Numerical Simulation Of Nanoparticles Movement In Shear Flow By Coupled Fluid-solid Model With Lattice Boltzmann Method

The catalyst layer as a core component of fuel cells is the main place where the electrochemical reaction takes place,which determines the speed of the electrochemical reaction and the performance of fuel cells.Generally,different formulations are used to prepare the catalyst ink by stirring or ultrasonic oscillation,and then the porous catalyst layer is prepared by brushing or spraying to further make a fuel cell.The performance of the catalyst layer is compared by testing cell performance.However,this method is not only inefficient,but also fails to provide theoretical guidance for the preparation of high-efficiency catalyst layers.The right way is to establish the relationship between the preparation process and microstructure of the catalyst layer and cell performance.As the first step in the preparation of porous catalyst layers,the preparation of catalyst ink is crucial.So far,the theoretical researches related to the preparation process of catalyst ink are extremely lacking.It is urgent to carry out research on the agglomeration and dispersion mechanism of components under internal or external force as well as control method in the preparation of catalyst ink,which can provide theoretical guidance for producing high-performance fuel cells.Meanwhile,the process of the catalyst ink preparation involves the coupled fluid-solid mechanism of nanoparticles in shear flow,and the related research also has important academic value.In this paper,the carbon black particles commonly used in the preparation of catalyst ink for fuel cell are taken as the research object.The interaction mechanism between carbon black particles is explored from a mesoscopic perspective.The effect of the shear force acted by the fluid on the carbon black particles and particle groups is analyzed.The mechanism of agglomeration and dispersion of carbon black particles and particle groups under shear flow is clarified.The effects of flow characteristics,particle size,solvent type,etc.on the agglomeration and rheological characteristics of system including the dispersion of system,number and size of particle agglomerations,relative effective viscosity of system,etc.are revealed.The results can provide guidance to the preparation of catalyst ink.The main research works and results are as follows:(1)The lattice Boltzmann method,Lees-Edwards boundary and periodic boundary are used to construct the shear flow model.Combined with the smoothed profile method,a coupled fluid-solid model is further constructed.On this basis,the inter-particle model is constructed by applying the DLVO theory with the consideration of the attractive and repulsive forces between particles.Through the simulation of the movement of two particles under shear flow,it is found that the two particles will separate after colliding when the particle Reynolds number is greater than a critical value,otherwise the particles will agglomerate.The critical particle Reynolds numbers for the particle separation or agglomeration in water,ethanol and isopropanol solvents are 0.8,0.18 and0.6,respectively.Different from the results of the traditional collision model which deviates from the actual physical observation phenomena,the DLVO model can reflect the influence of the solvent on the movement of particles.(2)Based on the inter-particle model,the coupled fluid-solid model for the multi-particle is constructed by using the principle of summation.By analyzing the particle distribution,the number of particle agglomerations,the number of particles in the particle agglomerations,average particle degree of agglomeration structure and the relative effective viscosity of the aqueous and non-aqueous systems under different particle Reynolds numbers,it is found that the increase of the particle Reynolds number is beneficial to improve the dispersion and relative effective viscosity of the system.As the particle Reynolds number increases,the volume of the agglomerations decreases while the number and the internal porosity of the particle agglomerations increase.However,the change trend is gradually slowed down after the particle Reynolds number is higher than the critical value.(3)Based on the coupled fluid-solid model for the multi-particle,the effects of particle volume fraction and particle radius on the agglomeration and rheological properties of the system are further considered.It is found that an increase in the volume fraction of particles is not conducive to the dispersion of system,but increases the relative effective viscosity of the system.As the volume fraction of particles increases,the number of the particle aggregations increases and the particles in the aggregations are gradually connected tightly.Under the constant particle number,the larger particle radius is not conducive to the dispersion of system but increase the relative effective viscosity of the system.On the contrary,under the constant particle volume fraction,the larger particle radius is beneficial to improve the dispersion of system.With the increase of particle radius,the internal structure of the system has developed from a cluster network structure to a dispersed distribution structure.