| Solid-liquid stirred tanks are widely used in chemical industry processes.In order to strengthen the mixing effect,it is very important to explore dynamic characteristics of each phase,interaction mechanisms between different phases,as well as design,optimize and scale-up the structure of the internals of the stirred tank.In this thesis,particle motion characteristics in different types of solid-liquid stirred tanks have been ystematically studied for industrial needss,including particle cluster in laminar stirred tanks,particle suspension and drawdown in turbulent stirred tanks,and particle residence time in continuous stirred tanks.The experimental photographing technique was used to explore the motion and cluster of particles in the high-viscosity laminar stirring tank.Results show that the density of particles is the key factor that determines whether particles can form a cluster directly,that is,only buoyant particles can realize it.Particles with different diameters have different critical Reynolds numbers for clustering,and the clustering of monodisperse particles and the separation of polydisperse particles can be controlled by changing Reynolds number.Due to the lack of research on the mechanism of particle clustering,the circular and "petal-shaped" clustering motion trajectories of particles are further analyzed by the CFDDPM method.The mechanism of particle clustering is analyzed,and it is found that the pressure gradient force on the buoyant particles pulls particles to follow the streamline of fluid within the isolated mixing region.Low-density particles occupy the inner central annular orbit preferentially,and high-density particles occupy the outer helical orbit.It provides a theoretical basis for the potential application of particle clustering.In view of the defects and limitations of the existing numerical models,a discrete element model coupled volume of fluid(DEM-VOF)parallel algorithm is developed based on FLUENT and EDEM in this thesis.On the one hand,it is used to study the problem of uneven particle suspension in the unbaffled turbulent solid-liquid stirred tank with free surface.The relationship between particle suspension and flow field is analyzed,such as the distribution of fluid velocity,turbulent kinetic energy and turbulent dissipation rate.A profiled bottom stirred tank is designed and optimized,which can improve the suspension uniformity of the particles by more than 20%.On the other hand,the DEM-VOF parallel algorithm is used to study the mechanism of the drawdown of floating particles and optimize the structure of the stirred tank based on the industrial demand that floating particles are difficult to drawdown.It is found that the drawdown process of particles can be divided into three stages: pull-down stage,dispersion stage and cycle stage.The mechanisms of the drawdown of particles include the force,collisions between particles,and the turbulence of free liquid surface.Baffles and larger impeller clearance can improve the pulldown efficiency.However,the larger impeller clearance will also hinder the dispersion of particles.The asymmetrical installation of the baffles has a significant effect on accelerating particle dispersion.The above results point out the direction for designing a stirred tank with high mixing efficiency.In view of the lack of research on the residence time of particles in continuous stirred tanks,studies on the residence time of particles in a continuous stirred tank are carried out by CFD-DPM simulations.Results have shown that the residence time of particles is different from that of fluids obviously.The average residence time of particles increases with the increase of particle diameter and particle density,and decreases when increasing feed rate.The average residence time of the particles decreases firstly and then increases with the increase of the rotation speed,where the turning point is the critical suspension rotational speed.It provides a data basis for macro-control of particle residence time and product quality. |
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