Particle flocculation in a stirred tank: A microscopic test by coupled CFD-DEM approach

Particle flocculation in a stirred tank was numerically investigated by a coupled CFD-DEM approach, in which a microscopic test in a typical zone was first proposed instead of solving the full-scale particle field. The flocculation kinetics was described by the Johnson-Kendall-Roberts (JKR) theory, followed by calibration of surface energy parameter. A Volume of Fluid (VOF) model was employed to capture the interface between liquid and air. The two-way coupling of fluid and particle was achieved by resolving pressure gradient force and Gidaspow's drag force in the momentum equations. Based upon the qualitative and quantitative validation tests in air-water interface pattern and fluid tangential velocity, respectively, the particle coordination number and flocs fractal properties (e.g. fractal dimension, voidage, effective density) were investigated considering the effect of impeller speed. Results show that the number of particles first increases with particle coordination number and then decreases, and is further positively correlated with the stirring intensity in lower coordination numbers (0-3) while negatively correlated in higher coordination numbers (>= 5). The influences of turbulence dissipation rate on the average particle coordination number were also investigated in radial, tangential, and axial directions. The mean floc size decreases with impeller speed, however, the fractal dimension increases in a certain range. The voidage increases with the daughter-particle number in a floc while the effective density of flocs is inversely proportional to the floc size. It is recommended to adopt operations that facilitate the one-by-one attachment mode and syneresis for forming relatively large and compact flocs.