The Transport Properties Of Fractal Porous Material And Its Application In Fuel Cells

Porous materials have structural properties such as low relative density and high specific surface area,as well as good permeability and other transport properties.They are widely used in engineering practices and industrial production such as energy,photovoltaics,environment,biology,machinery,and aerospace.The transport properties of porous materials depend on their structural characteristics,but it is difficult to accurately describe the complex microstructure of porous materials using traditional methods based on Euclidean geometry.Studies have shown that the structural properties of a large number of natural and artificial porous materials exhibit self-similar fractal scale characteristics,so this paper uses fractal geometry theory to characterize the multi-scale pore structure of porous materials,and establish a physical model of pore scales in the fluid transport process of porous materials.The tortuosity and permeability of fractal porous materials are studied,and the application of porous material seepage characteristics in proton exchange membrane fuel cells is discussed.The main research contents of this article include:(1)Analysis of tortuosity of fractal porous materials: A fractal capillary bundle model of fluid transport of porous materials was established,and the fractal scale relationship between tortuosity and porosity of porous materials was theoretically derived,and the value of Sierpinski carpet model The pore-scale fluid transport process of porous materials is simulated,and the quantitative relationship between tortuosity and porosity is discussed.The correctness of the fractal capillary bundle model and the Sierpinski carpet model is verified by comparing experimental data.The results show that the tortuosity of the porous material depends not only on the statistical characteristics of the pore structure,but also on the topological and morphological characteristics of the pore structure.The tortuosity of porous materials increases as the porosity decreases and the fractal dimension of the tortuosity increases,while the increase in the ratio of the fractal dimension of the pore diameter to the pore diameter decreases the tortuosity.In addition,the smooth surface and slip effect of solid particles can further reduce the tortuosity,and the anisotropy caused by the movement of the particles in the vertical flow direction will increase the tortuosity.(2)Permeability study of fractal porous materials: Isotropic and anisotropic Sierpinski carpet models with different fractal dimensions were constructed.The seepage characteristics of fractal porous materials were numerically simulated,and the permeability and pore structure characteristics were discussed.Quantitative correlation.The results show that the permeability of the porous material depends on the porosity,pore size fractal dimension and pore size range,and its predicted value is in good agreement with existing theoretical models and experimental data.Under the condition of a certain porosity,the increase in the fractal dimension of the pore size leads to an increase in the proportion of small pores,the pore structure tends to be complicated,and the flow resistance increases,which in turn reduces the permeability.The increase of the particle anisotropy factor in the vertical flow direction can increase the permeability of the porous material.As the porosity decreases,the flow obstruction effect of the solid particles increases,but at the same time the slip effect improves the fluid throughput.(3)Effect of transport characteristics of fractal porous materials on proton exchange membrane fuel cell performance: A multi-scale multi-field coupling model of proton exchange membrane fuel cells was constructed using the quantitative relationship between porous material permeability and pore structure,and the microstructure of porous materials was discussed And the seepage characteristic is quantitatively related to the electrical performance of the fuel cell.The results show that as the porosity and permeability decrease,the current density of the fuel cell decreases;while at the same porosity,the decrease in fractal dimension will increase the current density of the fuel cell.This article establishes a physical model of the transport process of porous materials at the pore scale,which is helpful to understand the transport mechanism of porous materials and provides a theoretical basis for the application of optoelectronic porous materials,porous materials for oil and gas reservoirs,and fiber porous materials.The multi-scale and multi-physics coupling model of fuel cell is constructed by using the quantitative relationship between porous material seepage characteristics and microstructure,which has important theoretical and practical significance for optimizing fuel cell structure and reducing fuel cell cost.