性介質在較小孔隙率時可獲得較佳的熱傳效率。多孔性介質結構中孔隙率太大時，反而會因為流場的二次迴盪使得熱傳效能降低。且得知熱傳效率會受到往復運動與流體變化的影響。所得最大熱傳增益約20%左右。以實際流場變化與數值模擬之流線圖互相驗證，兩者結果非常吻合。

The aim of this study is to investigate the variations of flow and thermal fields in the cooling channel within the reciprocating channel, and the heat transfer rate on the heated crown of the channel is discussed. A Galerkin finite element formulation with ALE method is adopted to investigate the variations of the flow and thermal fields induced by the reciprocating motions of the channel with porous media. The heat transfer rates on different models that base on working fluid is air and data runs are performed for Reynolds numbers, frequencies, amplitudes, temperature differences and channel with porous media relative angles from horizon will be shown and discussed. Heat transfer rates are affected by cooling fluid flowing and reciprocating motion mutually. The enhancement of heat transfer rate is definite and the most achievement is about 20% in this work. Comparisons of both experimental and numerical results have good agreements and the channel with porous media can help the enhancement of heat transfer rate more. The porous media under the larger porosity situation could obtain more heat dissipation than the traditional solid one. The porous media with a very small mean porosity could result in reduction thermal performance.