Simulation Of Dilute Pneumatic Conveying By CFD-DEM

Pneumatic conveying is a method of transportation of granular particles in a pipeline using a gas stream, and has been extensively employed across a wide range of industries, including the agriculture, mineral, chemical, pharmaceutical, coating and metallurgical industries. It can be broadly divided into two broad categories:dilute phase and dense phase. It is important to understand the mechanism of the particle flow in dilute pneumatic conveying pipelines for its design and optimization of the process.A three-dimensional CFD-DEM model is used to simulate dilute pneumatic conveying. In CFD-DEM, the particle phase is obtained by Discrete Element Method (DEM), and the flow of continuum gas phase is determined by Computational Fluid Dynamics (CFD) according to the Navier-Stokes equations. The coupling between the two phases is achieved through the calculation of the momentum sink of the drag force. Compared with other simulation models, CFD-DEM which accounts for the particle collisions and can obtain the detail informations of the particle motion and force, is recognized as an effective method to study the fundamentals of pneumatic conveying. But there are some problems on the simluation stability and efficiency. Some new techniques have been implemented to solve these problems include:(1) A methodology for determining voidage based on the part volume of the particle is used to improve the accuracy of the voidage calculation to avoid the simulation instability; (2) A larger time step has been used to speedup the simulation with the reduction of the stiffness according to the effect of the stiffness on the results of dilute pneumatic conveying simulation; (3) A simplified CFD-DEM model which ignores the effect of the solid volume fraction on the gas phase is introduced for its less compute-intensive.Compared with the experiment data, the traditional CFD-DEM model in dilute pneumatic conveying is verified, and various drag force models also have been compared. The simplified CFD-DEM model has been validated by compared the predictions of pneumatic conveying with different types of bends for the traditional CFD-DEM model. It is indicated that the simplified CFD-DEM model can predict the particel rope formation and dispersion in horizontal-vertical bend case and vertical-horizontal bend case quantitatively with the traditional model, and in horizontal-horizontal bend case qualitatively. The results illustrate that there are dramatic effets of the layout direction of bends on the particle rope features. The particle rope disperses and the particles finally ditribute uniformly in vertical pipe in horizontal-vertical bend case. The rope disperses quickly and the particles deposit to the bottom of the horizontal pipe as the effect of gravity in vertical-horizontal bend case. The particles move along a spiral line and keep close to the inside of the pipe wall at downstream of the bend in horizontal-horizontal case.The particle rope formation and dispersion are sensitive to the operational conditions. The effect of gas velocity, solid mass flow rate, bend radius ratio and coefficient of the wall friction is studied by compared the particle rope strength, dispersion rate, particle velocity and the collision information in different types of bends. It is indicated that the gas velocity can change the particle velcoity, and the solid mass flow rate and the bend radius ratio will effect the particle rope strength and dispersion rate. There is a significant influence to the rope dispersion for the coefficient of the wall friction. In all cases, the collisions mainly occur in the bend section and the particle-wall collision is the major collision type in dilute phase. The change trend of the particle rope dispersion rate with the bend radius ratio obtained by CFD-DEM is consistent with the epxeriment result. It illusrates that collison has an obviously influence to the particle flow even though in dilute pneumatic conveying.In multi-sphere element method, the non-shpere particle can be represented by several spherical elements. The results depends strongly on the number and the arrangement of the sperical elements. There are significant differences between the results of 4-element and 7-element arrangements for corn-shape particle. The simplified CFD-DEM model is also applicable on non-spherical dilute pneumatic conveying simulation according to the results of the horizontal-vertical case for both models. The particle rope dispersion and particle velocity at downstrem of the bend are different for sphere, corn-shape, plate and cylinder particles. The multi-sphere element method can obtain the same flow features of the plate particle with the polyhedron method. Meanwhile, The plate particles also form a stronger rope than spherical in horizontal-horizontal case.The pressure drop in inclined pipe increases firstly and then reduces with the raise of the angle, and the particle velocity in inclined pipe is smaller than that in horizontal and vertical pipe. In the design of the pipe layout, the inclined pipe can reduce the transportation distance and the angle of the bend, meanwhile the particle collision intensity and the pressure gradient also decrease. In addition, a combination of two bends forms a complicated particle rope which disperses slowly. The length of the pipe between two bends plays a key role on the particle rope features, and it should be determined carefully to avoid a dramatic low velocity that the particles approach to the second bend with.