擺動矩形體在流動場中之流場與熱傳特性研究

Abstract

本研究以葛拉金有限元素法，採用Arbitrary Lagrangian-Eulerian (ALE)座標系統，首先探討流場中與流體逆向或同向運動的物體所誘導流場和溫度場之變化，以瞭解這類問題的運動機制，並比較不同的物體運動速度之影響；進而研究矩形柱體以垂直於流體流動方向振動所產生的漩渦剝離機制和熱傳的增益，詳細探討漩渦剝離的形成和發展過程，並針對不同的矩形柱振動速度、振幅、障礙比和展弦比，比較其熱傳增益。再利用因物體在流場中振動會提升熱傳增益，而散熱鰭片可以增加散熱面積之概念，設計一種可應用於電子元件散熱的擺動式鰭片散熱裝置。 綜合所獲得的研究結果，可以很清楚的觀察到當物體在流動的流體中運動時，物體推擠流體和流體填補因物體移動所產生空洞的現象。而矩形柱在流場中振動時，流場和溫度場的變化受矩形柱振動與剝離漩渦交互作用的影響，漩渦剝離之頻率會逐漸改變成與矩形柱振動的頻率相同，使流場和溫度場隨時間成週期性變化，且當矩形柱之振動速率與振幅提高時，流場變化較劇烈，熱傳量會提升。對於鰭片在流場中擺動時，當鰭片擺動速率較大時，會在鰭片上表面與下表面交錯形成回流區和再附著區，並逐漸往下游遷移，使得原本附著在鰭片上之速度和溫度邊界層受到擾動而被破壞和縮小，熱傳效果會有效提升。

In this study, a Galerkin finite element formulation with Arbitrary Lagrangian-Eulerian (ALE) kinematic description method is adopted to investigate the variations of the flow and thermal fields induced by a body moving or oscillating in a flow. At first, a body moving in opposition to or in the same direction as a flowing fluid in a channel is studied. Then a heated transversely oscillating rectangular cylinder in a cross flow is investigated. The effects of Reynolds number, oscillating amplitude, oscillating speed, blockage ratio, and aspect ratio on the flow structures and heat transfer characteristics are examined. Finally, a new model for enhancing the heat transfer of a finned heat sink is proposed. In this method, extremely thin fins are installed in the finned heat sink, and these fins may swing back and forth in a flow as the fluid flows through them. Based upon the above procedures, the results show that the body may press the fluid near the body, and the fluid will simultaneously replenish the vacant space induced by the movement of the body. These phenomena are apparently different from those of regarding the moving body as stationary one in the flowing fluid. For the rectangular cylinder oscillating in a cross flow, the interaction between the oscillating rectangular cylinder and vortex shedding from the rectangular cylinder dominates the state of the wake. The vortex shedding frequency is changed to match with the rectangular cylinder oscillating frequency gradually, and the flow and thermal fields may approach a periodic state with time. The heat transfer rates are enhanced remarkably as the oscillating amplitude and speed of the rectangular cylinder are increased. For fins swinging in a flow, the velocity and thermal boundary layers may be contracted and disturbed as the fins swing with a large speed, and the heat transfer rates are remarkably affected by the swinging speed and amplitude of the fins.