B.H. Xu

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.

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.

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.

Stress distribution in a sandpile formed on a deflected base

This paper presents a study of the pressure distribution beneath a sandpile by means of discrete element method. Simulations were performed with spheres of different properties for wedge-shaped sandpiles formed on bases of different degrees of deflection. The results are analyzed in terms of the stress distribution, and normal and shear pressure distributions beneath a sandpile. It is shown that the interparticle forces in a sandpile are highly disordered and mainly propagate with large force chains. Base deflection has a significant effect on the normal pressure distribution when a sandpile is formed with multisized particles and small sliding friction coefficients. However, it is not the sole factor leading to a normal pressure distribution with a dip, particularly when a sandpile is formed with monosized particles.

SIMULATION OF AGGREGATE DEFORMATION AND BREAKUP IN SIMPLE SHEAR FLOWS USING A COMBINED CONTINUUM AND DISCRETE MODEL

Abstract The evolution of aggregates suspended in liquid and subjected to simple (elongational) shear flows is reported under different shear rates and for varying strengths of interparticle interactions. This is carried out using simulations via the combined continuum and discrete model, offering a combined approach of distinct element method and computational fluid dynamics. In such a model, the motion of individual particles is obtained by solving Newtons second law of motion and flow of continuum fluid by the locally-averaged Navier–Stokes equations. The results demonstrate the dependence of both the dominating aggregate breakup mechanism and final aggregate morphology on shear rates and strength of interparticle interactions.

The combined-continuum-and-discrete-model (CCDM) for simulation of liquid-particle flows

Abstract The Combined-Continuum-and-Discrete-Model (CCDM) is a technique that can simulate microscale behaviour of fluid-particle systems. Previous studies have focused on gas-solids flows; however, the technique is equally applicable to liquid-solid systems providing the model is expanded to account for complex fluid-particle interaction forces and changes to interparticle contact behaviour caused by the liquid medium. In this work, liquid-fluidized beds have been simulated using CCDM. Results indicate that modifications to account for the effect of the liquid have little impact on macroscopic system qualities such as minimum fluidization velocity and bed expansion, but a significant improvement in terms of the microscale particle mixing behaviour produced by the model.

Acoustic emission monitoring from a lab scale high shear granulator—A novel approach

A new approach to the monitoring of granulation processes using passive acoustics together with precise control over the granulation process has highlighted the importance of particle-particle and particle-bowl collisions in acoustic emission. The results have shown that repeatable acoustic results could be obtained but only when a spray nozzle water addition system was used. Acoustic emissions were recorded from a transducer attached to the bowl and an airborne transducer. It was found that the airborne transducer detected very little from the granulation and only experienced small changes throughout the process. The results from the bowl transducer showed that during granulation the frequency content of the acoustic emission shifted towards the lower frequencies. Results from the discrete element model indicate that when larger particles are used the number of collisions the particles experience reduces. This is a result of the volume conservation methodology used in this study, therefore larger particles results in less particles. These simulation results coupled with previous theoretical work on the frequency content of an impacting sphere explain why the frequency content of the acoustic emissions reduces during granule growth. The acoustic system used was also clearly able to identify when large over-wetted granules were present in the system, highlighting its benefit for detecting undesirable operational conditions. High-speed photography was used to study if visual changes in the granule properties could be linked with the changing acoustic emissions. The high speed photography was only possible towards the latter stages of the granulation process and it was found that larger granules produced a higher magnitude of acoustic emission across a broader frequency range.

DEM Simulation of Particle Motion in a MiPro Granulator

High shear mixers/granulators are used in a variety of industries these days, a prime example been the pharmaceuticals. It is important to gain a fundamental understanding of the motion of particles inside these devices for smooth operations and strict quality control. This work shows a preliminary study on a laboratory scale granulator (MiPro, ProCept Belgium). The granulator has three identical blades consisting of a horizontal section which runs along the bottom of the bowl and a vertical section which runs partway up the side of the bowl, each blade is inclined at 45°. The motion of particles is quantified by velocity fields and bulk porosity. The results show that complicated flow patterns occur due to the complex impellor design. The highest velocities are found to coincide with the tip of the impellor blades, the particle bed is less porous ahead of the blades, with a region of high porosity directly behind the vertical section of each blade.

Development of a Computer Simulation Technique for Predicting Heat Transfer in Multiphase Liquid-Particle Flow Systems

Many of the highly active waste liquors that result from the reprocessing of spent nuclear fuel contain particulate solids of various materials. Operations for safe processing, handling and intermediate storage of these wastes often pose significant technical challenges due to the need for effective cooling systems to remove the heat generated by the radioactive solids. The multiscale complexity of liquid-particle flow systems is such that investigation and prediction of their heat transfer characteristics based on experimental studies is a difficult task. Fortunately, the increasing availability of cheap computing power means that predictive simulation tools may be able to provide a means to investigate these systems without the need for expensive pilot studies. In this work we describe the development of a Combined Continuum and Discrete Model (CCDM) for predicting the heat transfer behaviour of systems of particles suspended in liquids.Copyright