Studies On The Formation And Structure Of Agglomerates In Fluid-particle Flow

The particle-fluid flow featuring spatio-temporal multi-scale structures is typically a complex system,in which agglomerates are the most commonly observed heterogeneous structure.The characteristic of agglomerates has a significant effect on the normal operation of fluidized beds or other reactors.At present,the research on agglomerates is mainly related to the macro-statistical properties,such as size and concentration distributions.However,the studies on the formation process and structural characteristics of agglomerates are very limited.In this thesis,a fluid-particle flow containing millions of cohesive particles was simulated using discrete particle method(DPM),a method for recognizing and quantifying the agglomerates was then proposed.Taking full advantage of DPM that the location and velocity of any particles at any moment can be tracked,the structure of agglomerates are characterized via their size,shape and stability,the process of particles entering or leaving agglomerates can be tracked,the growth and breakage process of all agglomerates therefore can be monitored quantitatively.The mechanism of agglomerates was finally explored in term of force analysis.Chapter 1 reviewed the state-of-the-art of the studies related to agglomerates,the discrete particle method used in this thesis and experiments was then summarized in chapter 2.It was shown that the agglomerates can be observed in both experiment and simulation.The average velocity as well as the volume fraction of particles in the central region of the reactor has a fair agreement.Chapter 3 proposed a method for recognizing and quantifying agglomerates,the influences of reactor thickness,feed state,and cohesive strength on the simulation results were explored,and the structure characteristics of agglomerates were investigated from the aspects of size,shape and stability.The results indicated that the number of particles in an agglomerate can represent its size effectively,the average coordination number and the distribution of coordination number of particles in agglomerates indicated the shape of agglomerates.The average surface distance between coordinated particles and the average residence time of particles in agglomerates indicated the stability of the agglomerates.In Chapter 4,we quantified the growth and breakage of agglomerates.It was shown that the growth and breakage of agglomerates can be continuous or abrupt,although most situations are continuous.In addition,the non-uniform distributions of cohesive force,drag and collisional force had a significant impact on the net change rate,size and shape of agglomerates.In Chapter 5,we summarize the main conclusions and present a perspective on future work.