Meskos Stavros, Stefanov Stefan, Valougeorgis Dimitris
Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bontchev St. bl. 4, 1113 Sofia, Bulgaria.
Department of Mechanical Engineering, University of Thessaly-Pedion Areos, 38334 Volos, Greece.
Micromachines (Basel). 2019 Mar 7;10(3):178. doi: 10.3390/mi10030178.
The mixing process of two pressure driven steady-state rarefied gas streams flowing between two parallel plates was investigated via DSMC (Direct Simulation Monte Carlo) for different combinations of gases. The distance from the inlet, where the associated relative density difference of each species is minimized and the associated mixture homogeneity is optimized, is the so-called mixing length. In general, gas mixing progressed very rapidly. The type of gas surface interaction was clearly the most important parameter affecting gas mixing. As the reflection became more specular, the mixing length significantly increased. The mixing lengths of the HS (hard sphere) and VHS (variable hard sphere) collision models were higher than those of the VSS (variable soft sphere) model, while the corresponding relative density differences were negligible. In addition, the molecular mass ratio of the two components had a minor effect on the mixing length and a more important effect on the relative density difference. The mixture became less homogenous as the molecular mass ratio reduced. Finally, varying the channel length and/or the wall temperature had a minor effect. Furthermore, it was proposed to control the output mixture composition by adding in the mixing zone, the so-called splitter, separating the downstream flow into two outlet mainstreams. Based on intensive simulation data with the splitter, simple approximate expressions were derived, capable of providing, once the desired outlet mixture composition was specified, the correct position of the splitter, without performing time consuming simulations. The mixing analysis performed and the proposed approach for controlling gas mixing may support corresponding experimental work, as well as the design of gas micro-mixers.
通过直接模拟蒙特卡洛法（DSMC）研究了在两个平行平板间流动的两种压力驱动稳态稀薄气流在不同气体组合下的混合过程。从入口开始，每种物质的相关相对密度差最小且相关混合物均匀性最佳的距离就是所谓的混合长度。一般来说，气体混合进行得非常迅速。气体与表面的相互作用类型显然是影响气体混合的最重要参数。随着反射变得更加镜面反射，混合长度显著增加。硬球（HS）和可变硬球（VHS）碰撞模型的混合长度高于可变软球（VSS）模型，而相应的相对密度差可忽略不计。此外，两种组分的分子量比对混合长度影响较小，对相对密度差影响更重要。随着分子量比降低，混合物的均匀性变差。最后，改变通道长度和/或壁温影响较小。此外，有人提出通过在混合区添加所谓的分流器来控制输出混合物的组成，该分流器将下游流分离成两个出口主流。基于带有分流器的大量模拟数据，推导出了简单的近似表达式，一旦指定了所需的出口混合物组成，就能提供分流器的正确位置，而无需进行耗时的模拟。所进行的混合分析以及所提出的控制气体混合的方法可能会支持相应的实验工作以及气体微混合器的设计。
