In this study experimental and numerical investigations with the discrete element method (DEM) on the mechanical interactions of spheres and polyhedral dices in a rotating drum are conducted. In DEM ...the dices are approximated by polyhedra and smoothed polyhedra respectively and hence allow examining the influence of sharply-edged and smooth particle geometries on the mechanical behavior. Simulation results are in good general agreement with the experiments and hence demonstrate the adequacy of DEM as well as polyhedral and smoothed polyhedral approximation schemes to simulate non-spherical particle geometries. It was observed that an increase of particle angularity leads to an increase of the dynamic angle of repose. On the other hand, while spheres mix faster than the polyhedral dices, no significant difference in the mixing behaviors of the dices can be observed.
Display omitted
•Increase of the dynamic angle of repose with decreasing particle sphericity•Faster mixing of spheres, no significant differences between the dices•Good DEM predictions of dynamic angle of repose, but overprediction of mixing rates•Sharply-edged geometries promote interlocking and formation of pile ups.
A numerical investigation of gas and solid flow as well as the liquid accumulation in a scaled Blast Furnace (BF) model was carried out using the discrete element method (DEM) coupled with ...computational fluid dynamics (CFD). This three dimensional simulation considers the interaction of the solid flow of a layered burden column with the counter flowing process gas. The influence of the cohesive zone is modeled by a material dependent permeability, the molten iron and the slag are considered as one additional liquid phase in the hearth. The method used is compared with a two dimensional numerical slot model presented in former publications by Zhou et al. and then applied to a different three dimensional setup. The comparison of the models exhibits considerable differences concerning the general multiphase flow behaviour. Moreover, it is shown that three dimensional models are required to correctly resolve the actual spatial flow structure and its influence on the shape of the coke free region in the hearth. Finally, the three dimensional model is used to show the influence of the liquid discharge on the stress distribution along the walls.
Snapshot of gas and solid flow pattern in a scaled three dimensional Blast Furnace model. Display omitted
•Three dimensional DEM-CFD modeling of a scaled blast furnace model.•Comparison of the model with previous publications.•The impact of different liquid levels on the particle arrangement in the lower part is investigated.•Effect of simplified liquid iron discharge is shown.
Display omitted
•CFD-simulations of rotary cement kilns considering coating layers are conducted.•Numerical models for RDF, coating and clinker bed are presented and combined.•The impact of coating ...regions on RDF combustion and clinker properties is investigated.•Light and evenly distributed coating profiles are found to be beneficial for the process.•Heavy and locally concentrated coating can result in a high free lime content of 2 wt%.
The formation of regions of solid coating, where agglomerated clinker material adheres to the refractory lining of the kiln wall, is very common during cement clinker production. While a thin coating layer protects the refractory lining, strong deposit formation can impair the material flow through the kiln. In this study, the impact of these coating layers on the clinker production process within a rotary kiln is investigated with CFD simulations. The fuel injected at the main burner is a mixture of pulverized coal and refuse derived fuel (RDF). Advanced models were developed to accurately describe the trajectories and thermal conversion of non-spherical RDF particles in the gas phase. These models are based on a detailed fuel analysis of major RDF fractions. A blocked-off region approach is used to consider different coating profiles within the simulation domain. The thermochemical processes in the clinker bed of the kiln are approximated with a one-dimensional model that calculates heat and mass exchange with the gas phase, the incorporation of fuel ashes into the bed and the chemical-mineralogical reactions of the clinker. The blocked-off region approach is also employed to account for the clinker bed geometry in the kiln, which greatly depends on the considered coating profile. Two cases, one with a thin and evenly distributed coating profile and one with a thick and locally concentrated coating, are simulated. The resulting impact on RDF conversion, gas phase properties and clinker phase formation are assessed and compared to a reference case without any coating. Results show that the insulation effect of a thin coating profile increases the gas phase temperature in the kiln and helps to reduce the free lime content of the final clinker product. In the case of heavy coating, a temperature shift towards the solid material inlet of the kiln occurs, which outweighs the beneficial insulation effect of the coating in the sintering zone and leads to lower local gas phase temperatures. In combination with reduced clinker residence times, this results in a slight increase of the free lime content in the clinker.
The current work presents an initial approach of using a particle based method (Discrete Element Method, DEM) to simulate municipal solid waste (MSW) incineration on grates. Therefore, an in-house ...DEM code has been coupled with FLUENT. Models have been formulated for drying, volatile release and char conversion. The volatiles released are converted in the furnace above the waste bed which has been calculated with FLUENT. A comparison of simulations with measurements in an existing MSW incineration plant of CO2, O2, H2O and temperatures above the bed is presented. Agreement is fair considering the measurement uncertainties and the complexity of the process. In a sensitivity study concerning the influence of waste composition (heating value, number of waste fractions), waste particle size distribution and radiative flux onto the waste bed on conversion has been carried out. Finally, the most common grate systems, backward stoking, forward stoking and roller grates have been compared briefly, including different types of furnace geometries above the bed. The results demonstrate that the approach developed gives new insight into the complex interaction of waste movement, waste conversion and gas phase combustion above the bed which can’t be obtained with other approaches, like continuum models for the waste bed.
In this study 3D DEM-simulations of hopper discharge using non-cohesive, monodisperse spherical and polyhedral particles as well as particle shapes generated by the multi-sphere method are carried ...out. For this purpose an overview of the Common Plane algorithm for contact detection between polyhedral particles is given and an important refinement of the contact point definition is presented. In the hopper the effect of increasing particle angularity on the flow properties is investigated. Moreover, three different hopper designs are chosen, to further examine the influence of hopper angle and hopper opening size on the flow properties in combination with varying particle shapes. It is demonstrated that particles with an increasing angularity reduce the mass flow rate from the hopper and in case of the flat bottom hopper (α=0°) increase the residual quantity after discharge. In all simulations significant differences between polyhedral and clustered particles were observed, which indicates that the type of particle shape approximation is a parameter that has to be considered in DEM-simulations of hopper discharge.
In this study 3D DEM-simulations of hopper discharge using non-cohesive, monodisperse spherical and polyhedral particles as well as particle shapes generated by the multi-sphere method are carried out. In the hopper the effect of increasing particle angularity as well as hopper angle and hopper opening size on the flow properties is investigated. Display omitted
► Simulation of hopper discharge with multi-sphere and polyhedral particles. ► Reduction of mass flow with increasing particle angularity. ► Significant differences between multi-sphere and polyhedral particles. ► Improved contact point definition within the Common Plane algorithm for polyhedra.
In this study experimental and numerical investigations with the discrete element method (DEM) on the discharge of spheres and polyhedral dices from a hopper are conducted. In DEM the dices are ...approximated by polyhedra and smoothed polyhedra respectively and hence allow examining the influence of sharply-edged and smooth particle geometries on the discharge properties. Simulation results are in good general agreement with the experiments and hence demonstrate the adequacy of DEM as well as polyhedral and smoothed polyhedral approximation schemes to simulate non-spherical particle geometries. Compared to spheres the dices exhibit an increased flow resistance and readiness to form pile-ups at the bottom walls of the hopper. Both phenomena are better approximated using polyhedral approximations of the dices, showcasing the influence of the selected shape approximation scheme on the numerical results.
In this study experimental and numerical investigations with the discrete element method (DEM) on the discharge of spheres and polyhedral dices from a hopper are conducted. In DEM the dices are approximated by polyhedra and smoothed polyhedra respectively. Simulation results are in good general agreement with the experiments and hence demonstrate the adequacy of the DEM to simulate non-spherical particle geometries. Display omitted
► Good agreement between experimental and numerical results of hopper discharge. ► Angular particles increase flow resistance and formation of pile-ups. ► Shape approximation in DEM has a non-negligible influence on simulation results.
Granular materials in industrial applications or nature usually consist of a wide variety of differently formed particles. Very often the shape of the involved objects does not even reveal any ...symmetry. For the simulation of these particle systems the Discrete Element Method based on spherical or symmetric shaped particles like ellipses/ellipsoids or superquadrics is therefore inadequate. A numerical implementation addressing this problem was found by introducing particles clustered from simpler geometries like spheres. Based on this approach, known as the multi-sphere method, it is possible to ensure computational efficiency for contact detection and force calculation on the one hand, but also allow a suitable representation of shape on the other hand. Today this approach is implemented in several commercial DEM-packages. However, studies on the validity of this approach have only been performed in limited cases. To address this situation, an experimental study performed by Gorham and Kharaz D.A. Gorham, A.H. Kharaz, The measurement of particle rebound characteristics, Powder Technology 112 (3), 193–202 (2000) is modeled using the multi-sphere approach. The spherical aluminum oxide particle used in the investigation is approximated by a number of smaller spherical bodies. Macroscopic collision properties are calculated and compared to experimental and numerical results obtained for a rigid spherical particle.
Granular materials in industrial applications or nature usually consist of a wide variety of differently formed particles. An efficient numerical implementation for DEM was found by introducing particles clustered from simpler geometries like spheres. Based on this approach, known as the multi-sphere method, it is possible to ensure computational efficiency for contact detection and force calculation on the one hand, but also allow a suitable representation of shape on the other hand. Today this approach is implemented in several commercial DEM-packages. However, studies on the validity of this approach have only been performed in limited cases. To address this situation, an experimental study performed by Gorham and Kharaz D.A. Gorham, A.H. Kharaz, The measurement of particle rebound characteristics, Powder Technology 112 (3), 193–202 (2000). is modeled using the multi-sphere approach. The spherical aluminum oxide particle used in the investigation is approximated by a number of smaller spherical bodies. Macroscopic collision properties are calculated and compared to experimental and numerical results obtained for a rigid spherical particle.
Display omitted
In this study experimental and numerical investigations with the Discrete Element Method (DEM) on the mechanical interactions of particles with varying sphericity and aspect ratio in a rectangular ...hopper are conducted. In the DEM the test particles are approximated by four commonly used approximation schemes.
A decrease of particle sphericity or an increase of the aspect ratio results in an more uneven, intermittend particle flow and overall lower discharge rate. It was deducted from the measurement results that changing these geometric particle properties elevates the shear strength of the particle bed and, hence, has a significant influence on the discharge properties of a hopper.
Simulation results are in good general agreement with the experiments and thus demonstrate the adequacy of the DEM to predict the mechanical interactions in granular media consisting of non-spherical particles. The results presented in this study show only a minor influence of the method used to approximate particle shape within the DEM. Obviously the discharge characteristics are much stronger related to macroscopic geometric parameters than the fine scale resolution of particle geometry.
Display omitted
•Increase of shear strength with decreasing sphericity or increasing aspect ratio.•Good DEM predictions of the hopper discharge properties.•Minor influence of approximation scheme used in DEM on simulation results.
For the simulation of dense granular systems the Discrete Element Method based on a soft-sphere approach is commonly used. In such simulations collisions between particles take a finite time. The ...equations of motion are applied for each particle and solved numerically. Therefore models for the forces acting between particles in contact need to be specified. In this paper the focus is set on normal contacts. Based on macroscopic and microscopic accessible parameters like coefficient of restitution, collision time, force, displacement and displacement rate a wide range of commonly used force schemes are reviewed. Results obtained from these commonly used models are compared to experimental data on collisions of different metal alloys, ice and marble as reported in literature. Due to obvious limitations extensions are presented. The new extended models based on linear and non-linear models are compared to experimental data and their accuracy and applicability are discussed.
For the simulation of granular systems the Discrete Element Method is commonly used. For this simulation method, models for the forces acting between particles in contact are needed. Based on macroscopic accessible parameters like coefficient of restitution, collision time, force, displacement and displacement rate for a wide range of commonly used normal force schemes are reviewed. Results obtained are compared to experimental data on collisions of different metal alloys, ice and marble as reported in literature. Due to limitations extensions are presented.
Display omitted
The combined “Blocked-Off & Discrete Ordinates” (BO/DO) approach is used within a DEM/CFD frame to model the radiative heating of stationary and moving (rotating with respect to the mesh) cubic ...particles. A single cube, placed inside a larger cubic box, is heated by radiation and its temperature evolution over time is investigated. Two different models for the particle heating are considered: the first one assumes a uniform cube temperature while the second model calculates the intra-particle heat transfer leading to a non-uniform temperature distribution as it occurs in thermally thick particles. Three different particle materials, wood, styrofoam and steel, are considered.
Initially, the radiative heating of stationary cubes, approximated by the BO approach in a CFD mesh, is compared to the heating of cubes enclosed by a corresponding body conformal mesh as a reference. The results show that deviations in cube temperature mainly result from the artificial increase of the particle surface in the BO. When applying a particle surface area ratio to compensate for the artificial surface area increase deviations in temperature can be reduced to 5.2% compared to body conformal mesh.
In a further step, the cube is constantly rotating which changes the orientation of the cube's surfaces relative to the radiation source. The evolution of the cube's surface temperatures, which is dependent on the orientation, is modelled by the BO/DO approach. As expected, materials with low thermal conductivity show larger temperature gradients over time than materials with high thermal conductivity. The temperature difference between different surfaces turns out to be in the ranges 50/14/0 K (for styrofoam/wood/steel) and 134/65/0 K between surface and core, thus reflecting the correct physical behavior. The evaluation of the BO/DO approach concerning its capability to account for radiative heat transfer in DEM simulations for particles with a regular (cubic) geometry, as examined in the current paper, forms the basis to extend the description to arbitrarily shaped particles, a great potential of the BO/DO approach as will be shown in an outlook example.
Display omitted
•Transient simulations of radiative heating of moving, complex particles in DEM/CFD.•Using the Blocked-Off approach in combination with the DO model.•Due to the BO approach, temperature deviates by up to 5.2%.•Particle rotation has a large impact on the heating of the particle.•Low thermal conductivity materials have large intra-particle temperature gradients.
