Study On Fluidized Nanoparticle Agglomeration Properties: Adhesion,Fragmentation,and Drag Force

Nanoparticles,with their ultrafine particle size and high specific surface area,have widespread use in industrial fields,such as chemistry,energy,environment and particle synthesis.As a new nanoparticle processing technology,fluidized bed(FB)has attracted more and more attention.Due to the strong adhesion,nanoparticles often exist in the form of loose agglomerates,which are dynamically broken and reassembled in fluidized beds.Therefore,the in-depth understanding of the characteristics of nanoparticle agglomerates is of great significance to effectively control the fluidization of nanoparticles.In this paper,the agglomeration characteristics of nanoparticles were studied from the aspects of adhesion,fragmentation and drag force,which provided a theoretical basis for the improvement of nanoparticle fluidization and the development of nanoparticle fluidization model.The main research conclusions are as follows:The force-displacement curves of SiO₂,Al₂O₃ and TiO₂ nanoparticle agglomerates were measured by Atomic Force Microscopy(AFM).The adhesive force and apparent Young's modulus of agglomerates are compared during loading and unloading.It is found that the adhesive force of SiO₂ agglomerates is lower than that of the other two agglomerates.The adhesive force and apparent Young's modulus of the agglomerates during unloading are greater than those during loading,and increase with the normal load.When the normal load is 2n N?10n N,for the loading process,the adhesive force of SiO₂ agglomerates can be ignored,and the adhesive force of Al₂O₃ agglomerates is similar to that of TiO₂,ranging from 0.1n N to 0.9n N.For the unloading process,the adhesive forces of SiO₂,Al₂O₃ and TiO₂ agglomerates are 0.01n N?0.15n N,2n N?8n N and 1n N?11n N,respectively.When the normal load is 100n N?500n N,during the loading process,the apparent Young's modulus of SiO₂,Al₂O₃ and TiO₂ agglomerates are 30k Pa?200k Pa,40k Pa?210k Pa and 60k Pa?650k Pa,respectively.During the unloading process,the apparent Young's modulus of SiO₂,Al₂O₃ and TiO₂agglomerates are 0.6MPa?3MPa,0.4MPa?3.5MPa and 0.7MPa?11MPa,respectively.The increase of normal load leads to the augment of contact area and solid content of the agglomerates,resulting in the increase of Young's modulus and the unloading adhesive force of the agglomerates.The fragmentation characteristics of SiO₂,Al₂O₃ and TiO₂ agglomerates were studied by the experiments of free-falling agglomerates collision.It is found that for different agglomerates,the collision rebound and breakage characteristics of agglomerates are quite different.Obvious deformation and collapse of SiO₂ agglomerates are found after collision with a few small fragments.In addition,the SiO₂ agglomerates do not roll or rebound during the collision process.Al₂O₃agglomerates roll after collision with no obvious collapse and deformation,and many large fragments were observed.There are a few large fragments after the collision of TiO₂ agglomerates,with no rolling and rebound,nor obvious collapse and deformation.For the same kind of agglomerates,the equivalent diameter of the maximum fragments increases with the falling height.For different agglomerates,the equivalent diameter of the maximum fragments follows the order:SiO₂<TiO₂<Al₂O₃.The size distributions of SiO₂,TiO₂ and Al₂O₃ fragments are in the range of 10?m?90?m,25?m?230?m and 25?m?210?m,respectively,which could provide a basis for the parameters of the fragmentation kernel function in the population balance model.Based on the method of directly constructing the agglomerate structure,the fluid flowing through the agglomerates was numerically simulated and the characteristics of the drag coefficient were statistically analyzed.Agglomerates with different structures were generated with the fractal dimension(₁))ranging from 1.8 to 2.9.The influences of the structural characteristics and direction of the agglomerates on drag coefficients were studied.It is found that the fluid tends to bypass rather than pass through the dense agglomerates.With the increase of the porosity of agglomerates,the fluid penetrates deeper inside the agglomerates,and the contact area between the fluid and the agglomerates increases.At the same Reynolds numbers,with the decrease of fractal dimension,the pressure of agglomerates per unit projection area decreases.However,due to the high specific surface area of the loose agglomerates,the contact area with the fluid is larger,which leads to the increase of the drag force.The drag coefficient of the agglomerates is influenced by the structural characteristics of the agglomerates,the direction of fluid flowing through the agglomerates and the Reynolds number.When different agglomerates have close projected areas,the drag coefficient of agglomerates under the same Reynolds number is positively related to the radius of gyration(₂))and the porosity()of the agglomerates,and negatively related to the fractal dimension(₁))and the dimensionless density(/)of the agglomerates.