A Study of Flow Structures and Heat Transfer Characteristics from an Oscillating Rectangular Body in a flow
|Keywords:||有限元素法;移動網格;漩渦剝離;回流區;振動矩形體;finite element method;moving mesh;vortex shedding;recirculation zone;oscillating rectangular body|
|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。
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.
|Appears in Collections:||Thesis|