Yuan Chao, Zhang Hongna, Li Xiaobin, Oishi Masamichi, Oshima Marie, Yao Qinghe, Li Fengchen
School of Aeronautics and Astronautics, Sun Yat-sen University, Guangzhou 510275, China.
School of Mechanical Engineering, Tianjin University, Tianjin 300350, China.
Micromachines (Basel). 2021 Apr 22;12(5):477. doi: 10.3390/mi12050477.
Oscillatory flow has many applications in micro-scaled devices. The methods of realizing microfluidic oscillators reported so far are typically based on the impinging-jet and Coanda effect, which usually require the flow Reynolds number to be at least at the order of unity. Another approach is to introduce elastomeric membrane into the microfluidic units; however, the manufacturing process is relatively complex, and the membrane will become soft after long-time operation, which leads to deviation from the design condition. From the perspective of the core requirement of a microfluidic circuit, i.e., nonlinearity, the oscillatory microfluidic flow can be realized via the nonlinear characteristics of viscoelastic fluid flow. In this paper, the flow characteristics of viscoelastic fluid (Boger-type) in a T-shaped channel and its modified structures are studied by two-dimensional direct numerical simulation (DNS). The main results obtained from the DNS study are as follows: (1) Both Weissenberg (Wi) number and viscosity ratio need to be within a certain range to achieve a periodic oscillating performance; (2) With the presence of the dynamic evolution of the pair of vortices in the upstream near the intersection, the oscillation intensity increases as the elasticity-dominated area in the junction enlarges; (3) Considering the simplicity of the T-type channel as a potential oscillator, the improved structure should have a groove carved toward the entrance near the upper wall. The maximum oscillation intensity measured by the standard deviation of flow rate at outlet is increased by 129% compared with that of the original standard T-shaped channel under the same condition. To sum up, with Wi number and viscosity ratio within a certain range, the regular periodic oscillation characteristics of Oldroyd-B type viscoelastic fluid flow in standard T-shaped and its modified channels can be obtained. This structure can serve as a passive microfluidic oscillator with great potential value at an extremely low Reynolds number, which has the advantages of simplicity, no moving parts and fan-out of two.
振荡流在微尺度设备中有许多应用。迄今为止报道的实现微流体振荡器的方法通常基于冲击射流和柯恩达效应，这通常要求流动雷诺数至少为单位量级。另一种方法是将弹性膜引入微流体单元；然而，制造过程相对复杂，并且膜在长时间运行后会变软，这导致偏离设计条件。从微流体回路的核心要求即非线性的角度来看，振荡微流体流可以通过粘弹性流体流动的非线性特性来实现。本文通过二维直接数值模拟（DNS）研究了粘弹性流体（博格型）在T形通道及其改进结构中的流动特性。DNS研究得到的主要结果如下：（1）魏森贝格（Wi）数和粘度比都需要在一定范围内才能实现周期性振荡性能；（2）随着交叉点上游一对涡旋的动态演化，振荡强度随着连接处弹性主导区域的扩大而增加；（3）考虑到T型通道作为潜在振荡器的简单性，改进结构应在上壁附近有一个朝向入口雕刻的凹槽。在相同条件下，与原始标准T形通道相比，可以通过出口处流速的标准偏差测量的最大振荡强度提高了129%。综上所述，在Wi数和粘度比在一定范围内时，可以得到标准T形及其改进通道中奥尔德罗伊德-B型粘弹性流体流动的规则周期性振荡特性。这种结构可以作为一种在极低雷诺数下具有巨大潜在价值的无源微流体振荡器，具有结构简单、无运动部件和二路分支的优点。
