A Study On The Micro-and Nano-Scale Gas Flow Properties In Fractal Porous Media

Porous media(porous material)have been widely applied in the field of material electronics such as micro-nano electromechanical system,semiconductor field effect transistor,fuel cell,chemical engineering,fiber fabric,aerospace composite etc.The internal transport properties of porous media are significant for the efficient exploitation and utilization of materials,design and improvement of devices.Recently,with the rapid development of micro-and nano-scale technology,microelectronics and micro-processing technique,micro-and nano-scale porous media have attracted broad interests.However,most studies are focused on the transport process of porous media in conventional scale,and the study on internal transmission in micro-and nano-scale porous media are few.The gas transport in micro-and nano-scale porous media may involve non-slip flow,slip flow,transition flow and free molecular flow.Although some modified theoretical models have been proposed to characterize the single phase flow and diffusion in micro-and nano-scale porous media,the empirical constants in these models are difficult to determine and the physical meanings and microscopic seepage mechanisms are not clear.The unified mathematical model and seepage theory are needed.Thus,the study on the gas transport properties and mechanisms in micro-and nano-scale porous media is one of research hotspots and difficulties for multidisciplinary fields.Many natural and synthetic porous materials have been shown to have self-similar fractal structures,and the pores indicate fractal scaling laws within a wide range of scales.Therefore,fractal geometry has been used in the current work to characterize the multiscale pore structure,the microscopic seepage flow models are developed to study the gas flow and diffusion in micro-and nano-scale porous media via theoretical analysis and numerical simulation.The main contents of this thesis are listed as follows.(1)Gas flow in micro-and nano-scale porous media:A fractal capillary bundle model and fractal tree-like network model are developed to derive the effective gas permeability of micro-and nano-scale porous media,and the effect of gas slippage effect on the gas permeability are systematically studied.The reliability of the model is verified by experiments.A numerical study is carried out on the gas flow in a two-dimensional Sierpinski carpet based on the finite element method.Theoretical and numerical results are shown that the fractal dimension of pore and tortuosity can increase and decrease the effective permeability,and slip effect has a significant effect on the gas permeability in the micro-and nano-scale porous media.(2)Gas diffusion in micro-and nano-scale porous media:Both molecular diffusion and Knudsen diffusion mechanisms are taken into account for the multiscale porous media,and the analytical expression for effective diffusion coefficient is presented.The relationship between gas diffusion coefficient and microscopic structures of porous media is illustrated compared with numerical results and related experiments.Gas diffusibility in micro-and nano-scale porous media not only depends on pore structures but also related to the gas properties.(3)Tortuosity in porous media:The hydraulic tortuosity of porous media is numerically calculated by the finite element method on the two-dimensional Sierpinski carpets with circular and square particles.The tortuosity is calculated in theoretically and compared with the experimental and numerical simulation results.The relationship between the porosity,the gas slip effect and the particle shape on the tortuosity is discussed.Relationship between the tortuosity and the porosity of the porous medium is obtained.The results show that the gas slip effect can significantly reduce the tortuosity of the porous medium.The present work is of great theoretical and practical significance for understanding the gas transport mechanisms in micro-and nano-scale porous media and the development of microscopic seepage flow theory as well as the applications of micro-and nano-scale porous media.