電熱力流場混合器的數值模擬與設計

Abstract

本文探討含特殊造型平面電極的微流道內電解質流體所產生的漩渦流動。採用流力及電熱力的理論，透過COMSOL軟體進行模擬，並加以實驗驗證。最後，利用此漩渦流動的特性設計出各式微流混合器，並分析其混合效能。 實驗裝置是採用標準微機電技術製作，其結構是一矩形截面直微流道，並在流道底壁建置一對具夾角之平面電極，電極通以10伏特的交流電，以驅動流道內的電解液運動，再將所記錄的影像以PIV方法分析，可獲得流場。經與計算結果比較，雖局部定量結果差異頗大，但定性結果相符，而初步確認可採數值模擬研究此微流裝置。 計算結果提供三維微流道內漩渦流場的詳細性質，其中重要結論含：(1)微流道內流體的最大溫度差雖然只有0.32K，但因為具有甚大的溫度梯度而能對流體的運動造成很大的影響。(2)溫度不會隨交流電的頻率變化而有所改變，但是卻會隨著流體導電率的增加而呈線性成長。而流場的速度量值會隨著交流電頻率的增加而變小，但卻隨著流體導電率的增加而變大。(3)電極夾角的角度對微流道內的溫度場與流場的整體趨勢影響不大，只會稍微改變高溫區域和流場漩渦的位置，但是對於速度量值而言，電極夾角的影響很大，夾角為 時有最大的漩渦速度。 混合器方面，本文依照流場的計算結果設計出多種不同的電極陣列，並進行了詳細的計算與分析。在平均背景流速100 及電壓30 下，其中一項交錯電極設計(二行，共六片電極)的混合器之混合效能可達89.6%。

The goal of this thesis is to study the vortex motion of electrolyte in a micro channel with designed electrodes built on one of its walls. The problem is formulated based on fluid mechanics subjected to electro-thermal force, solved by using the CMSOL software, and validated by comparing with an experiment. The result of such a vortex motion is employed to the design of a micro-mixer, which is also analyzed in details numerically. The apparatus of the experiment is fabricated by using the standard MEMS techniques. It is a micro-channel of rectangular cross section with a pair of electrodes on its bottom wall. The fluid motion is generated by actuated the electrodes at 10 volts (20 volts peak-to-peak), and is recorded and analyzed using the method employed in PIV. The numerical results agree qualitatively and fairly quantitatively with the experiment, with considerable discrepancies at a limited local region. The main findings of the present three-dimensional calculation are as follows. (1) Although the temperature differences are within 0.32K, its effect on fluid motion is significant because of large temperature gradient. (2) The temperature remains unchanged as the electric frequency varies but increases linearly with the medium conductivity. The velocity magnitude decreases as the frequency increases but increases as the medium conductivity increases. (3) The angle between the electrode pair does not have significant effect on the temperature and the appearance of the vortex structure, but it does affect the velocity magnitude. The velocity magnitude obtains its maximum when the angle is 180°. Various electrodes configurations for mixing enhancement are designed and simulated based on the results of vortex structures. For a background velocity at and an applied voltage at 30 volts, the mixing efficiency can reach 90% for a staggered electrode (two rows) design with six electrodes.