Lattice Boltzmann Study Of Binary Gas Mixture Flow In Microsystem

Binary gaseous mixture microsacle flows occur commonly in nature and a variety of engineering and technical fields, for example, the enhanced oil recovery, geologic seques-tration of carbon dioxide, the spreading of pollutants in groundwater reservoirs and Micro-electro-mechanical systems (MEMS). Such kind of flow involves a variety of species effect and interaction. Since expensive and degradingness of the experimental study, numerical method becomes an important tool in the study of these flows. In recent decades, Lattice Boltzmann method (LBM) has been demonstrated as an effective way in the study of the bi-nary mixture microscale flow due to its mecroscopic character and applicability in complex boundary. Therefore, we chose this method in our study in the binary mixture microscale flow in this thesis. Works on binary gas mixtures are still very few. There are still some basic issues that remain unresolved. So in this thesis, by improving and developing some models for binary gas mixtures within the framework of multiple relaxation times (MRT) lattice Boltzmann equation (LBE), we do some detailed studies of these flows. The contents of the present thesis can be described as follow:Firstly, the flow of the whole the binary gas mixture is studied. The MRT LBE mod-el with effective relaxation times is used in the study of the pressure distribution in the microscale gaseous flow. Different rarefaction and compression effects are considered. Specially, the effect of aspect ratio which is always ignored in the previous study is also investigated. The numerical results show that:in slip and transition regimes the increase of Knudsen number tends to diminish the curvature of the pressure distribution; it is also shown that the relationship between the pressure deviation and the aspect ratio follows a second-order power law approximately.Secondly, the interaction between different species in gas mixture is studied by the sim-ulation of gas separation in the binary gas mixture microscale flow. The effective relaxation times and slip velocities which includes the wall truncation are firstly used in previous the binary gas mixture LBE model. Based on this new model, the separation phenomenon is studied under different gas mixture and Knudsen number. The numerical results show that: Separation phenomenon is more obvious for a mixture with large molecular mass ratio; the effect of rarefaction and compression are just the opposite. Separation process is enhanced when the Knudsen number becomes large, but fierce as the pressure ratio decreases.Thirdly, the interaction between gas and boundary is studied in this part of the paper by simulating binary gas mixture flows in roughness micro channels. We firstly extend the Diffuse-Bounce-Back (DBB) boundary to previous model condition for the realization of some complicated boundaries in the binary gas mixture LBE model. Based on the above modification, the roughness effect is studied by commutating the volume flux in roughness micro channel. The numerical results show that:The increase of relative roughness height and surface roughness intensive leads to a decrease of volume flow rate; The flow rates decrease with the increase of the Knudsen number when Kn<0.1, then when Kn is larger than0.1, the volume flow rates nearly have very no change.Finally, in the present thesis, a three dimensional (3D) numerical study of binary mix-ture flows in micro channels with different cross sections is performed by extending the above MRT LBE model to3D. A3D binary gas mixture model in firstly developed from above two dimensional (2D) model. The effect of gas-gas and gas-surface interaction to the volume flow rate are both considered in the simulation of binary gas flows in micro chan-nel with different cross sections. The numerical results show that:the volume flow rate in the micro channel with triangle cross section is larger than that in the micro channel with square cross section, but the influence of cross section on the separation is just the reverse. Meanwhile, the effect of the cross section will be weaker in a larger Kn number.To sum up, the flow problems with binary gas mixture in microsystems are studied numerically by the LBM, and the obtained results enhance the understanding of flow pattern and internal mechanism of such complex flows. Several valuable efforts are made in the present thesis to promote the applications of LBM in rarefied gas mixture flows, and these works can serve as a solid basis for further studies.