DEM-VOF simulations on the drawdown mechanisms of floating particles at free surface in turbulent stirred tanks

Abstract

• The drawdown process is divided into three stages. • The force acting on floating particles is one main drawdown mechanism. • Particle collision and turbulence kinetic energy of free surface are also mechanisms. • OED was used to study the effects of particle properties and stirred tank geometries. The drawdown of floating particles in turbulent stirred tanks is widely encountered in industrial processes. However, the data oriented drawdown mechanisms and the dependence of particle dispersion on particle properties and stirred tank structures are lacking. Especially in the simulation works, the effect of the free liquid surface on the drawdown of particles is commonly ignored. In this work, discrete element model coupled with volume of fluid method (DEM-VOF) is used to investigate the influence of particle properties and stirred tank structures on the drawdown processes with an orthogonal experimental design (OED). Both up-pumping and down-pumping of a 45° pitched blade (PBTU and PBTD) in the flat-bottomed stirred tank are studied. The method has been verified and validated with the invasive image-based experiments. Here, it is shown that the drawdown processes of PBTU and PBTD are different. The drawdown process is divided into pull-down stage, dispersion stage and cycle stage. Particle-particle collision, the total force, the coupling force and the turbulent kinetic energy of the free liquid surface are the main drawdown mechanisms. Baffles and the impeller clearance are key factors. The optimal combinations of factors level for the most uniform distribution and the least power consumption are obtained by OED, which provides reference for further industrial applications.