DEM-CFD Method Research Articles

Validation study on a coarse-grained DEM-CFD simulation in a bead mill

A bead mill is often employed in powder processes. In the bead mill, the bead-particles move rapidly under high shear rate due to the moving structures. The discrete element method (DEM) coupled with the computational fluid dynamics (CFD) has been applied to simulate the bead-particle behavior for the optimization of the bead mill design and operating conditions. However, the existing DEM-CFD method faces essential problems regarding the substantial limit of the number of computational particles, stable calculations, and modeling of rapidly moving structures. To solve these problems, we newly investigate a numerical model combining the coarse-grained DEM-CFD, an implicit algorithm for the drag force term, and a scalar field-based wall boundary in the calculation of a bead mill commonly employed in industrial powder processes. In this study, our proposed model is shown to successfully simulate the macroscopic behavior of the bead-particles through the validation test of the coarse-grained DEM-CFD simulation in the bead mill system. Specifically, the macroscopic characteristics such as the particle location, the particle velocity, and total kinetic energy, are shown to agree well between the original particle system and coarse-grained particle systems. Consequently, this study illustrates the combination of the coarse-grained DEM-CFD, an implicit algorithm for the drag force term, and the scalar field-based wall boundary is essential for simulating the typically industrial bead mill, where an enormous number of bead-particles should be calculated in liquid under high shear rate.

Effects of ellipsoidal and regular hexahedral particles on the performance of the waste heat recovery equipment in a methanol reforming hydrogen production system

Hydrogen production from methanol has attracted attention due to its wide range of raw material sources and mature technology. Using waste heat of industrial high temperature solid particles like blast slag and steel slag etc. To provide vaporization heat and reaction heat for the reaction between methanol and water is an emerging technology for hydrogen production from methanol, which can save additional thermal energy resources. Herein, the performances of equipment that uses the waste heat of ellipsoidal and regular hexahedral particles to provide a heat source for methanol to hydrogen were explored by the DEM-CFD method. Compared with spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles have poor fluidity in the stagnant area, and the empty area is enlarged and irregular in shape. The average velocity peaks of the ellipsoidal and regular hexahedron particles are larger than those of spherical particles, and the overall mean velocity fluctuation of ellipsoidal particles is similar to that of spherical particles while the regular hexahedron particles' is larger. The average temperature drop rate of the ellipsoidal and regular hexahedral particles is slower than that of spherical particles, the uniformity of temperature distribution is worse than that of spherical particles. The ellipsoidal and regular hexahedral particles’ average effective heat transfer coefficient is smaller than that of spherical particles, and the heat transfer effect is weaker than that of spherical particles. The effective heat transfer coefficient of ellipsoidal particles is 2.95 W/(m−2∙K−1) lower than that of spherical particles and the effective heat transfer coefficient of hexahedral particles is 6.09 W/(m−2∙K−1) lower than that of spherical particles. Therefore, compared with the spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles produce less hydrogen.

Solid-liquid Two Phase Flow Simulation on Particle Motion in the Axial Flow Pump

Based on the DEM-CFD method, the solid-liquid two-phase flow in an axial flow pump was simulated, and the velocity and trajectory of particles passing through the impeller and deflector were investigated. It was found that the particles accelerated about 150% after passing through the impeller. Five solid concentration schemes were set up to explore the influence of solid concentration on particle velocity and average residence time. The maximum velocity of particles at different concentrations tended to 10 m/s; The average residence time of particles in low concentration scheme (2%) is 120% of that in other schemes. The research in this paper can be used as a reference to reduce the erosion wear in the pump and enhance the heat transfer effect.

Effect of Packing Structure Evolution on the Flow Characteristics in a Binary Composite Packed Bed Based on DEM-CFD Method

The evolution of mesoscale structures of particle packing in binary composite packed beds and their effects on flow characteristics and wall effects were investigated using the discrete element method (DEM) and computational fluid dynamics (CFD). The DEM model was used to build a series of randomly mixed packing structures of particles in accordance with the dynamic change of mass ratio between particles in two size ranges, which were then confirmed by the findings of an X-ray tomography (CT) scan. The results show that the packing structure of b25s75 was conducive to reducing the influence of wall effect in packed bed reactors. For b25s75, the dimensionless distance of radial porosity fluctuation from the wall is 0.3705, which is the smallest among the five packing models, indicating that this structure plays a suppressive role on the wall effect. In addition, the uniformity of velocity and temperature distributions in both the radial and axial directions of different packing structures were compared. The standard deviations of radial relative velocity distributions in the packed beds of b100, b75s25, b25s75 and s100 are 0.28, 0.178, 0.139 and 0.156, respectively, indicating that the stacking mode of b25s75 can make the fluid flow and the gas–solid interactions more uniform.

Experimental and Numerical Study of Pressure Drop Characteristics of Soybean Grain under Vertical Pressure

The vertical pressure lead to increased airflow resistance through the grain bulk, which affected the efficiency of ventilation and drying. The effects of vertical pressures at 50, 150, and 250 kPa on the pressure drop characteristics of soybeans were studied using experiment and numerical simulation. The random packing and different compression states for soybean packed beds were generated by the Discrete Element Method (DEM). The Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) were coupled to investigate the radial velocity and pressure drop of soybean bulk. The simulation results showed that the radial porosity had an oscillating distribution, and the radial average dimensionless velocity was consistent with the distribution trend of porosity. The increase in vertical pressure causes a decrease in porosity and an increase in local velocity. The PathFinder code was used as a supplementary method to calculate the pore path and pore characterization parameters, and the resistance coefficient term in the Forchheimer equation was determined. The compression of soybeans measured by the experiment mainly occurred within two hours after loading. The pressure drop of soybeans increased with the vertical pressure, with the average pressure drop at vertical pressures of 50, 150, and 250 kPa being 36%, 57%, and 92% higher than the uncompressed state (0 kPa). The pressure drop of soybeans calculated by the DEM-CFD method and the Forchheimer equation under different vertical pressures were in close agreement with the experimental results, and an average relative difference was found to be less than 10%. These results provide guidance for estimating the pressure drop of soybeans at different grain depths.

Numerical and experimental research on the degradation mechanism in a dust-free crusher using DEM-CFD method

Numerical and experimental research on the degradation mechanism in a dust-free crusher using DEM-CFD method

Multi-phase flow simulation of landslide dam formation process based on extended coupled DEM-CFD method

The landslide-river interaction and the impulse waves involved in the landslide dam formation process may not be insignificant and have not been extensively investigated and simulated. This paper presents a numerical investigation on the formation process of landslide dams and resulting free surface flow dynamics in the impacted river via coupled discrete element method (DEM) and computational fluid dynamics (CFD) with the volume of fluid (VOF). The accuracy and validity of the extended coupled method are verified using a series of test cases involving three-phase interaction and free surface evolution. It is then applied to simulate the landslide dam formation processes related to landslide and river flow scenarios of different kinematic characteristics. Furthermore, quantitative analysis is performed to describe the complex evolution of the dam morphology and dynamic evolution of impulse waves. It is found that the impact between the landslide, river flow and valley drives the dam formation process. The landslide velocity considerably influences the propagation of impulse waves, while the river flow velocities control the dam morphology in opposite ways in the upstream and downstream. This research provides a practical modeling framework to understand the formation mechanism of landslide dams and support applications in hazard prediction and mitigation.

Numerical Simulation on the Longitudinal Breach Process of Landslide Dams Using an Improved Coupled DEM-CFD Method

An accurate investigation of the landslide dam breach process is crucial for the understanding the breach mechanism and disaster prediction. However, the numerical research on the landslide dam breach process to date is rarely reported, especially regarding the soil-water flow coupling effect incorporated in the erosion process. This paper presents a numerical investigation on the longitudinal breach process of landslide dams via a coupled discrete element method (DEM) and computational fluid dynamics (CFD) with the volume of fluid (VOF). Moreover, a virtual sphere model is proposed to overcome the computational instability caused by the particle size approaching the mesh size. The accuracy and validity of the improved coupled method are verified using a series of single particle sedimentation cases. By employing this method, the longitudinal breach process of landslide dams featuring different materials and hydrodynamic conditions has been simulated. It is found to satisfactorily reproduce the longitudinal breach process of landslide dams including surface flow erosion, backward erosion, head-cut erosion, and water and sediment rebalance or complete breach. The effects of the inflow discharges and dam materials on the erosion process are systematically resolved. The breach flow can cause the rotation trend of particles and lead to the increase of tangential contact force at the initial stage of the dam breaching. During the breach process, both the strength and density of the force chain continue to attenuate. The results obtained from the improved coupled DEM-CFD simulations can reasonably explain the particle-fluid interaction mechanisms, physical and morphological evolution and breach process at both macroscopic and mesoscopic scales.

Numerical Study of an Annular Bead Mill by the Advanced DEM-CFD Method

Numerical Study of an Annular Bead Mill by the Advanced DEM-CFD Method

Numerical investigation on the mixing mechanism in a cross-torus paddle mixer using the DEM-CFD method

Studies on powder mixing in industrial mixers are crucial for the improved criteria of mixing equipment design and optimization within various mixing unit operations. In the present study, a series of simulation for the powder mixing in an elaborately designed mixer, called a cross-torus paddle mixer, is conducted using the Advanced DEM-CFD method. The effects of various operational parameters on the mixing performance are analyzed and discussed in detail. Through these investigations, it is found that rotational speed, filling level, liquid viscosity, and paddle obliquity can significantly affect mixing performance. Owing to a much-enhanced convective motion of particles, a higher rotational speed exhibits a more efficient mixing process. Smaller number of particles inside the vessel shows the better mixing performance, which is attributed to an enhanced diffusive motion of particles. Decreasing the liquid viscosity or increasing the paddle obliquity can also result in a more efficient mixing in a diffusive dominant mode. By performing the simulation of mixing behavior in a mono-dispersed system, the present work provides significant evidence and novel insights to further understand the mixing mechanisms.

Visualization study on the coarse graining DEM for large-scale gas–solid flow systems

Gas–solid flows are ubiquitous in industrial systems. The coupled model of the discrete element method (DEM) and computational fluid dynamics (CFD) is one of the techniques for the simulation of the gas–solid flows. To enhance the applicability of the DEM-CFD method, the coarse graining DEM has been developed to simulate large-scale powder systems. The coarse graining DEM is a scaling law model and hence can simulate large-scale systems using a smaller number of particles than the actual one. Although the coarse graining DEM enables to effectively simulate large-scale powder systems on a single PC, visualized image is lack of reality due to reduced spatial resolution. Here a novel visualization method is developed to produce realistic images. In the proposed technique, the virtual particles are located by considering the motion and location of the coarse grain particles. To show the adequacy of the proposed method, two types of visualization based on the computational results are made: a blow-up of powder due to injected gas flow and a spouted bed. In these systems, the visualized images are shown to be improved by the proposed visualization technique. The effectiveness of the proposed method is proved by the agreement of the images based on the calculation results between the standard DEM-CFD method and coarse graining DEM.

On artificial density treatment for the pressure Poisson equation in the DEM-CFD simulations

The discrete element method (DEM) coupled with computational fluid dynamics (CFD) is regarded to be a standard approach in a numerical calculation of gas-solid flow. When the DEM-CFD method is applied to the simulation of industrial systems, we cannot shy away from the modeling of arbitrary shape wall boundaries. Very recently, we have developed an innovative wall boundary model in the DEM-CFD simulations, where the signed distance function (SDF) and the immersed boundary method (IBM) are used for the wall boundary in the DEM and CFD. The SDF makes it possible to create an arbitrary shape wall boundary based on the scalar field. The IBM can be used for the wall boundary model by reflecting the number of the SDF points on the local volume fraction in CFD grid. Thus, the arbitrary shape wall boundary can be easily created in the DEM-CFD method by combining the SDF with the IBM. On the other hand, the authors' group has pointed out the necessity of the density scale technique in CFD. In the density scale technique, the density scale coefficient is considered in the calculation of the pressure Poisson equation. When the SDF/IBM is applied to CFD without density scaling technique, an unnatural fluid flow might occur through the wall boundary object. The effect of the density scaling coefficient on a gas-solid flow has not been examined so far. From this background, the influence of the density scale coefficient on a gas-solid flow is investigated in the present study. Through this study, introduction of the density scaling technique is designated to be crucial when the IBM is used for the wall boundary in the DEM-CFD simulation. The density scaling coefficient is shown to be decided not by theory but by experience, and the value should become 100 to 1000 in a typical gas-solid flow system. Besides, it is illustrated that the calculation cost is not influenced by the value of the density scale coefficient in the DEM-CFD simulations.

Coarse graining DEM simulations of a powder die-filling system

Application of numerical simulations are desired to improve the quality of powder products and to optimize the production process in powder die-filling. In the powder die-filling, complex gas-solid interaction should be simulated since the solid particles are significantly influenced by the gas flow under moving wall boundary. The discrete element method (DEM) coupled with computational fluid dynamics (CFD) has been widely utilized in the simulation of gas-solid flows, and the adequacy has been proved through countless studies. On the other hand, the existing DEM-CFD method is extremely difficult to simulate the gas-solid flow where the wall boundary is moving. Besides, in the existing DEM-CFD method, huge number of computational particles cannot be simulated on a single PC. Hence, numerical simulations of the industrial powder die-filling becomes a challenging topic from a viewpoint of evolution of the DEM-CFD method, because modeling of the moving wall boundary as well as modeling of large number of computational particles should be considered simultaneously. Very recently, the authors' group has developed the Integrated DEM-CFD method including the arbitrary shape wall boundary model and the scaling law model. In the Integrated DEM-CFD method, the wall boundary is modeled by the signed distance functions and the immersed boundary method, and besides the coarse graining DEM is employed as the scaling law model. Adequacy of the Integrated DEM-CFD method has been proved through verification tests. In the current study, applicability of the Integrated DEM-CFD method is examined in the powder die-filling. Adequacy of the Integrated DEM-CFD method is shown by agreement between the original particle system and the coarse graining particle system in the powder die-filling. Through the verification tests, macroscopic characteristics of the powder die-filling are shown to be reproduced by the Integrated DEM-CFD method. Consequently, the Integrated DEM-CFD method is illustrated to be effective for the simulation of the industrial powder die-filling systems.

A Dynamic Coarse Grain Discrete Element Method for Gas-Solid Fluidized Beds by Considering Particle-Group Crushing and Polymerization

The discrete element method (DEM) coupled with computational fluid dynamics (CFD) is used extensively for the numerical simulation of gas-solid fluidized beds. In order to improve the efficiency of this approach, a coarse grain model of the DEM was proposed in the literature. In this model, a group of original particles are treated as a large-sized particle based on the initial particle distribution, and during the whole simulation process the number and components of these particle-groups remain unchanged. However, collisions between particles can lead to frequent crushing and polymerization of particle-groups. This fact has typically been ignored, so the purpose of this paper is to rationalize the coarse grain DEM-CFD model by considering the dynamic particle-group crushing and polymerization. In particular, the effective size of each particle-group is measured by a quantity called equivalent particle-group diameter, whose definition references the equivalent cluster diameter used by the energy-minimization multi-scale (EMMS) model. Then a particle-group crushing criterion is presented based on the mismatch between the equivalent diameter and actual diameter of a particle-group. As to the polymerization of two colliding particle-groups, their velocity difference after collision is chosen as a criterion. Moreover, considering the flow heterogeneity induced by the particle cluster formation, the EMMS drag force model is adopted in this work. Simulations are carried out by using a finite volume method (FVM) with non-staggered grids. For decoupling the Navier-Stokes equations, the semi-implicit method for pressure linked equations revised (SIMPLER) algorithm is used. The simulation results show that the proposed dynamic coarse grain DEM-CFD method has better performance than the original one.

Flexible discretization technique for DEM-CFD simulations including thin walls

The discrete element method (DEM) coupled with computational fluid dynamics (CFD) method is regarded as a standard approach for a simulation of a gas-solid mixture system. In the DEM-CFD method, the local volume average technique is employed, and hence the fluid motion is calculated based on the void fraction. Although the accuracy of the DEM-CFD method has been improved through lots of studies, inflexibility may become a problem due to the local volume average technique. Specifically, calculations of a gas-solid flow involving thin walls is substantially impossible even by the improved DEM-CFD method. This is because the thin wall cannot be represented when its thickness becomes as large as one grid size due to usage of the local volume average technique. In order to solve this problem, a flexible discretization technique is newly proposed, where the signed distance function and the immersed boundary method are introduced into the dual grid model. In this technique, two kinds of grids are used to calculate the void fraction and the fluid flow. Thus, this technique makes it possible to simulate a gas-solid flow involving a thin wall. Verification and validation tests are performed to show the adequacy of this technique. Through this study, the proposed technique is illustrated to reproduce the exact solution and experimental results in the gas-solid flow involving the thin wall. Consequently, the proposed technique is shown to yield reasonable results in gas-solid flows involving the thin walls.

Numerical Study of Bead Milling by the Advanced DEM–CFD Method

Numerical Study of Bead Milling by the Advanced DEM–CFD Method

Study of heat transfer by using DEM–CFD method in a randomly packed pebble-bed reactor

The pebble-bed reactor is one of the most promising designs for the nuclear energy industry. In this paper, a discrete element method–computational fluid dynamics (DEM–CFD) approach that includes thermal conduction, radiation, and natural convection mechanisms was proposed to simulate the thermal-fluid phenomena after the failure of forced circulation cooling system in a pebble-bed core. The whole large-scale packed bed was created using the DEM technique, and the calculated radial porosity of the bed was validated with empirical correlations reported by researchers. To reduce computational costs, a segment of the bed was extracted, which served as a good representative of the large-scale packed bed for CFD calculation. The temperature distributions simulated with two different fluids in this DEM–CFD approach were in good agreement with SANA experimental data. The influence of the natural convection mechanism on heat transfer must be taken into account for coolants with strong convective capacity. The proposed DEM–CFD methodology offers a computationally efficient and widely applied method for understanding the heat transfer process in a pebble-bed core. The method can also be easily extended to assess the passive safety features of newly designed fluoride-salt-cooled pebble-bed reactors.

Research on rotating speed’s influence on performance of Deep-Sea lifting motor pump based on DEM-CFD

Metal minerals in deep sea are considered to be the most important mineral resources in the 21st century. With the development of science and technology, deep-sea mining has gained more and more attention, and the pipeline lifting method is the most promising for mining. It is particularly important to use the pump as a key component in the pipeline upgrading. In this paper, the DEM–CFD method is used to study the solid–liquid two-phase fluid flow in the Deep-Sea lifting motor pump. Data about the distribution of pressure on the suction surface and pressure surface as well as the distribution of particles in the pump under different rotation speed can be obtained. Tests verify the efficiency and head of the pump by numerical simulation. It provides a theoretical method for the study of solid–liquid two-phase fluid flow in deep-sea mining.

Numerical investigation on the influence of air flow in a die filling process

Abstract Die filling is an important manufacturing process that is widely involved in material science and technology in chemical engineering. Application of numerical technologies is desired in optimizing a die filling process. The Discrete Element Method (DEM) is widely used to simulate granular flows in die filling. However, the existing DEM has a problem regarding the computation of air flow in a moving shoe system, even though the DEM is coupled with computational fluid dynamics (CFD). This is because that the simulation of a gas-solid flow with a moving boundary requires an extremely complex algorithm. In order to solve this problem, The Advanced DEM-CFD method is proposed to calculate granular particles interacting with air meanwhile during the whole die filling process. In the Advanced DEM-CFD method, a compact algorithm is employed to simulate a gas-solid flow in a moving boundary system, where wall boundary is modeled by Signed Distance Function and Immersed Boundary Method for solid phase and fluid phase. Validation tests are performed to show the adequacy of the Advanced DEM-CFD method. The results show that the simulations well agree with the experiments, and hence our numerical approach is adequate to reproduce actual die filling processes.

Flow characteristics analysis of purge gas in unitary pebble beds by CFD simulation coupled with DEM geometry model for fusion blanket

Helium is used as the purge gas to sweep tritium out when it flows through the lithium ceramic and beryllium pebble beds in solid breeder blanket for fusion reactor. The flow characteristics of the purge gas will dominate the tritium sweep capability and tritium recovery system design. In this paper, a computational model for the unitary pebble bed was conducted using DEM-CFD method to study the purge gas flow characteristics in the bed, which include porosity distribution between pebbles, velocity field distribution, pressure field distribution, pressure drop as well as the wall effects on velocity distribution. Pebble bed porosity and velocity distribution with great fluctuations were found in the near-wall region and detailed flow characteristics between pebbles were displayed clearly. The results show that the numerical simulation model has an error with about 11% for estimating pressure drop when compared with the Ergun equation.