Mixing Mechanisms of Viscoelastic Impinging Flow in Cross-Shaped Channels
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摘要: 采用激光诱导荧光（PLIF）技术对十字型通道中黏弹性流体（聚环氧乙烷溶液）进行可视化研究，重点考察了雷诺数（Re）、聚合物溶液质量分数（w）和通道尺度对流体流动模式、振荡特性以及混合效果的影响。结果表明，对于牛顿流体（水），随着雷诺数增加（20<Re<500），500 μm、6 mm和1 cm的十字型通道内均依次呈现分离流、稳态吞噬流、涡脱落振荡以及非稳态吞噬流；而对于黏弹性流体，随着聚合物溶液质量分数增加（0.01%≤w≤0.30%），流体弹性效应增强，在低聚合物溶液质量分数下微通道内出现了惯性弹性非稳态振荡，导致混合增强，而在较高质量分数下出现了弹性主导的非稳态振荡，弹性非稳态振荡在维森贝格数和雷诺数较低时具有周期性特征，随着维森贝格数和雷诺数增大，振荡周期减小且趋于不规则。对于6 mm和1 cm十字型通道，随着聚合物溶液质量分数的增加，涡脱落振荡和非稳态吞噬流的临界雷诺数增大，在较高聚合物溶液质量分数下非稳态吞噬流消失。Abstract: Recently, the microchannel has attracted wide attention due to its safety, reliability and high mixing performance. Most of the current researches focus on Newtonian fluids, but the liquids used in practical applications are often elastic and viscous. Therefore, the planar laser-induced fluorescence (PLIF) technique was used to visualize viscoelastic fluid (polyethylene oxide solution) in cross-shaped channels, focusing on the influence of Reynolds number, channel sizes and polymer solution concentration on flow regimes, oscillatory characteristics and mixing effect. For Newtonian fluid (pure water), the separation flow, steady engulfment flow, vortex shedding oscillations and unsteady engulfment flow were found with the increase of Reynolds number (20<Re<500) in all cross-shaped channels (500 μm, 6 mm and 1 cm). For viscoelastic fluid, as the polymer solution mass fraction increased (0.01%≤w≤0.30%), the fluid elastic effect was enhanced, where an inertioelastic unsteady oscillation occurred at low concentrations which enhanced mixing, and an elastic-dominated unsteady oscillation emerged at higher concentrations within the microchannel.The elastic-dominated unsteady oscillation had periodic characteristics at low Wi and Re. As Wi and Re increased, oscillation period decreased and the flow tended to be irregular. For the 6 mm and 1 cm cross-shaped channels, the critical Reynolds number of vortex shedding oscillations and the unsteady engulfment flow increased with the increase of concentrations, and the unsteady engulfment flow disappeared at higher concentrations.
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图 1 （a）实验流程图及（b）十字型通道示意图
Figure 1. Schematic diagram of (a) experimental flow chart and (b) cross-shaped channel
图 2 500 μm十字型微通道内纯水流动模式
Figure 2. Flow patterns for pure water in the cross-shaped channel of 500 μm
图 3 500 μm十字型微通道内惯性弹性非稳态振荡的PLIF图（w=0.03%）
Figure 3. PLIF images of inertial elastic unsteady oscillations in the cross-shaped channel of 500 μm (w=0.03%)
图 4 PLIF瞬时图（Re=0.37，Wi=2.26，w=0.30%）
Figure 4. Instantaneous PLIF snapshots (Re=0.37, Wi=2.26, w=0.30%)
图 5 不同质量分数聚合物溶液和不同通道尺度下的流动模式相图
Figure 5. Map describing flow patterns with different mass fractions of polymer solution and channel sizes
图 6 不同Wi下归一化浓度变化时间序列及其功率谱（w=0.30%）
Figure 6. Time series of normalized concentration and corresponding spectra at different Wi (w=0.30%)
图 7 不同质量分数流体的斯特劳哈尔数随雷诺数的变化
Figure 7. Strouhal numbers for different mass fractions of fluids at various Reynolds number
图 8 3种尺度通道内不同质量分数流体Is随雷诺数的变化
Figure 8. Is varies with Reynolds numbers for different mass fractions of fluids in three different channel sizes
表 1 PEO溶液流体性质参数
Table 1. Fluid property parameters of PEO solutions
w/% η/(mPa·s) λ/10−3 s η0/η 0.01 1.12±0.02 12±1 0.897 0.03 1.59±0.02 23±1 0.632 0.10 3.90±0.05 44±2 0.258 0.30 25.00±0.10 84±4 0.040 
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