CFD-DEM simulation of complex particle-fluid flows

Complex three dimensional particle-fluid flow operations (for example pneumatic conveying systems, cyclonic separator unit operations, circulating fluidised beds and dense medium cyclones) are encountered in many industries including energy, mineral, metallurgical, petroleum, environmental, pharmaceutics and materials, and is core in process/chemical engineering. One approach to optimise these industrially important operations is to develop a fundamental understanding of the underlying physics and mechanics of each operation. With this knowledge, the application of each technology to particular circumstances can be optimised to maximise business profitability. In this treatise presented in this thesis, a fundamental model, i.e., combined discrete element method (DEM) and computational fluid dynamics (CFD) (DEM-CFD or CFD-DEM) model, has been attempted in a numerical study of complex three dimensional particle-fluid flows. In the DEM-CFD model, the motion of particles is modelled as a discrete phase, described by Newton's laws of motion applied at an individual particle scale, and the flow of fluid (gas or liquid) is treated as a continuum phase, described by the local averaged Navier-Stokes equations applied at a computational cell scale, while allowing the mutual interaction (e.g., momentum, heat and mass transfer) between particle and fluid phases. This method is recognised as an effective approach of studying the fundamentals of particle-fluid flow by various investigators. It is now increasingly used worldwide, but so far, its application is limited to relatively simple flow systems, mainly because the code development for the solution of fluid phase can be very challenging and time-consuming when complicated dense particle-fluid flow systems are considered. In this approach developed in this thesis, taking advantage of the available CFD development, a DEM-CFD model has been developed and extended, and for the first time combined with the most popular commercial CFD software, i.e. Fluent, as the platform, achieved by incorporating explicit DEM code and a coupling scheme between DEM and CFD into Fluent through its User Defined Functions (UDFs). The applicability of the developments is demonstrated in the study of the particle-fluid flow in various complex 3D systems including pneumatic conveying bends, cyclone separator, circulating fluidised bed (CFB), and dense medium cyclones (DMCs). By comparing the numerical findings with experimental measurements, it is shown that the numerical results qualitatively capture some of the flow features typical to those flow systems considered, such as: roping and particle segregation in the pneumatic conveying bends; strands flow and reduction of tangential velocity after loading solids in the gas cyclone separator; core-annulus flow structure and axial solid segregation in the CFB; and “surging”, performance deterioration with decrease of medium-to-coal ratio and particle size, and the extraordinary effect of coal type in the DMCs.Quantitative agreements between simulated results with those measured were also obtained in various aspects, such as the periodical fluctuation of pressure drop in the pneumatic conveying bend, the decrease of pressure drop with increase of solid loading rate in the gas cyclone separator, the separation error and density as a function of particle size in the DMC.The internal flow structures are analysed by use of particle velocity and concentration, fluid/slurry flow field (velocity, pressure and density), particle-fluid, particle-particle and particle-wall interaction forces, which leads to an improved understanding of the flow mechanisms of these systems. The obtained intensity of particle-particle and particle-wall interactions can be used to study the attrition of solids and the erosion (wear) of walls in those systems respectively