本文主要是針對軸對稱、侷限、不可壓縮微噴流衝擊至等熱通量加熱平面的流場與熱傳特性進行數值研究。文中採用穩態、二維圓柱座標，藉由紊流理論的數值模擬探討微噴流衝擊流場及溫度場之特性以驗證物理模式之可行性。本文中紊流統御方程式是以控制體積法為基礎，配合有限差分法及冪次法則來離散成差分方程式。對於紊流的結構則是以雙方程紊流模式來描述，並採用SIMPLE運算法則求解壓力－速度結合的問題。

本文研究的參數包含衝擊高度/噴口直徑之比值(H/D = 2、4)、雷諾數(Re = 210、419、524)、熱通量(q" =50 、100 、150 )，工作介質為空氣。數值預測結果分別以加熱區表面的紐塞數、無因次溫度、流場速度向量及熱傳係數分佈圖來表示。根據文獻中沈(2003)、顏(2004)所進行之微噴流衝擊實驗結果顯示，在此微尺度下，雖然雷諾數不大，但由於其所對應的速度已相當快，流場已呈紊流狀態；同時為了驗證文獻中Pence et al.(2003)所提到在此微尺度下，運用層流與紊流理論來進行數值模擬，其表面溫度的差異不大；故本文採用紊流理論來探討此流場。由流場速度向量圖可以發現，在衝擊高度低(H/D = 2)和雷諾數小(Re = 210)的流場，當微噴流之層流區衝擊到加熱表面時，其衝擊後之壁面流會在表面形成一個穩定與結構性之大尺度渦流。然而，當衝擊高度及雷諾數增加時，由於微噴流在剪流層的不穩定性擾動無足夠的能量生成大尺度渦流，故此現象就會消失；這與文獻中沈(2003)、顏(2004)提到之微噴流衝擊流場結構的特性吻合。雷諾數對於流場特性及熱傳效應也有相當顯著的影響；紐塞數在加熱區表面的值會隨著雷諾數的增加而增加，且在大約r/R ＝1.2的地方都會產生偏離中心線的極大值，這也與Pence et al.(2003)在文獻中所提到的一致，證明本數值研究之正確性。本文數值預測可以提供予微機電系統及CPU之散熱設計作參考。

The investigation of the flow field and heat transfer characteristic of an axisymmetric, confined, incompressible microjet impinging on a flat surface with an uniform heat flux has been carried out numerically in this study. The numerical simulation of a steady, 2D cylindrical coordinate, turbulent flow heat transfer is adopted to test the accuracy of the physical model. The turbulent governing equations are solved by using the Control-Volume based finite-difference method with the power-law scheme, and the well-known turbulence model to describe the turbulent structure. The SIMPLE algorithm is used to solve the pressure-velocity coupling.

The parameters studied include nozzle to impinging surface spacing (H/D=2、4)、Reynolds number (Re=210、419、524), and also heat flux (q" =50 、100 、150 ). The working medium is air. The local Nusselt number distributions along with the nondimensional temperature, heat transfer coefficient distributions and the velocity vector plots near the impingement surface are predicted and analyzed. According to the experimental studies of microjet impingement by Shen (2003) and Yen (2004), even the Reynolds numbers are low, the corresponding velocities are very fast. And also, in order to verify the results of Pence et al. (2003) that there was no obvious change in the surface temperature along the impingement surface using a turbulence model, therefore turbulent flow is adopted for this numerical calculation. From the velocity vector plots, it is found that there is a coherence structures of large vortex observed at the low impinging distance (H/D=2) and low Reynolds number (Re=210). Owing to the fact that instability waves in the shear layer do not have enough energies to roll up into the ring vortex for the microjet, the large vortex will disappear by increasing the impinging distance and the Reynolds number, the same flow structure characteristics as presented by the experiments of Shen (2003) and Yen (2004). The effects of Reynolds number on heat transfer and flow field are significant. The Nusselt number increases with increasing Reynolds number, and the local maxima is observed near r/R=1.2 rather than at the centerline, showing good agreement with Pence et al.(2003) and supporting the validity of the present study. Numerical predictions obtained from this study will provide physical insight into the MEMS system and the cooling of CPU.

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