Computational Fluid Dynamics Discrete Element Method Model Research Articles

Silver Recovery from Crystalline Silicon Photovoltaic Solar Cells Using Continuous Stirred-Tank Reactors.

Silver can be recycled from the end-of-life crystalline silicon photovoltaic (PV), yet the recycling and its technology scale-up are still at an early stage especially in continuously operations e.g., continoursely stirred tank reactors (CSTR). Here, the silver recovery from the solar cells is technically understood and optimized in the CSTR system from the point of view of silver recovery efficiency, through integrating experimental and numerical investigations. Specifically, based on the experiments, a kinetics model is developed and scanning electron microscopy surface morphology is characterized; and a computational fluid dynamics-discrete element method (CFD-DEM) particle-scale model is integrated with the kinetics model and validated against the fluid-flow pattern and silver leaching performance results from lab measurements. The validated CFD-DEM model is then applied to understand the particle-scale behavior of silver leaching in the CSTR system in terms of hydrodynamics and AgNO3 distribution under different impeller speeds. The simulation results show that the silver leaching performance is improved in an improved CSTR design with a lower impeller position and doubled impeller layers. This work reveals the effectiveness and underlying hydrodynamics of silver leaching in CSTR systems and lays a foundation for improving silver recovery in PV recycling.

Computational modelling of gas–liquid–solid multiphase free surface flow with and without evaporation

Gas–liquid–solid multiphase systems are ubiquitous in engineering applications, e.g. inkjet printing, spray drying and coating. Developing a numerical framework for modelling these multiphase systems is of great significance. An improved, resolved computational fluid dynamics-discrete element method (CFD-DEM) framework is developed to model the multiphase free surface flow with and without evaporation. An improved capillary force model is developed to compute the capillary interactions for partially floating particles at a free surface. Three well-known benchmark cases, namely drag coefficient calculation, the single sphere settling, and drafting-kissing-tumbling of two particles are conducted to validate the resolved CFD-DEM model. It turns out that the resolved CFD-DEM model developed in this paper can accurately calculate the fluid–solid interactions and predict the trajectory of solid particles interacting with the liquid phase. Numerical demonstrations, namely two particles moving along a free surface when the liquid phase evaporates, and particle transport and accumulations inside an evaporating sessile droplet show the performance of the resolved model.

Numerical prediction on minimum fluidization velocity of a supercritical water fluidized bed reactor: Effect of particle shape

Supercritical water fluidized bed reactor (SCWFBR) is a novel gasification reactor for hydrogen production. Compared with traditional hydrogen production technique, the SCW-based gasification has outstanding advantages such as high hydrogen production efficiency, no emission of gaseous pollutants, low water consumption and strong material adaptability. In this paper, based on Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations, numerical investigation is conducted on the effect of particle aspect ratio (α) on the minimum fluidization velocity (Umf) of a SCWFBR containing cylindrical biomass particles. The coupled CFD-DEM model is firstly validated against previously published experimental data. Then, the Umf for six types of particle shapes are calculated under different working conditions of temperature ranging from 633 to 693 K, pressure ranging from 25 to 27 MPa and α ranging from 1 to 6. Numerical results show that Umf increases with the temperature, but decreases with the pressure. Moreover, Umf is increasing with α. The underlying mechanisms are revealed by investigating the micro-structure of different shaped particles. It is shown that the increase of Umf with α is caused by the weakened fluid-solid interaction force and enhanced inter-particle locking. And the weakened fluid-solid interaction force is mainly due to the increase of the voidage. Finally, a predictive correlation of Umf for the SCWFBR is proposed considering α which demonstrates strong predictive capability for cylindrical particles.

Study on tribocharging of CaCO3 particles based on CFD-DEM gas-solid two-phase flow coupling

Unsuccessful prediction of particle tribocharging in tribocharger is partly due to the complexity of tribocharging process. In this work, computational fluid dynamics-discrete element method (CFD-DEM) modeling approach has been used to investigate particle charge in a cyclone tribocharger. Then particle image velocimetry (PIV) method is used to calculate the particle charge-to-mass ratio. The CaCO3 particle charge in a cyclone tribocharger is simulated and predicted under different gas velocities, particle sizes and moving distances. The simulation results carry out the quantitative and qualitative relationship between different parameters. At the same time, the charge-to-mass ratio between the particle simulation and the experimental results under different gas velocities is compared, and the results are all in the magnitude of 10−4. Experimental results in terms of particle charge-to-mass ratio are thus obtained. The CFD-DEM model is verified by comparison with the experimental results

Numerical and experimental validation of a detailed non-isothermal CFD-DEM model of a pilot-scale Wurster coater

Even though the Wurster technology is commonly used in the pharmaceutical industry for applying coating to beads, additional research work is still necessary to fully understand and control the process. Various simulation approaches have been used extensively in the recent years to investigate different fluidized beds, with and without spray application. The Computational Fluid Dynamics- Discrete Element Method (CFD-DEM) coupled approach is especially suitable to simulate the coating process because it resolves both the fluid and granular flow, and provides detailed particle-scale information.This paper primarily highlights the ability of the CFD-DEM coupled code to capture the thermodynamics and water mass loading of the gas phase in the Wurster coating application by directly comparing the experimental results with the CFD-DEM simulation results, for four different experimental runs and unscaled particle size. It also reports on the four specially designed experiments, giving the measurements of the air temperature and humidity at four points in the coater.The code is verified as well, using a simple simulation test case, with the objective of ensuring the energy and mass conservation in the CFD-DEM communication protocol.The paper also presents the time-averaged contour plots of temperature, humidity and particle volume fraction distribution for four different experiment/simulation cases and provides insights into their spatial distribution in the Wurster coater.

CFD-DEM model study of gas–solid flow in a spout fluidized bed with an umbrella-like baffle

In this paper, a new flow regime is identified by applying an umbrella-like baffle in a rectangular spout fluidized bed and studied by Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach. The simulations indicate the new flow regime features a multistage spouting – first spouting and second spouting. It is studied in aspects of spout robustness, mixing efficiency and formation mechanism. In the multistage spouting, the first spout and second spout always deflect but in opposite directions largely. This dynamic balance of the two spouts enables the high robustness in the multistage spouting pattern. Moreover, the efficiency of the gas–solid mixing is improved as four extra mesoscale particle circulations are formulated, reducing solids residence time in the dense region. The formation mechanism and regulation of the second spouting is also explored. It is indicated that the baffle opening angle of >60° can ensure the continuum and compressible property of particle flow.

A DEM-based approach for modeling the evolution process of seepage-induced erosion in clayey sand

In this paper, a predictive model for simulating temporal behaviors of clayey sand during seepage-induced erosion has been developed by coupling discrete element method (DEM) with computational fluid dynamics (CFD). In this model, the particle–fluid coupling simulation is solved by a “fixed coarse-grid” scheme in 3D particle flow code (PFC3D), and the suffusion of clay matrix in the initiation of erosion is converted to a degradation process of bonding strength between particles according to a degradation law. The law is derived from the well-known shear stress threshold law dealing with soil internal erosion, which is based on two erosion parameters—the critical shear stress and the erosion coefficient. Then the degradation law is implemented in the CFD–DEM model via developing customized code using the Python language. The ability of the model to predict the interfacial erosion of soils is confirmed by two numerical tests. The results are seen to match the empirical criteria, such as revealing a clearly defined critical tangential shear stress, beyond which erosion occurs, and a positive correlation between the rate of erosion and the pressure gradient. It is believed that the numerical model is able to reproduce the time-dependent evolution process of seepage-induced erosion in clayey sand.

Eulerian-Lagrangian simulation of air-steam biomass gasification in a three-dimensional bubbling fluidized gasifier

A CFD-DEM (computational fluid dynamics – discrete element method) model, with heat transfer, and homogeneous and heterogeneous chemical reactions accounted for, is implemented here to numerically study the biomass gasification in a three-dimensional bubbling fluidized bed. The results show that the biomass inlet on the right sidewall results in laterally asymmetrical profiles of important bed hydrodynamics and gas concentrations. Increasing the temperature, steam-biomass ratio and equivalence ratio all enlarge the gas voidage, fluid force, collision force and dispersion coefficient of the solid phase, but have different effects on the different homogeneous reaction rates. Vigorous horizontal and vertical solid dispersions occur in the lateral central and sidewalls of the bed, respectively. The results of the current work provide valuable insights regarding the hydrodynamics of the bubbling fluidized bed for biomass gasification, and are expected to be useful for the operation, scale-up and optimization of such systems for sustainable energy production.

CFD–DEM–PBM coupled model development and validation of a 3D top-spray fluidized bed wet granulation process

In pharmaceutical manufacturing, fluidized bed granulation is one of the common processing options available to achieve better flowability of powders through size enlargement of primary particles. In fact over the last 50 years, various fluidized bed operations including freezing, drying, impregnation, coating, etc. have become a common place in the chemical processing industry due to the high level of contacts between fluids and solids attainable in a fluid bed system. These complex interactions between the fluid and particles also mean that simulating fluidized beds are still a challenging endeavor. Generally, Computational Fluid Dynamics (CFD) packages are employed to model the pressure drops in fluids; however, the presence of high concentration of solids and the complexity of granulation behavior require more advanced particle models than are available with CFD software. As a result, coupled frameworks that utilize the strength of particulate simulations such as Discrete Element Method (DEM) and bulk granulation modeling such as Population Balance Model (PBM) in conjunction with CFD information are the next steps to developing practical fluid bed granulation models.This paper aims to provide a comprehensive description of the development and validation of a coupled CFD–DEM–PBM framework for a fluidized bed wet granulation operation. A two-way coupled CFD–DEM model is developed for a 3-dimensional, lab-scale, top spray fluid bed granulator to study the effects of process parameters such as inlet air flow rate and inlet air temperature on the particle flow dynamics and the residence time in the spray zone. A one-way transfer of data from CFD–DEM to PBM is then applied to relate the effects of particle-fluid interactions to granulation behavior occurring within the fluidized bed system. Mechanistic rate expressions were developed in the PBM to create links between CFD–DEM results and PBM rate kernels which can express the effect of critical process parameters (CPPs) such as air flow rate, inlet air temperature, binder spray rate, etc. to experimentally measured critical quality attributes (CQAs) including granule size distribution and liquid content values. From comparison with experimental results, the framework presented shows good accuracy at capturing the dynamics of the system. The presented framework demonstrates a practical process model development methodology by efficiently coupling multi-phase simulation techniques which can be used for effective process design, development and scale-up purposes.

CFD-DEM modeling of rod-like particles in a fluidized bed with complex geometry

In industrial applications, fluidized bed reactors with complex geometry are often used to process non-spherical particles, e.g., the biomass utilization. It is well known that CFD-DEM (Computational Fluid Dynamics/Discrete Element Method) is a powerful tool for studying the fluidization. However, few researchers had ever investigated the fluidization of non-spherical particles in the complex geometry using CFD-DEM. In this study, a robust and efficient CFD-DEM algorithm is correspondingly proposed, in which the non-spherical particles are represented by rod-like particles and are described by the super-ellipsoid model. An approach is incorporated into the CFD-DEM model to detect the contact between the rod-like particles and the complex geometry. The CFD-DEM coupling method has also been modified for the coupling between DEM and CFD with the unstructured mesh. Subsequently, the efficiency of the coupling method of CFD-DEM is tested. To validate the prediction accuracy of the CFD-DEM model, the simulations of the fluidization of rod-like particles in a fluidized bed with a horizontal immersed tube that represents the complex geometry are then compared with the corresponding experiments. The results demonstrate that the proposed CFD-DEM model has a rather high efficiency and accuracy for modeling the fluidization of rod-like particles in a fluidized bed with complex geometry.

CFD–DEM simulation of a pseudo-two-dimensional spouted bed comprising coarse particles

In the present study, computational fluid dynamics (CFD) and the discrete element method (DEM) are used in conjunction with the Eulerian–Lagrangian method to simulate a pseudo-two-dimensional spouted bed comprising coarse 6-mm particles. The open-source OpenFOAM code is used to solve the governing equations. The predicted vertical particle velocity along the bed axis, particle velocity profiles in the radial direction, power spectral density, time-averaged particle velocity vectors, bed pressure drop, and solid flow pattern are evaluated and compared with existing experimental data. Good agreement is found between the CFD–DEM results and the measured data. It is also shown that the present CFD–DEM model accurately predicts the particle flow pattern throughout the bed. It is found that the drag force, solid stresses, and gravity play important roles in the CFD–DEM simulation of a spouted bed comprising coarse particles.

Development of an unresolved CFD–DEM model for the flow of viscous suspensions and its application to solid–liquid mixing

Although viscous solid-liquid mixing plays a key role in the industry, the vast majority of the literature on the mixing of suspensions is centered around the turbulent regime of operation. However, the laminar and transitional regimes face considerable challenges. In particular, it is important to know the minimum impeller speed ( N j s ) that guarantees the suspension of all particles. In addition, local information on the flow patterns is necessary to evaluate the quality of mixing and identify the presence of dead zones. Multiphase computational fluid dynamics (CFD) is a powerful tool that can be used to gain insight into local and macroscopic properties of mixing processes. Among the variety of numerical models available in the literature, which are reviewed in this work, unresolved CFD-DEM, which combines CFD for the fluid phase with the discrete element method (DEM) for the solid particles, is an interesting approach due to its accurate prediction of the granular dynamics and its capability to simulate large amounts of particles. In this work, the unresolved CFD-DEM method is extended to viscous solid-liquid flows. Different solid-liquid momentum coupling strategies, along with their stability criteria, are investigated and their accuracies are compared. Furthermore, it is shown that an additional sub-grid viscosity model is necessary to ensure the correct rheology of the suspensions. The proposed model is used to study solid-liquid mixing in a stirred tank equipped with a pitched blade turbine. It is validated qualitatively by comparing the particle distribution against experimental observations, and quantitatively by compairing the fraction of suspended solids with results obtained via the pressure gauge technique.

Design of microfluidic channels for magnetic separation of malaria-infected red blood cells.

This study is motivated by the development of a blood cell filtration device for removal of malaria-infected, parasitized red blood cells (pRBCs). The blood was modeled as a multi-component fluid using the computational fluid dynamics discrete element method (CFD-DEM), wherein plasma was treated as a Newtonian fluid and the red blood cells (RBCs) were modeled as soft-sphere solid particles which move under the influence of drag, collisions with other RBCs, and a magnetic force. The CFD-DEM model was first validated by a comparison with experimental data from Han et al. 2006 (Han and Frazier 2006) involving a microfluidic magnetophoretic separator for paramagnetic deoxygenated blood cells. The computational model was then applied to a parametric study of a parallel-plate separator having hematocrit of 40% with a 10% of the RBCs as pRBCs. Specifically, we investigated the hypothesis of introducing an upstream constriction to the channel to divert the magnetic cells within the near-wall layer where the magnetic force is greatest. Simulations compared the efficacy of various geometries upon the stratification efficiency of the pRBCs. For a channel with nominal height of 100 µm, the addition of an upstream constriction of 80% improved the proportion of pRBCs retained adjacent to the magnetic wall (separation efficiency) by almost 2 fold, from 26% to 49%. Further addition of a downstream diffuser reduced remixing, hence improved separation efficiency to 72%. The constriction introduced a greater pressure drop (from 17 to 495 Pa), which should be considered when scaling-up this design for a clinical-sized system. Overall, the advantages of this design include its ability to accommodate physiological hematocrit and high throughput - which is critical for clinical implementation as a blood-filtration system.

CFD–DEM modelling of particle ejection by a sensor-based automated sorter

The efficiency of sensor-based automated sorting depends on both correct identification and separation of different types of particles. It is known that the distribution of particles fed to the sorter will affect both of these. When different particles are in close proximity, they can be “agglomerated” or seen as a single particle during identification and also have an increased probability of being unintentionally co-ejected. Both factors will have a negative effect on separation efficiency.The aim of this work was to model the air ejection manifold of a sensor-based automated sorter and to investigate the relationship between particle proximity and unintentional co-ejections. The airflow from a single air ejection valve of a sorter was modelled using computational fluid dynamics (CFD) software and calibrated against a Tomra Sorting Solutions optical sorter. It was found that the air ejection manifold could be accurately represented in CFD code. Particles were modelled using the discrete element method (DEM) software and the effect of particle position, relative to an air ejection valve, on accurate ejection was examined using an integrated CFD–DEM model. The results of these models matched reasonably well with physical measurements. The models created can be used as a basis for the prediction of sorter efficiency.

Computational fluid dynamics–discrete element method analysis of the onset of scour around subsea pipelines

In this study, we used a computational fluid dynamics (CFD)–discrete element method (DEM) model to analyze the triggering of sand movement below an offshore pipeline. The advantage of this CFD–DEM model is that it considers the motion and force applied to every individual particle, thereby facilitating detailed simulations of seepage flow and the breakdown of the water–sand mixture beneath the pipeline. The simulation results were similar to previous experimental studies, which showed that scouring occurs after the driving pressure beneath the pipe generates a floating gradient in the sand layer near the wake. Furthermore, the computed critical current velocity that leads to the onset of scour was in reasonable agreement with the observational data. Thus, we consider that CFD–DEM is a promising numerical tool for future investigations of the mechanism that underlies the onset of scour.

CFD–DEM simulation of the filtration performance for fibrous media based on the mimic structure

In order to obtain the comprehensive filtration information, such as the deposition morphology of particles, evolutions of the instantaneous pressure drop and efficiency, 3-D mimic models of fibrous media had been established using a stochastic algorithm and computational fluid dynamics (CFD) - discrete element method (DEM) model was adopted to simulate the gas-solid flow characteristics in fibrous media exposed to particle loading in this work. Meanwhile, the simulation results had been validated by the macro-empirical model proposed in the previous literature. In addition, the influence of various parameters including porosity, face velocity and particle size on the filtration performance of fibrous media had also been investigated. It is concluded that simulation results of the CFD–DEM model are good agreement with predictive values of the macro-empirical model. Moreover, the porosity of fibrous media, face velocity and particle size have an important effect on the filtration performance of the fibrous media.

CFD‐DEM Model for Simulating Solid Exchange in a Dual‐Leg Fluidized Bed

AbstractThe domain coverage method (DCM) is proposed to establish a computational fluid dynamics‐discrete element method (CFD‐DEM) model based on irregular mesh. The gas field was solved by Fluent software and the DEM model was coupled with Fluent software by user‐defined functions. Gas turbulent viscosity was calculated by the coupled k‐ϵ two‐equation model and the soft‐sphere collision model was used to get particle contact force. The CFD‐DEM model based on irregular mesh was firstly verified to be reasonable by comparing the simulated injected bubble with that simulated by Bokkers et al. The solid exchange behavior was studied numerically in a 2D dual‐leg fluidized bed (DL‐FB). The simulation results were compared with experimental results and proved that the CFD‐DEM model is established successfully based on the efficient DCM. The DEM model is expanded to be used on irregular mesh in fluidized beds with complex geometries.

Particle scale modelling of the multiphase flow in a dense medium cyclone: Effect of fluctuation of solids flowrate

Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in the coal industry. The flow dynamics/fluctuation in a DMC is important but has not been studied previously. In this work, the dynamics is studied by numerically with special reference to the effect of the fluctuation of solid mass flowrate. The simulation is carried out by use of a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In the model, the motion of discrete mineral particle phase is obtained by DEM which applies Newton’s equations of motion to every individual particle and the flow of medium (mixture of water, air and fine magnetites) phase by the traditional CFD which solves the Navier–Stokes equations at a computational cell scale. The simulated results are analysed in terms of medium and coal flow patterns, and particle–fluid, particle–particle and particle–wall interaction forces. It is shown that under high fluctuation frequency and current conditions, the performance of DMC is not sensitive to both the fluctuation amplitude and period of coal flow at the DMC inlet. However, under low fluctuation frequency, as fluctuation amplitude increases, the separation performance deteriorates slightly and the flow is obviously affected at the spigot. A notable finding is that the near-gravity particles that tend to reside at the spigot and/or have longer residence time in the DMC would be affected more than other particles. The work shows that this two-way coupled CFD–DEM model could be a useful tool to study the dynamics of the flow in DMCs.