Compuational Study Of Pneumatic Conveying By Discrete Element Model

The pneumatic transport of granular material is a common operation frequently employed to transport solid particles from one location to another in chemical, metallurgical and agricultural industries. It is environmentally friendly, flexible and can be fully automated. But it may also involve high power consumption, wear, abrasion, blockage and particle degradation. Hence understanding the physics of the pneumatic conveying can help to optimize the design and operation of the process.A three-dimensional combined approach of the discrete element model (DEM) for particles and CFD computational fluid dynamics (CFD) for fluids is developed in this work, which can be used to simulate pneumatic transportation of solid particles along conveying pipelines such as straight pipes and bends. In CFD-DEM, the motion of discrete particles is obtained by DEM which applies Newton's laws of motion to every particle, whereas the flow of continuum fluid is described by the local averaged Navier-Stokes equation that is solved by the traditional CFD. The approach can reproduce the assembly physics, and generate the detailed particle-scale dynamics information such as trajectories and velocity of and forces on particles for the fundamental understanding of pneumatic covneying. The CFD model is based on the finite volume method in non-staggered body-fitted grids. In order to improve numerical efficiency and stability, some new modelling techniques have successfully been implemented in the developed model. These include: (1), the point-locating approach for relating particle positions to grid cells, (2) the least-square interpolation of gas properties, (3) suitable periodic boundary conditions for both gas and solid phases in the flow direction, and (4) the concept of spherical cell in the calculation of porosity and particle-fluid interaction force. The comparison of numerical and experimental results shows that the developed CFD model can quantitatively predict the flow fields in the lid-driven cavity, tundish, ladle, and the developed CFD-DEM can quantitatively predict the slug velocity, height of settled layer, and particle velocity in horizontal slug flow, and the pressure drop and mean voidage in vertical dilute-phase pneumatic conveying.The developed CFD-DEM model is applied to the study of the horizontal stable slug flow, the unstable motion of horizontal slugs, and the flow regimes in a vertical pipe, followed by the detailed force analysis in each case. It is shown that that the current model can capture the key flow behaviours in pneumatic conveying as follows: (1) the exchange of particles between a slug and a settled layer, gas pressure characteristic, inter-relationship between gas velocity, slug velocity and particle velocity, and effects of gas and solid flowrates on slug velocity and slug length in the horizontal stable slug flow; (2) the unstable motion of slugs including start-up, formation, and combination, and the periodic characteristic of slug flow in horizontal pneumatic conveying; and (3) the flow regimes and flow transition in the vertical pneumatic conveying. These results are consistent with the experimental observations.The following results can be obtained by the force analysis: (1) in the horizontal stable slug flow, the movement of a slug is macroscopically controlled by the axial particle-fluid and particle-wall interactions, whereas the particle-particle interaction microscopically causes a slug to sweep up particles in a settled layer. The magnitudes of these interaction forces increase with gas and solid flowrates; (2) the structure of normal contact force inside a horizontal slug has a uniform, non-uniform, and uniform distributions in turn in the axial direction when the slug achieves a stable state from a static state; (3) In the vertical pneumatic conveying, the mechanisms underlying the relationship between pressure drop and gas velocity can be explained using the particle-fluid force and gas-wall friction force, and a new phase diagram in terms of the key forces can be established to identify dilute-phase flow and dense-phase flow in vertical pneumatic conveying.