往復運動薄塊對噴流中壁面熱傳影響之研究

Abstract

本研究利用數值方法分析噴流中往復運動薄塊對壁面附近速度與溫度場的影響；並以流場可視化技術與熱傳實驗，驗證數值計算的結果，且進一步探討往復運動薄塊對噴流中壁面熱傳的增益。在數值方面，採用Arbitrary Lagrangian-Eulerian (ALE)座標系統，配合葛拉金有限元素法，處理此種流體與固體介面間相互影響的問題。首先探討往復運動薄塊提昇噴流中壁面熱傳效率的機制，進而比較不同的往復運動速度、往復運動距離及往復運動薄塊與壁面間隙等重要參數，對提高壁面熱傳效率的影響。在實驗方面，以流場可視化技術觀測流場結構，同時量測壁面溫度，估算壁面紐塞數，與數值計算結果相互印證。 由數值與實驗結果得知，壁面上往復運動薄塊推擠前方流體而破壞其前方之邊界層。同時，牽引後方流體填補薄塊往復運動產生的空間，因而生成新的邊界層，並且引導流體流向高溫壁面。邊界層的生成與低溫流體流向高溫壁面，為提高熱傳效率的主要機制。此外，隨著往復運動速度加快，邊界層的破壞與生成較頻繁，使得熱傳量大幅提高。另一方面，由於往復運動與噴流結構相互影響，在獲得最大熱傳增益的前提下，存在一最佳往復運動距離。當絕熱之往復運動薄塊與高溫壁面接觸時，壁面散熱面積減小，然而前述提高熱傳的機制較明顯，因此相較於往復運動機構與高溫壁面分離時，仍有較高之熱傳增益。

The study investigates the heat transfer enhancement by a reciprocating block on the heated plate under a confined slot jet. The characteristics of flow and thermal fields are analyzed both numerically and experimentally. In the numerical analysis, a Galerkin finite element formulation with arbitrary Lagrangian-Eulerian method is adopted to solve the flow and thermal fields. The flow structure is observed by flow visualization technique, and the heat transfer rate is estimated by measuring the temperature of the heated plate. The numerical and experimental results show that the motion of the block destroys the boundary layer on the heated plate before the reciprocating block, and the boundary layer reforms behind the reciprocating block. The boundary layer reformation and the cold fluid, induced by the reciprocating block, flowing toward the heat plate are the major heat transfer enhancement mechanisms. Besides, when the block moves faster, the boundary layer reforms frequently. As a result, the heat transfer enhancement becomes huge. However, when the block moving distance approaches to the width of slot jet inlet, the reciprocating block obstructs the jet flowing to the heat plate. And, the heat transfer rate of the heat plate decrease.