DEM-LES/DNS Coupling Simulation Of Particle Collision In Complex Two Phase Flows

The research is divided into three main parts, using the coupling methods of discrete element method-large eddy simulation/direct numerical simulation (DEM-LES/DNS) to investigate the particle-particle collision and related phenomena in particulate/granular flows, fluidized beds and swirling jets, respectively. To say specifically, the DEM is used for simulation of particulate/granular flows; the DEM-LES method is used for simulation of fluidized beds; and the DEM-DNS method is used for simulation of swirling jets.Firstly, for the study of particulate/granular flows, an experimental study of particle-particle collision rates is carried out by using high speed camera and and the PTV algorithm. The statistical correlation between the particle-particle collision rate and particle concentration, diameter and mean relative velocity is obtained. Meanwhile, the effect of gravity on the particle-particle collision rate is analyzed and discussed. Secondly, a numerical study on particle-particle collision rates in homogeneous and isotropic turbulence for particles with different Stokes numbers are performed to investigate the effect of turbulence on the statistical correlations of collision rates by using the direct numerical simulation and hard sphere model. Meanwhile, the effects of particle-particle collision on particle clusing/coagulation phenomenon for small Stokes numbers are illustrated. Then, the hard sphere model is used to study the characteristics on particle motion in a rotating tumbler with wavy boundaries based on the energy analysis and angular momentum analysis. The effects of wave number, amplitude and velocity of rotation of the wavy boundary on the motion pattern and collision pattern of particle clusters are analyzed. Finally, the soft-sphere model is used to study the mixing characteristics of particles within a tumbler, including the fractal feature of mixing interface and the information entropy characteristics of mixing, etc.For the study of fluidized beds, at first, the DEM-LES coupling method is developed to simulate the small scale three-dimensional fluidized beds with immersed tubes, analyzing the variation characteristics of gas-particle forces and the distribution characteristics of particle-tube collisions and tube erosions. Then, the same method is used to simulate a two-dimensional pulsed fluidized bed to investigate the effects of pulsed fluidization on modulation of the characteristics of gas-particle interactions, pressure drops and particle fluctuations, etc. Meanwhile, the effect of pulsed fluidization on change of distribution characteristics of particle-tube collision is analyzed. Finally, the effect of locally blowing gas flow on the particle-tube collision characteristics is studied in detail by the DEM-LES coupling method, including the cases with several immersed tubes and a tube bank in a two-dimensional bed respectively, and the case with immersed tubes in the three-dimensiona bed. The mechanisms of modification of particle-tube collision characteristics by the disturbance of locally blowing gas are illustrated.For the swirling jets, a direct numerical simulation study of vortex breakdown is carried out at first, and the coherent oscillation characteristics of flow variables within the breakdown region are observed. Secondly, the same method is used to study the spatial structure of vortex cores of swirling jets, and the instrinsic topology structures of vortex cores at different swirl numbers are illustrated. Moreover, the statistical characteristics of the flow fields are studied, including the mean velocity profiles, fluctuation profiles, etc. Then, the forces on particles, particle distribution and dispersion, the influencing facters on particle dipersion are studied systemically. A quantification analysis of the relative importance of particle forces is carried out. The distribution patterns of particles and the influencing facters on particle dispersion are revealed and discussed. Finally, the distribution charactersics of particle-particle collision in gas-solid swirling jets are investigated in detail by the DEM-DNS coupling method, including the collision distribution in the configuration space and velocity space, the relationship, e.g. correlation and modulation, between collision and turbulence, and the effects of Reynolds stress on particle-particle collision etc.