Study On Suspened Characteristics Of Floating Particles In Stirred Tank

Solid-liquid suspension operation is widely used in daily life and industrial production.The suspension of floating particles in the stirred tank is more common in a number of chemical unit operation applications such as food processing technology,polymerization,fermentation and wastewater treatment.In the previous studies,the mechanism of floating particles under critical conditions is relatively rare.Moreover,the research on system of floating particles of solid-liquid mixing mainly focuses on the two-phase mixing with small particle size particles,the analysis of particles in large particle size and solid-liquid flow field has not been achieved.In order to further improve the understanding of the behavior and mechanism of the critical drawdown motion of floating particles,in this paper,PIV and CFD simulation methods were used to investigate the hydrodynamic characteristics of the fluid and the motion of the floating particles in large particle size under low Reynolds number.In this paper,a certain ratio of sucrose-sodium chloride solution and 9.6mm PMMA spherical particles are used for refractive index matching.The just drawdown impeller speed and complete drawdown speed at different operating conditions were investigated.The flow characteristics of the fluid and particle characteristics in the stirred tank with particle volume fraction ranging from ?=0 vol.%to ?=8 vol.%is investigated by two-dimensional PIV technology.The method of PIV and CFD numerical simulation is used to obtain the drawdown mechanism of floating particles under laminar flow.Under the just drawdown condition,the tangential velocity of the fluid and the collision between the particles are the main factors leading to the particle drawdown.The research results can provide effective data and model support for the critical motion mechanism of floating particles in complex solid-liquid mixing system.In a square stirred tank,the fluid flow and the particle suspension characteristics of the floating particle system under the just drawdown condition are studied.This will support a foundation for the study of particle suspension characteristics under the complete drawdown condition.PIV technique was used to analyze the velocity field of the fluid,and the flow field and particle motion characteristics were predicted by DPM(Discrete Phase Model)and DEM(Discrete Element Method Collision Model)model in FLUENT software.The conclusions are as follows:(1)It is easier to achieve the drawdown of floating particles at down impeller pumping mode.When increase the off bottom clearance from the bottom the just draw down speed decreased.The experimental investigation has shown that when increasing the particle volume fraction the just draw down speed increased.(2)Compared with single-phase flow field,the radial velocity attenuation of the multiphase flow field at a particular location is about 20%and the axial velocity attenuation is about 13%under the same just drawdown impeller speed.(3)The tangential velocity at the location near the liquid level under different operating conditions is much greater than the radial and axial velocity.The conclusions of the PIV experiments and CFD simulation at the complete drawdown conditions are as follows:(1)when gradually increases of the particle volume fraction,the presence of particles causes a certain effect on the flow field in the tank,CFD simulation results are in good agreement with PIV experimental values.(2)The change of the phase angle of the blade in the stirring tank of the floating granule system will affect the flow flield in the stirred tank.Based on the study of laminar flow for the critical drawdown and complete drawdown conditions of floating particles,a preliminary exploratory analysis of the critical drawdown behavior of single floating particles is carried out by experiments and LBM direct numerical simulations,which will provides a reliable method for the theoretical study of the critical drawdown motion of single particle.In the study of the critical drawdown behavior of single particles in the laminar flow,the high-speed imaging technique is used to capture the trajectory of the single particle.At the same time,the LBM simulation method was used to predict the particle motion directly.The conclusions are as follows:(1)when the impeller at downward pumping mode,it needs a short time to achieve the drawdown of single particle.The larger the particle size,the shorter the time the particles reach the impeller area in stirred tank.(2)The flow field distribution,the critical drawdown process and the drawdown motion trajectory of the single particle simulated by LBM are in good agreement with the experimental results.