垂直通道內振動對高溫面熱傳增益之研究

Abstract

本研究利用數值方法分析垂直管道內高溫面加入水平振動的因子後，對高溫面散熱效率的影響。又採用的數值模擬為Arbitrary Lagrangian-Eulerian（ALE）座標描述系統，配合葛拉金有限元素法（FEM），處理此種流體與固體介面間相互影響的問題。首先探討在垂直管道內振動高溫面於自然對流領域內提升高溫面熱傳效率的機制，並且針對不同的振幅與雷利數求得加入振動條件後，高溫面散熱效果可以優於穩態非振動高溫面散熱效果的臨界振動頻率；進而探討當垂直管道流進入混合對流與強制對流範圍時，在不同的振幅與葛瑞秀夫數下，亦可發現加入振動條件後，振動高溫面之散熱效果可以優於穩態非振動高溫面散熱效果的臨界振動頻率。 雖然一般的觀念大都認為存在振動的熱表面其熱傳的效果會比沒有振動的情形下要好，但是由研究結果得知，在不同的振幅與雷利數或葛瑞秀夫數搭配時，加入振動後的熱傳現象仍可能比沒有振動時的熱傳效率差，也因此可以定義且找出臨界振動頻率以提供研究振動熱傳者之參考。最後，有關臨界振動頻率的經驗公式在強制對流、混合對流以至於自然對流的形式皆不相同，本文皆已分別推導出來，且在三種的對流形式中都有相當平滑的曲線可以獲得。

The study investigates the heat transfer enhancement by a oscillation motion of heat surface in a vertical channel flow. In the numerical simulation analysis, a Galerkin finite element method (FEM) with arbitrary Langrangian-Eulerian coordinate (ALE) is adopted to solve the flow and thermal fields for moving boundary problem. Firstly, the mechanism of heat transfer enhancement by oscillation heat surface is discussed in natural convection scene. For different oscillation amplitude and Rayleigh number, the frequency that the heat transfer rate of oscillation heat surface is higher than non-oscillation condition is found and called critical oscillation frequency. The limitation enhancement of similar physical model is still researched by this study in mixed and forced convection scenes for different amplitude and Grashof number. Generally, that a heat surface subject to a oscillation motion is advantageous to convective heat transfer is a well-known tuition. However, based upon the results indicated above the heat transfer rates with certain combinations of the oscillation amplitude and Rayleigh number or Grashof number are possibly smaller than that of the stationary condition. Then the critical oscillation frequency could be regarded as a criterion of whether the heat transfer rate of the heat surface subject to a oscillation motion being larger than that of the stationary state or not . Finally, the so called critical oscillation frequency can be expressed as a function of oscillation amplitude and Grashof number in all the natural, mixed, and forced convection scenes.