Research On Particle Agglomeration And Disaggregation In Vortex Pneumatic Flow Field

Powder separation has shown unique advantages due to its simple operation,high efficiency,and low cost in the field of powder production.However,ultrafine particles with a particle size of less than 10μm have a large specific surface area and large surface energy,and are prone to agglomeration,which brings great difficulties to the preparation of ultrafine powders.Studying the cause of powder agglomeration and disaggregation mechanism can provide a theoretical basis for taking disaggregation methods.The turbo air classifier is a common equipment for preparing ultrafine powder by separating coarse and fine particles through the pneumatic flow field.It has the characteristics of large output,high efficiency and wide use in the field of powder preparation.As the industry has higher and higher requirements for ultra-fine powders with small particle size and narrow distribution range,it is a critical problem to analyze the particle agglomeration in the turbo air classifier and make agglomerate disaggregation.In this paper,the force and influence factors of particles in the flow field of the classifier are analyzed,and the numerical simulation of the classifier is carried out.The simulation results are analyzed in combination with theory,and the reasons for agglomeration and the mechanism of agglomeration are explored,and the measures for agglomeration are proposed.The effect of particle motion characteristics on the flow field on agglomeration is studied.From the point of view of flow field shear force and collision,the mechanism of deblocking is analyzed.The structure is improved according to the mechanism of desorbing to improve the dispersion of the powder.The main contents are as follows:The powder is simplified as spherical particles,and the forces of ultrafine particles in the process of air flow classification are analyzed and calculated.The effect of particle size ratio,moving velocity,physical properties,air velocity and collision between particles on agglomeration are discussed.The results show:for dry particles,van der waals forces are the main cause of agglomeration.For wet particles,the agglomeration of particles with a particle diameter below 2.5μm is mainly due to van der Waals forcess,aggregation of particles with a diameter greater than2.5μm is mainly due to liquid bridge forces.The collision force are the main forces for agglomeration and disaggregation.The agglomerations formed by small particles and large particles are more difficult to disaggregation.The collision force between particles increases with the increase of the relative collision velocity,showing a linear relationship,and the slope is related to the particle size ratio.The fluid drag force of particles increases with the increase of air velocity.When the flow velocity is small,the drag force increases rapidly,and the drag force increases slowly with the increase of flow velocity.The CFD-DEM coupling is used to simulate the movement of aggregates and their disaggregation process in the annular region near the guide vane of the turbo air classifier.The degree of influence of different inlet air velocity on the disaggregation of aggregates is studied,and the mechanism of aggregate disaggregation of the in the area near the guide vane in the annular area is revealed.The simulation results show that disaggregation of the aggregates near the guide vanes in the flow field is caused by the collisions between the aggregates and the wall of guide vanes,but the shearing force of the flow field.When the rotating velocity of the rotor cage is 1200rpm and the inlet air velocity is 6,12,18,and24m·s^-1,respectively,the proportion of agglomerates after partial disaggregation does not exceed 4%,indicating that a single agglomerate has a greater degree of disaggregation in the flow field;the proportion of single particles increases from 71.7%to 88.39%and the proportion of the aggregates decreases from 24.8%to 10.51%with the increase of inlet air velocity.It indicating that the increase of inlet air velocity will improve the dispersion of powder near the guide vanes in the turbo air classifier.The introduction of dimensionless parameters relative to collision parameters verifies this conclusion.The theoretical and numerical simulation analysis are carried out on the two structures of guide vane tail with and without cylinder and the diameter of guide vane tail cylinder.The influence of guide vane tail cylinder on powder dispersion and convection field near the guide vane in the annular area of the classifier is studied.The conclusions are as follows:the cylinder at the tail of the guide vane will cause the powder to gather at the junction of the cylinder and the guide vane wall,and most of them still exist in the form of single particles after disaggregation.The powder dispersibility in the control volume with improved structure is better.When the inlet air velocity is less than or equal to 12m·s^-1,the improvement of the tail cylindrical structure can better disperse the powder;when the inlet air velocity is greater than or equal to 12m·s^-1,the effect on powder dispersion gradually becomes flat.When the diameter of the tail cylinder is less than 2mm,increasing the diameter of the tail cylinder of the guide vane has no obvious effect on the powder dispersion;when the diameter is greater than 2mm,the effect of increasing the diameter of the guide vane tail cylinder on powder dispersion will become more and more obvious.As the diameter of the tail cylinder increases,the tangential velocity in the annular area decreases more and the fluctuation becomes smaller,which is beneficial to the stability of the tangential velocity field.The tail cylinder of the guide vane has little effect on the radial velocity of the airflow.The cylinder at the tail of the guide vane makes more small vortices near the guide vane,which is conducive to powder collision and improve dispersion.