INFLUENCE OF EXPANDED CAVITY ON MIXING PERFORMANCE OF SQUARE-WAVE MICRO-MIXER
LiuZhaomiao2),, WangWenkai, PangYan (College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology, Beijing 100124, China) 中图分类号:O35 文献标识码:A
关键词:微混合器;混合强度;压降;结构优化;数值模拟 Abstract Micro-mixer has great application potentials in many fields such as material synthesis, pharmaceutical preparation and biochemical detection due to its advantages of saving reagents, higher mixing index and easy integration. In order to further improve the mixing performance, to ensure the safety of the mixing process and the accuracy of the biochemical reaction results, a new square wave micro-mixer with extended cavity was designed. Under the premise of considering the mixing index and pressure drop, the effects of the width of slit, the length of slit, and the height of extended cavity on the mixing performance of micro-mixer were analyzed by experiment and simulation. The optimal structural parameters were achieved under different Reynolds number (). Compared with the square-wave micro-mixer, the mixing strength of the square-wave micro-mixer with the extended cavity is higher when . Moreover, the gap of mixing index between two square-wave micro-mixer reaches maximum, up to 12%, at . Under the same Reynolds number, the pressure drop of the square-wave micro-mixer with the extended cavity is lower than that of the square-wave micro-mixer. Meanwhile, the analysis for the internal flow field of the square-wave micro-mixer with the extended cavity was carried out. It is found that the eddy current is introduced on the basis of the laminar flow state of the fluid because of the existence of the extended cavity structure, which means the change of the flow state of the fluid in the channel and the enhancement of convection effect, thus the mixing performance is further improved.
本文借助高速摄影实验设备(如图2所示)对微混合器内流体的混合效果与流动状态进行研究分析,并与数值仿真结果进行对比. 高速摄影实验设备的型号为基恩士VW-9000. 微混合器芯片采用聚二甲基硅氧烷材质加工而成,去离子水和黑色墨水溶液(用去离子水稀释10倍)作为两种流体工质. 显示原图|下载原图ZIP|生成PPT 图2高速摄影系统示意图 -->Fig. 2Sketch of high-speed photography system -->
利用微注射泵将稀释后的黑色墨水溶液和去离子水等速注入微混合器入口1和入口2中,同时使用高速摄像镜头拍摄流体混合过程的光学图像. 对 m, m和 m的微混合器进行实验和数值模拟,并计算得到 在0.1 60下的 值,如图3所示. 显示原图|下载原图ZIP|生成PPT 图3实验和数值模拟值对比 -->Fig. 3Comparison of mixed index values in experimental and numerical simulation -->
从图3中可以看出,实验和数值模拟的 趋势基本一致. 当 在0.1 1区间时, 随 的增大呈下降趋势. 当 在1 60区间时, 随 的增大迅速提高,两者吻合较好. 分别为0.1, 1, 20和40时,微混合器 截面上的实验和数值模拟混合溶液质量分数分布图,如图4所示,流体流动的状态、混合程度均吻合良好. 时,入口流速较小,流体在通道中混合时间相对较长,不同流体工质分子间的扩散时间延长,导致分子扩散较为充分,因此 较高. 时,入口流速虽然比 时大,但混合时间变短,不同组分流体间的接触面变得清晰, 降低. 继续变大,分子扩散逐渐被对流扩散所取代成为影响 的主要因素,随着 的增大,通道内开始出现涡流,有助于提升不同流体工质间的混合,进而 逐步提高. 显示原图|下载原图ZIP|生成PPT 图4不同下微混合器-截面上混合溶液质量分数分布图 -->Fig. 4Distribution of mass fraction of mixed solution in - cross section of different under micro-mixer -->
3 结果与讨论
3.1 窄缝宽度对混合性能的影响
不同 下, 和 随 在0.1 60范围时的变化( m, m)分别如图5和图6所示. 由图5可知,随着 的减小, 增强. 主要有以下两个原因:一方面,随着 的减小,窄缝对流体的挤压效果更加显著,导致分子之间的扩散距离和扩散时间缩短,分子扩散更为充分;另一方面, 减小,使射流效应增强,更有益于混沌对流,对流的产生更有助于打破层流,导致不同流体工质之间的接触面积增大,促进了流体混合,进而 增大. 当 为50 m时, 在0.1和在10 60区间下, 均大于90%. 显示原图|下载原图ZIP|生成PPT 图5不同下对的影响 -->Fig. 5Effect of on under different -->
由图6可知,随着 的增大,不同 下的 均呈上升趋势. 随着 的减小,内部涡流导致能量损失增加, 增幅剧烈. 当 为150 m, 200 m, 250 m和300 m时,微混合器通道内的 没有明显变化. 当 为100 m且 时, 的最大值为32 kPa,对压力泵和微混合器强度的要求并不大;同时相对于前四种 ,在 时的 均大于90%. 因此,将 m确定为最佳值. 显示原图|下载原图ZIP|生成PPT 图6不同下对的影响 -->Fig. 6Effect of on under different -->
3.2 窄缝长度对混合性能的影响
不同 下, 和 随 在0.1 60时的变化规律( m, m)分别如图7和图8所示. 图7表明 越长, 越大. 当 m时,在不同 下,微混合器的 均最大,其中在 达到最大值,可达99.2%. 这是由于在此 下流体混合时间相对延长,更有助于分子扩散, 最高. 由图8可知, 与 成正比, 越长,窄缝对流体的挤压时间相对增长,阻力增加. 其中 =60时, m的 kPa,相比于 m时增加了48%. 虽然 m时的 略微高于其他 下的情况,但并不会影响到实际应用,因此选择最优的 600 m. 显示原图|下载原图ZIP|生成PPT 图7不同下对的影响 -->Fig. 7Effect of on under different -->
显示原图|下载原图ZIP|生成PPT 图8不同下对的影响 -->Fig. 8Effect of on under different -->
3.3 扩展腔高度对混合性能的影响
取 为50, 100, 150, 200, 250和300 m,在 为0.1 60下进行数值模拟( m, m),得到 和 的结果分别如图9和图10所示. 图9表明 在0.1 1之间时,随着 的增大, 不断下降,这是由于随着 的增大,不同组分流体间的接触面积减小,导致分子扩散不充分, 降低. 当 , 在 为50 m时达到最大,可达77%,在 m时最小,为70%. 在5 60时, m时的 最大,其中 时, 可高达99%. 这是由于在此 下涡流引起的对流扩散效应最为显著. 显示原图|下载原图ZIP|生成PPT 图9不同下对的影响 -->Fig. 9Effect of on under different -->
由图10可知,随着 的增大,微通道对流体的阻碍作用减弱, 逐渐降低. 其中 时, m下的 值与 m下的 值相比,减小了5.2%. 考虑到实际工况条件, 一般不会小于5,所以选取最优的 m. 显示原图|下载原图ZIP|生成PPT 图10不同下对的影响 -->Fig. 10Effect of on under different -->
综上所述,带扩展腔的方波型微混合器的最佳结构参数为 m, m及 m.
3.4 两种不同结构微混合器混合性能比较
对 m, m及 m下的带扩展腔的方波型微混合器与相同尺寸下的方波型微混合器进行数值模拟,得到 在0.1 60下的 与 的关系曲线图,分别如图11和图12所示. 由图11可知,两种微混合器的 均随着 的增大先减小后增大,且均在 时, 达到最小. 随着 继续增大,对流效应逐渐取代分子扩散成为混合的主导作用,通道内产生涡流, 逐步提高. 在0.1 1时,方波型微混合器的 略高于带扩展腔的方波型微混合器,其中在 时两者 最大 显示原图|下载原图ZIP|生成PPT 图11对的影响 -->Fig. 11Effect of on -->
(1)带扩展腔结构的方波型微混合器能在流体层流的基础上引入涡流,有效地促进了不同流体之间的混合,有助于提高监测生化反应结果的安全性及准确性. 相比于方波型微混合器,带扩展腔的方波型微混合器在雷诺数 时具有更好的混合强度. 虽然两种微混合器在相同 范围0.1{Invalid MML}60区间内表现出类似的压降趋势,但是带扩展腔的方波型微混合器的压降略低于方波型微混合器. (2)微混合器混合性能的变化规律为设计微混合器提供了重要的理论参考. 优化后的微混合器的体积缩小,流体间的扰动和对流效应增强,不同流体之间的接触面积增大,分子扩散更加充分,混合效果得到进一步提高,更有助于集成和生化反应的顺利进行. The authors have declared that no competing interests exist.
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