Research On The Fractal Structure Characteristic Of Fluidized Agglomerate Formed By Nanoparticles

Nanoparticles,which the diameter are between 0.1nm and 100 nm,are difficult to go on normal fluidization.Because of the strong cohesive force,nanoparticles are easy to aggregate and fluidized in the form of agglomerate.Fluidization agglomerates are a kind of internal contains multi-level structure of pore,and scholars generally agree that their multistage structures have fractal properties.The dynamic characteristics of fluidized agglomerate s are closely related to their structures.Therefore,it is important to understand the structure of aggregates in the process of fluidization,which will provide a theoretical basis for further study of gas-solid two-phase flow of nanoparticles in the future.In this paper,we choose the transmission electron microscope image which can clearly show the internal structure of the agglomerate as the research object of this subject,and study the fractal characteristics by using the box-counting method,and then reveal the relationship between the fra ctal dimension and the internal structure of the agglomerate.Based on the calculation principle of box-counting method,the influence of box size sequence selection on algorithm precision is analyzed,and a new selection method of box size sequence is proposed,and the accuracy of new selection method is verified.The results show that the new selection method of box size sequence can not only provide enough fitting data points,but also can eliminate the fitting of the data ladder phenomenon,that is,hav e a faster calculation speed,but also has a higher calculation accuracy.Based on the TEM images of the nanoparticle agglomerates at different levels,the fractal characteristics all levels between the agglomerates and hierarchical structures were studied by means of the box-counting method.The formation mechanism of nanoparticle fluidized agglomerates is proposed.The primary nanoparticles form a aggregate structure of the chain structure by the particle-cluster diffusion mechanism.The aggregates are fo rmed into two-level agglomerates with three-dimensional porous structure by particle-cluster diffusion mechanism.The two-level agglomerates in the three-dimensional space in a more intensive arrangement of the formation of a three-dimensional complex pore structure of the three-level agglomerates.In view of the limitation of the traditional method of measuring fluidization porosity,A new method for calculating the porosity of fluidized clusters is proposed and described in detail.The porosity of th e clusters is calculated by using the fractal dimension which can effectively describe the internal structure information of the clusters.And the feasibility and accuracy of fractal dimension method are verified by image method and weighing method respect ively.It is found that the fractal dimension considers the distribution of the primary nanoparticles in the radial direction,and can describe the change of the internal structure of the agglomerate.And the standard deviation between the porosity of the fluidized agglomerates calculated by the fractal dimension method and the porosity calculated by the weighing method is very small,and the maximum relative error between the two method is only 0.5%.