複動式與獨立式合成噴流場之數值模擬分析

Abstract

液晶顯示器產業中扮演著關鍵角色的背光模組產業一直是開發創新的焦點，而由於發光二極體具備著低驅動電壓、不含汞、高色再現性等特性，將來取代冷陰極燈管已是必然的趨勢了。但為了要能達到高照度的需求，二極體即便產生高熱密度的問題，在空間有限及低噪音的前提下，具備構造簡單、控制容易等特點的合成噴流便為極具發展潛力的主動式散熱元件。本文以其操作頻率與振幅定義出Re、St 及S三個主要無因次參數，經數值模擬結果的分析後獲悉 主要會影響噴流的渦漩結構及其推進效率，而 主要是影響噴流速度與渦漩強度。最佳化的設計方向為大振幅且高頻率，在此前提下複動式設計對於獨立式合成噴流下游穩定噴流區的改善有限，但兩者在噴口附近的流場則差異甚鉅，複動式合成噴流速度較為集中亦較快且同時具有較長的高速區，因此倘若使用於衝擊冷卻應用之上複動式合成噴流的散熱效率應可較獨立式合成噴流獲得有效地提升。此外 還是影響噴流能否成形的關鍵參數，其倒數值必須大於某常數K才足以產生合成噴流，經本文分析複動式與與獨立式合成噴流的K值約為0.25及0.5。

In the backlight module industry, light emitted diode has already been an inevitable trend to replace the cold cathode light, because of characteristics such as the low voltage of urging, no mercury, and high color reproduction. In order to reach the demand for the high intensity of illumination, we should resolve the problem of density of high fever emerging in the diode which will influence the performance of LCD system. Because LCD has limited space and needs low noise, “synthetic jet” is an extremely potential active heat dissipation component possessing the characteristics such as simple structure and easy control. This article uses operating frequency and amplitude of moving boundary to define three main dimensionless parameters—Re,St,and S. After analyzing simulation results, we get two conclusions. On the one hand, increasing St will strengthen the structure of vortex ring and enhance thrusting efficiency. On the other hand, increasing S will enhance the intensity of vortex and increase the jet speed. Under the optimum premise, large amplitude and high frequency, the design of double acting for improving the steady jet region of single synthetic jet is limited. However, the near field of double acting synthetic jet has more concentrating velocity, higher speed, and longer high speed region than single synthetic jet. Therefore, it will have better performance on the application of impinging cooling. In addition, the reciprocal of St must be greater than a constant K to ensure jet formation. According to the results of this article, the values of K of double acting and single synthetic jet are 0.25 and 0.5 eventually.