Numerical And Experimental Studies On The Hydrodynamic Characteristics Of Fluidized Nanoparticle Agglomerates

Gas fluidization is one of the best techniques available to disperse and process nanoparticles.Due to the strong cohesive effect between nanoparticles,they are always fluidized in the form of agglomerates.Due to the high porosity and the complex hierarchical internal structure,the traditional drag coefficient model of solid particles may not be suitable for the fluidized nanoparticle agglomerate,and the effect of agglomerate structure on its hydrodynamic characteristics must be considered.In this thesis,numerical and experimental methods are employed to study the interaction between gas and agglomerates,and to reveal the relationship between the structural characteristic and hydrodynamic characteristic of agglomerates,so as to establish the drag coefficient model of fluidized nanoparticle agglomerates,which is of important theoretical and practical significance.The hierarchical solid structures and multi-scale pore structures of fluidized nanoparticle agglomerates were qualitatively revealed by TEM images of slices of solidified agglomerates.The morphology and fractal dimension of aggregates and simple agglomerates,and the porosity and pore size distribution of multi-scale pores in agglomerates were quantitatively characterized by image analysis.The experimental results show that each stage of the hierarchical structures has multi-scale characteristics.The sizes of aggregates are between 300 and 1000 nm,and those of simple agglomerates are between 1 and 20 μm,and those of fluidized agglomerates are between 100 μm and 2 mm.The shapes of aggregates,simple agglomerates and fluidized agglomerates in the upper part of the fluidized bed are irregular,while the shapes of fluidized agglomerates at the bottom of the fluidized bed are nearly spherical.Each stage of the hierarchical structures has fractal characteristics.The fractal dimensions of aggregates and simple agglomerates are about 1.9 and 2.6,respectively.The pore structure of fluidized nanoparticle agglomerates is complex and heterogeneous and displays a multiscale feature.Nearly 50% of the pores intra simple agglomerates have sizes between 25 nm and 100 nm.Nearly 40% of the pores inter simple agglomerates have sizes of hundreds of nanometers and the others are of micrometer scale.Based on the fractal theory,the radially varing permeability model were established.Agglomerates with different fractal dimensions and diameters were created to simulate the gas flow around and through the agglomerate at different Reynolds numbers.The influences of fractal dimension,Reynolds number and agglomerate diameter on the velocity field,pressure field,and streamline inside and outside agglomerates were investigated.The influences of the above variables on the drag coefficient of agglomerates were analyzed.A drag coefficient model was established as a function of the fractal dimension,Reynolds number and agglomerate diameter.The simulation results show that with the increase of the Reynolds number,more fluid passes through the agglomerate and the mean velocity of fluid inside the agglomerate increases;With the increase of fractal dimension or agglomerate size,the fluid passing through agglomerates declines and the mean velocity of fluid inside the agglomerate decreases;With the increase of the Reynolds number,fractal dimension or agglomerate size,the pressure difference through agglomerate increases obviously.When the fractal dimension is less than 2.6,the drag ratio increases with the increase of the Reynolds number or fractal dimension.When the Reynolds number is less than 100 or the fractal dimension is 2.4,the drag ratio is less than 1.When the Reynolds number is larger than about 100 and the fractal dimension is 2.6 and 2.7,the drag ratio is larger than 1.With the increase of agglomerate size,the drag ratio first increases rapidly,and then remains almost unchanged.It is unreasonable to assume a single fluidized agglomerate as a solid spherical particle.The settling experiment of single fluidized nanoparticle agglomerate was carried out.The relationship between the settling velocity and the agglomerate size and agglomerate density was analyzed.The relationship between the drag coefficient of agglomerate and the Reynolds number was analyzed and compared with that of the equivalent solid sphere.The experimental results show that the difference of settling velocity between the fluidized nanoparticle agglomerate and the equivalent solid sphere is obviously affected by the agglomerate density.The similarities and differences of drag characteristics between the fluidized nanoparticle agglomerate and the equivalent solid sphere were analyzed based on the drag ratio.The difference of drag characteristics between the fluidized nanoparticle agglomerate and the equivalent solid sphere is closely related to the porosity of pores between simple agglomerates.When the porosity of pores between simple agglomerates is less than 0.58,between 0.58 and0.77,and greater than 0.77,the drag ratio is larger than 1,close to 1,and less than 1,respectively.The permeability of fluidized nanoparticle agglomerates should be calculated based on the size of simple agglomerates and the porosity between them.The drag coefficient model developed based on the simulation results was verified by the experimental data.Based on the established drag coefficient model,the two-fluid method was used to simulate the fluidization behavior of nanoparticle agglomerates at typical experimental conditions.The simulation results were compared with those simulated with the drag coefficient model of the solid sphere and the homogeneous permeable sphere.The effects of the fluidization velocity on the bed height,the axial distribution of the pressure and agglomerate concentration,and the radial distribution of agglomerate concentration,gas velocity and agglomerate velocity were investigated for fluidization of nanoparticle agglomerates with monotonous agglomerate size and wide agglomerate size distribution.The applicability of the drag coefficient model for the simulation of nanoparticle agglomerate fluidization was investigated by comparing with the experimental data.The simulation results show that although the drag coefficients of single particle are significantly different between the models of the fractal agglomerate,the permeable sphere and the solid sphere,the difference of the fluidization simulation is not obvious.The bed height,axial pressure distribution and axial agglomerate concentration distribution of the three models are close to each other,and the fluidization of the three models all show the annular-core structure.With the increase of the fluidization velocity,the agglomerate concentration at the bottom of the fluidized bed decreases,while that at the upper part of the fluidized bed increases,which is consistent with the experimental results.The results show that the axial pressure distribution for simulation with the fractal agglomerate model is consistent with the experimental results,both of which show axial stratification in the fluidized bed,and the agglomerate concentration decreases with the increase of the axial height.