by Ke Li

Institution: University of New South Wales
Department: Materials Science & Engineering
Year: 2014
Keywords: Pneumatic conveying; Discrete element method; Computational fluid dynamics; Friction coefficient; Restitution coefficient; Flow transition mode; Circulating fluidized bed; Dense ring, Large scale
Record ID: 1065372
Full text PDF: http://handle.unsw.edu.au/1959.4/53939


ABSTRACT This work presents a numerical study of horizontal pneumatic conveying by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM), with special reference to the use of periodic boundary condition (PBC) for computational efficiency. A new iterative method is proposed to generally control the solid flow-rate simulated in a CFD-DEM model with PBC. The characteristics of the flows in the start-up section are analyzed in detail and compared with those in the well-developed flow section for different flow regimes. On this basis, two semi-theoretical correlations are formulated to respectively predict the start-up section length and the relation between particle number and solid flow-rate. The applicability of PBC to the CFD-DEM modelling of pneumatic conveying, as well as the two correlations, is verified by comparing the measured and calculated results under different conditions. A numerical study is carried out to investigate the effects of friction and restitution coefficients of particles on horizontal and vertical pneumatic conveying with CFD-DEM method. The flow behaivor is analysed in details in terms of particle flow pattern, solid concentration, gas pressure drop, and gas and solid velocities. It is shown that a change in either friction coefficient or restitution coefficient of particles being conveyed can lead to three flow transition modes at a constant solid flow-rate depending on the gas velocity, which can be represented by Mode I for dilute-phase only, by Mode II for an unsmooth transition from dilute-phase to unstable-zone and finally to a slug-flow, and by Mode III for smooth transition from dilute-phase to slug-flow. A diagram is established to predict these transition modes. Moreover, the reverse flow is studied in detail and a diagram in terms of gas velocity and friction coefficient is plotted to demonstrate the existence of this phenomenon. The numerical study of the characteristics of fine particles in gas-solid flow is also proposed considering the riser and downer in a circulating fluidized. The flow behavior is analysed in details in terms of particle flow pattern, cluster behaviors, gas and solid velocities and forces acting on particles. In addition, the effects of operation conditions and material properties on the radial particle distribution in the downer are examined under well-controlled conditions. The results show that the CFD-DEM model can reproduce the main phenomena in the riser and downer including the clusters and annular dense distributions. The annular dense distributions can be attributed to the interactive effects of particle-fluid force, gravity and particle-wall friction force. Large gas velocity leads to non-uniform distribution of the radial particle distribution. The dense rings cannot be observed when the solid concentration is low and the pipe diameter is small. Large particle diameter and high restitution coefficients also eliminate the dense ring distribution. The increased particle friction coefficient leads the swift of…