突縮流道內黏彈流體混合的數值研究

Abstract

本研究利用COMSOL Multiphysics軟體，以數值方法研究黏彈流體在平面突縮/突張微流道混合器內的流場及混合現象，並和牛頓流體狀况作比較。此混合器基本上由三段不同矩形截面積的扁平微流道連結而成，以一段窄流道(稱頸部段)連結上下游兩段寬流道，窄流道寬度與其上游寬流道寬度的比值稱為收縮比。計算結果顯示，黏彈流體與牛頓流體的混合均主發生在頸部段，下游突張流道對促進混合貢獻不大。混合效率均隨收縮比增加而增加，但會趨於飽和；且黏彈流體的混合效率優於牛頓流體者。這些結果均和流場的第一法向應力差和第二法向應力差呈正相關，表示流體的黏彈和流體混合有密切關連。本文亦對不同流量Q的就况進行了計算，結果顯示在Q > 5 ml/hr後，濃度分布已不太變化。本研究的結果對於深入理解黏彈流體在微流道中的行為以及混合過程的控制具有重要意義，可供設計和優化微流道混合器提供重要參考。

Numerical study was performed for studying the flow field and mixing phenomenon of viscoelastic fluids in a planar abrupt contraction/expansion microchannel mixer, with the aid of COMSOL Multiphysics, and the results were contrasted with those of Newtonian fluids. The mixer is essentially a device constructed by connecting three microchannels with different rectangular cross sections; with a narrow channel (called the necking section) in the middle. The ratio between the width of the necking section to that of the upstream channel is called the contraction ratio. It was found that major mixing occurs in the necking region for both viscoelastic fluid and Newtonian fluid, with minor mixing in the downstream channel. The mixing efficiency increases with contraction ratio, but tends to be saturated for sufficiently large contraction ratio; and the mixing of viscoelastic fluid is better than that of Newtonian fluid. All these mixing results are correlated with the first and second normal stress differences of the flow, indicating that the mixing is related strongly to the viscoelastic characteristics. Simulations have also been performed for different inlet flow rates, and the concentration distribution varies slightly when Q > 5 ml/hr. The present result is important for understanding the mixing phenomena of viscoelastic fluid, and is helpful for the design and optimization of mixers, particular in microfluidics.