Aibing Yu

Numerical simulation of the gas-solid flow in a fluidized bed by combining discrete particle method with computational fluid dynamics

Abstract The gas-solid flow in a fluidized bed is modelled by a combined approach of discrete particle method and computational fluid dynamics (DPM-CFD), in which the motion of individual particles is obtained by solving Newtons second law of motion and gas flow by the Navier-Stokes equation based on the concept of local average. The coupling between DPM and CFD is achieved directly by applying the principle of Newtons third law of motion to the discrete particle and continuum gas which are modelled at different length and time scales. The equations of motion for a system of particles are solved by a collision dynamic model developed in this work which, in conjunction with the predictor-corrector method, allows stiff particles ( κ = 50,000 Nm −1 ) to be used with a reasonable computational time step (1.5 × 10 −5 s) while conserving the energy and momentum. The gas-phase equations are solved by the conventional SIMPLE method facilitated with the Crank-Nicolson scheme to give the second order accuracy in the time discretization. The proposed model shows its capacity of simulating the gas fluidization process realistically from a fixed to fully fluidized bed via an incipient fluidization stage. This is done by a series of numerical tests to reproduce the experimental procedures in determining the minimum fluidization velocity of 2400 particles ( ϱ p = 2700 kgm −3 , D = 4 × 10 −3 m) in a pseudo-three-dimensional central jet fluidized bed of dimensions 0.9 × 0.15 × 0.004 m. The hysteretic feature of bed pressure drop vs superficial gas velocity curve is obtained for the first time realistically from first principles, with the predicted minimum fluidization velocity in good agreement with experiment. It is demonstrated that the proposed model is able to capture the gas-solid flow features in a fluidized bed from the largest length and time scales relevant to the processing equipment down to the smallest ones relevant to the individual particles.

Rolling friction in the dynamic simulation of sandpile formation

The contact between spheres results in a rolling resistance due to elastic hysteresis losses or viscous dissipation. This resistance is shown to be important in the three-dimensional dynamic simulation of the formation of a heap of spheres. The implementation of a rolling friction model can avoid arbitrary treatments or unnecessary assumptions, and its validity is confirmed by the good agreement between the simulated and experimental results under comparable conditions. Numerical results suggest that the angle of repose increases significantly with the rolling friction coefficient and decreases with particle size.

Discrete particle simulation of particle–fluid flow: model formulations and their applicability

The approach of combining computational fluid dynamics (CFD) for continuum fluid and the discrete element method (DEM) for discrete particles has been increasingly used to study the fundamentals of coupled particle–fluid flows. Different CFD–DEM models have been used. However, the origin and the applicability of these models are not clearly understood. In this paper, the origin of different model formulations is discussed first. It shows that, in connection with the continuum approach, three sets of formulations exist in the CFD–DEM approach: an original format set I, and subsequent derivations of set II and set III, respectively, corresponding to the so-called model A and model B in the literature. A comparison and the applicability of the three models are assessed theoretically and then verified from the study of three representative particle–fluid flow systems: fluidization, pneumatic conveying and hydrocyclones. It is demonstrated that sets I and II are essentially the same, with small differences resulting from different mathematical or numerical treatments of a few terms in the original equation. Set III is however a simplified version of set I. The testing cases show that all the three models are applicable to gas fluidization and, to a large extent, pneumatic conveying. However, the application of set III is conditional, as demonstrated in the case of hydrocyclones. Strictly speaking, set III is only valid when fluid flow is steady and uniform. Set II and, in particular, set I, which is somehow forgotten in the literature, are recommended for the future CFD–DEM modelling of complex particle–fluid flow.

An experimental and numerical study of the angle of repose of coarse spheres

This paper presents a numerical and experimental study of the angle of repose of mono-sized coarse spheres, a most important macroscopic parameter in characterising granular materials. Numerical experiments are conducted by means of a modified discrete element method. Emphasis is given to the effects of variables related to factors such as particle characteristics, material properties and geometrical constraints. The results show that under the present simulation conditions, the angle of repose is significantly affected by the sliding and rolling frictions, particle size and container thickness, and is not sensitive to density, Poissons ratio, damping coefficient and Youngs modulus. Increasing rolling or sliding friction coefficient increases the angle of repose, while increasing particle size or container thickness decreases the angle of repose. Based on the numerical results, empirical equations are formulated for engineering application. The proposed simulation technique and equations are validated by comparing the physical and numerical experiments, where focus is given to the effects of particle size and container thickness.

Nanoarchitectured Design of Porous Materials and Nanocomposites from Metal‐Organic Frameworks

The emergence of metal-organic frameworks (MOFs) as a new class of crystalline porous materials is attracting considerable attention in many fields such as catalysis, energy storage and conversion, sensors, and environmental remediation due to their controllable composition, structure and pore size. MOFs are versatile precursors for the preparation of various forms of nanomaterials as well as new multifunctional nanocomposites/hybrids, which exhibit superior functional properties compared to the individual components assembling the composites. This review provides an overview of recent developments achieved in the fabrication of porous MOF-derived nanostructures including carbons, metal oxides, metal chalcogenides (metal sulfides and selenides), metal carbides, metal phosphides and their composites. Finally, the challenges and future trends and prospects associated with the development of MOF-derived nanomaterials are also examined.

Evaluation of the packing characteristics of mono-sized non-spherical particles

Abstract An experimental study has been carried out under both loose and dense random packing conditions to quantify the dependence of the packing characteristics such as porosity and packing size on particle shape. It is shown that the porosity of mono-sized particles is strongly dependent on both particle shape and packing method, and for a given packing method, the porosity-shape relation can be approximately by the proper use of the packing results of cylinders and disks. The results indicate that the equivalent packing diameter of a particle is only affected by particle shape and not sensitive to the method of evaluation including factors such as the packing method and the mixing composition. Based on the measurements, correlations have been formulated for predictive purposes. The formulated porosity-shape and packing size-shape relations are demonstrated to be useful in particle characterization and in porosity prediction of non-spherical particle mixtures.

Simulated and measured flow of granules in a bladed mixer—a detailed comparison

Abstract Experimental measurements and simulations of a granular flow system have been compared in detail. Positron emission particle tracking (PEPT) was used to study the motion of glass beads in a vertical axis mixer with slowly rotating flat blades. Unlike conventional techniques which are restricted to measuring the flow at the surface or a transparent wall, PEPT revealed the motion of material throughout the entire bed. The flow produced was three dimensional with vortices, and is more complicated than other granular flows that have been described previously. Discrete element method (DEM) was employed to simulate the same system using various sets of parameters for the bed material. As there are extensive three-dimensional flow data from both the experiment and the simulations, it has been possible to make comprehensive quantitative comparisons. This enabled the accuracy of the sets of assumptions for the DEM simulations to be examined. The simulations predicted the overall motion of the bed well. However no one set of assumptions was best, but that different sets predicted the detailed motion more accurately in different parts of the bed. None the less, it is evident that DEM models can now be used with some confidence to explore mixer design and performance.

On the modelling of the packing of fine particles

Abstract It is postulated that the effect of absolute particle size on particle packing be modelled by use of (i) an initial porosity to take into account the packing of mono-sized particles and (ii) the concept of packing size ratio as a measure of the particle-particle interaction in forming a packing of mixed powders. This hypothesis is tested by the experimental measurements of the standard poured and tapped packing densities of white fused alumina powders. The results indicate that simple measurements can lead to the formulation of equations to quantify initial porosity and the packing size ratio. The porosity of multicomponent mixtures of powders can be predicted by the modified linear packing model as facilitated by the equations formulated.

Microdynamic analysis of particle flow in a horizontal rotating drum

The flow of particles in a horizontal rotating drum is studied based on the results generated by Distinct Element Method (DEM). The simulation conditions are comparable to those measured by means of Positron Emission Particle Tracking (PEPT), with a drum being 100 mm in diameter, 35% filled by spheres of 3 mm diameter, and rotating at a speed from 10 to 65 rpm. The simulation method is validated from its good agreement with the PEPT measurement in terms of the dynamic angle of repose and spatial velocity fields. The dependence of flow behaviour on rotation speed is then analysed based on the DEM results, aiming to establish the spatial and statistical distributions of microdynamic variables related to flow structure such as porosity and coordination number, and force structure such as particle interaction forces, relative collision velocity and collision frequency. An attempt has also been made to explain the effect of rotation speed on agglomeration based on the present findings.

Numerical simulation of the gas-solid flow in a bed with lateral gas blasting

Abstract This paper presents a numerical study of the gas–solid flow in a bed by a Combined Continuum and Discrete Model (CCDM). Numerical experiments are carried out to simulate the motion of 10,000 spherical particles of 4 mm diameter caused by lateral gas blasting into a bed with its thickness equal to the diameter of particles. It is shown that, depending on the gas velocity, the bed can transform from a fixed bed to a fluidised bed or vice versa. Two zones can be identified in such a bed: a stagnant zone in which particles remain in their initial positions, and a mobile zone in which particles can move in various flow patterns. If the gas velocity is in a certain range, the mobile zone is confined in front of the gas inlet, forming the so-called raceway in which particles can circulate. If the gas velocity is higher than a critical value, fluidisation results, with the mobile zone growing by the combined effect of bubble penetration and shearing between moving and static particles until a stable state where the boundary separating the mobile and stagnant zones is unchanged. The dependence of raceway and fluidisation phenomena on gas velocity has been examined in terms of the size and shape of the mobile zone, gas-solid flow patterns and forces acting on individual particles. It is found that large interparticle forces occur along the boundary between the mobile and stagnant zones, whereas large fluid drag forces occur at the roof of a raceway or bubble. The predictions of transition between the static and dynamic states, and the complicated hysteretic behaviour in terms of either bed pressure drop or raceway size are in good agreement with the experimental observations.