具截面變化之微流道流場模擬

機構典藏 > College of Engineering > Graduate Institute & Department of Aerospace Engineering > Thesis > Item 987654321/35624

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Title:	具截面變化之微流道流場模擬
Other Titles:	Numerical simulation of microchannel flows with variable cross section
Authors:	高耿偉;Gao, Geng-wei
Contributors:	淡江大學航空太空工程學系碩士班陳慶祥;Chen, Ching-shung
Keywords:	微流道;截面變化;邊界層方程式;質流率;渦漩;可壓縮流;microchannel;boundary-layer equation;compressible flow;variable cross section;mass flow rate;vortex
Date:	2007
Issue Date:	2010-01-11 06:53:02 (UTC+8)
Abstract:	在現今微流道流場研究下幾何結構大都是等直徑管道，在本研究中除了使用邊界層方程式計算圓形管可壓縮內流場來縮減運算時間外，並且藉由管道截面變化使流場產生渦漩，加以探討在可滑動壁面與不可滑動壁面渦漩與質流率的比較，以及藉由fluent來驗證邊界層方程式的準確性。本研究首先探討如何利用邊界層方程式模擬出迴流，邊界層方程式把迴流當作與主流場相反的方向的邊界層，所以迴流速度剖面會比較接近於邊界層的速度剖面。此外在本研究中分為兩種幾何外型，一種是漸縮漸擴管道、另ㄧ種是突擴張管道，在漸縮漸擴段中需使用隱性解法使接近壁面的速度能沿著壁面流動，而在非漸縮漸擴中則使用顯性解法。本研究在r方向速度使用連續方程式，所以當r方向的邊界層對渦漩影響很明顯時則需使用耦合解同時解出x、r方向的速度，如突擴張壁面管道。漸縮漸擴微流道下發覺可滑動壁面速度比不可滑動壁面在迴流區會有較快速度，以及較後面的分離點與再接觸點和較低的溫度。並且在壓力分佈可滑動壁面的壓力損失會較小。此外在微流道中流體的分離條件不論工作流體為何還是決定於雷諾數的大小。在突擴張微流道下，邊界層方程式由於先天的假設，在計算垂直壁面的邊界層會有ㄧ定的困難，故所能探討的現象有限，不過値得注意在突擴張壁面流體的黏性生熱的現象會更為明顯。本研究使用fluent相互驗證，雖然本研究中使用邊界層方程式無法相當準確的定量迴流速度，不過可以定性迴流速度的趨勢。更何況本研究利用簡化的Navier-Stokes方程式來計算，所以求解速度會比N-S方程式快上百倍或千倍，所以本研究可以利用較多的網格數以及較快運算速度模擬出令人滿意的結果。 In recent years most studies of microchannel flows focus on channels with constant cross-section. This study uses the boundary layer equations to calculate compressible microchannel flows with variable cross-section. The commercial software package, FLUENT was used to verify the present numerical procedure. Reverse flows were generated downstream of the expansions. The effects of slip conditions on the friction and the reverse flow were discussed. In this study we first demonstrate how to use the boundary layer method to successfully simulate reverse flows inside the channel. We investigated two variable cross-sections, one is the hyperbolic wavy channel (or nozzle channel) and the other is the sudden expansion channel. In order to stabilize the reverse flow, the implicit scheme was used to calculate the flow in the variable cross-section and the explicit scheme was used for the other part of the flow. The continuity equation was used to obtain the radial velocity, and the coupled scheme must be used in simulating sudden expansion flows. The results show that the slip boundary conditions produce less pressure losses, faster axial velocities, longer recirculation zones, and longer reattachment points when compared with the corresponding flows with the nonslip boundary conditions. The length of the recirculation zones is determined by Reynolds number and is independent of the types of working fluids utilized. Due to the assumptions and modifications employed in the present method in simulating the reverse flows, the recirculation zones predicted might not be very accurate. But the results are still acceptable for engineering purposes. Furthermore, the major advantage of the present numerical procedure is its fast speed due to the parabolic character of the governing equations. It was found that the present boundary layer method is two to three orders of magnitude faster than the full Navier-Stokes simulation, yet it provides acceptable accuracy.
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