Numerical Simulation And Experimental Study On Multiphase Mixing Characteristics In Vertical Stirred Tank

Stirred tanks are integral part in many industrial applications,for example in chemical and metallurgical processes,food,water treatment,pharmaceutical engineering.because of its prominence,mixing in stirred kettles has been the focus of research.However,due to the complexity of the flow field,the mixing characteristics within the stirred kettle are not fully understood.Computational Fluid Dynamics(CFD),which not only visualize images but also display flow field information,which is suitable for studying the mixing characteristics of vertical stirred tank under turbulent state.In this paper,the coupled CFD-DEM technique,combined with the VOF model and the multiple reference system(MRF)model for data exchange at the intersection,will be used to study the gas-liquid,solid-liquid and gas-liquidsolid mixing characteristics in the stirred kettle,providing some guidance for the stirring operation.In order to verify the reliability of the simulation results and to determine the accuracy of the relevant parameters,a visual stirred reactor was firstly fabricated using Plexiglas for the experimental study.Secondly,the mass,density and collision recovery coefficients of the particles were measured.And the optimal combination affected by many parameters such as the ratio of blade diameter,height from bottom,number of blades was selected using orthogonal experiments.And the ratio of blade diameter,height from the bottom,number of blades and blade inclination Angle are 0.5,48 mm,4 and 45° respectively.For gas-liquid flow,the influence of different rotational speeds and liquid level heights was studied on the basis of the model verification by the combination of simulation and physical experiment.The flow pattern,liquid surface morphology,turbulent kinetic energy distribution and tangential velocity were analyzed to investigate the mechanism of the gas-liquid flow and the influence of the relevant parameters on the flow in the kettle.The results show that the critical radius is larger with the increase of rotation speed,and the critical radius is larger with the decrease of initial liquid level,but all smaller than the blade radius.For the solid-liquid flow,the simulation model is validated on the basis of the experimental study.The effects of different parameters,such as particle density,particle size,impeller speed,liquid level height on the particles mixing characteristics were studied by using the validated model.The internal mechanism of the influence of different parameters on particle mixing was investigated by analyzing the velocity distribution,turbulent kinetic energy,particle mixing degree and spatial volume distribution of the flow field.The results show that the particle density has the greatest effect on the degree of mixing.When the particle density is 1200kg/m~3,the mixing performance is the best.The particle size and liquid level height have little effect on the mixing performance.It is shown from the velocity component that the axial velocity and tangential velocity change sharply in the stirred tank with baffle.The turbulent kinetic energy in the stirred tank is symmetrically distributed and the turbulence near the blade is the highest.The improvement of particle suspension performance will cause the circulation center to move down.Finally,a preliminary study of the mixing characteristics of the gas-solid-liquid in the stirred tank was carried out.The analysis of the particle motion,the liquid velocity distribution and the gas volume distribution initially revealed the movement of the three phases of mixing.