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J S Dennis - One of the best experts on this subject based on the ideXlab platform.
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the origin of pressure oscillations in slugging fluidized beds comparison of experimental results from magnetic resonance imaging with a Discrete Element Model
Chemical Engineering Science, 2014Co-Authors: Christopher M Boyce, Stuart A Scott, Daniel J Holland, J F Davidson, J S DennisAbstract:A 3-D cylindrical Discrete Element Model with computational fluid dynamics (DEM-CFD) was used to investigate the origin of pressure oscillations in bubbling and slugging fluidized beds in which only one bubble can erupt at a time. Beds 50. mm in diameter with various settled bed heights and superficial velocities were simulated for direct comparison with experimental results obtained using magnetic resonance imaging (MRI) and a pressure sensor (Muller et al., 2007). The theoretical predictions matched experimental results well for the frequencies of (i) pressure oscillations near the distributor and (ii) bubble eruption at the bed surface. In both Model and experiment it was found that the frequency of pressure oscillations near the distributor matched the frequency of bubble eruption at the top of the bed, rather than the frequency of bubble formation at the distributor. Further simulations showed that pressure oscillations at all positions in the bed originated from the drag force imparted on the fluid by the particles, rather than from a pulse in pressure created upon bubble eruption and subsequently propagated down through the bed, as proposed by Muller et al. (2007). The theory also demonstrated that drag force and thus pressure drop was concentrated in "plugs", viz. regions packed with particles across a horizontal cross section of the bed. These regions were largest in size after bubble eruptions, causing the frequency of pressure oscillations to match that of bubble eruption. © 2014 The Authors.
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novel fluid grid and voidage calculation techniques for a Discrete Element Model of a 3d cylindrical fluidized bed
Computers & Chemical Engineering, 2014Co-Authors: Christopher M Boyce, Stuart A Scott, Daniel J Holland, J S DennisAbstract:Abstract A Discrete Element Model (DEM) combined with computational fluid dynamics (CFD) was developed to Model particle and fluid behaviour in 3D cylindrical fluidized beds. Novel techniques were developed to (1) keep fluid cells, defined in cylindrical coordinates, at a constant volume in order to ensure the conditions for validity of the volume-averaged fluid equations were satisfied and (2) smoothly and accurately measure voidage in arbitrarily shaped fluid cells. The new technique for calculating voidage was more stable than traditional techniques, also examined in the paper, whilst remaining computationally-effective. The Model was validated by quantitative comparison with experimental results from the magnetic resonance imaging of a fluidised bed analysed to give time-averaged particle velocities. Comparisons were also made between theoretical determinations of slug rise velocity in a tall bed. It was concluded that the DEM-CFD Model is able to investigate aspects of the underlying physics of fluidisation not readily investigated by experiment.
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adapting data processing to compare Model and experiment accurately a Discrete Element Model and magnetic resonance measurements of a 3d cylindrical fluidized bed
Industrial & Engineering Chemistry Research, 2013Co-Authors: Christopher M Boyce, Stuart A Scott, Daniel J Holland, J S DennisAbstract:Discrete Element Modeling is being used increasingly to simulate flow in fluidized beds. These Models require complex measurement techniques to provide validation for the approximations inherent in the Model. This paper introduces the idea of Modeling the experiment to ensure that the validation is accurate. Specifically, a 3D, cylindrical gas-fluidized bed was simulated using a Discrete Element Model (DEM) for particle motion coupled with computational fluid dynamics (CFD) to describe the flow of gas. The results for time-averaged, axial velocity during bubbling fluidization were compared with those from magnetic resonance (MR) experiments made on the bed. The DEM-CFD data were postprocessed with various methods to produce time-averaged velocity maps for comparison with the MR results, including a method which closely matched the pulse sequence and data processing procedure used in the MR experiments. The DEM-CFD results processed with the MR-type time-averaging closely matched experimental MR results, validating the DEM-CFD Model. Analysis of different averaging procedures confirmed that MR time-averages of dynamic systems correspond to particle-weighted averaging, rather than frame-weighted averaging, and also demonstrated that the use of Gaussian slices in MR imaging of dynamic systems is valid.
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validation of a Discrete Element Model using magnetic resonance measurements
Particuology, 2009Co-Authors: Christoph R Muller, Stuart A Scott, Daniel J Holland, Belinda C Clarke, A J Sederman, J S Dennis, L F GladdenAbstract:Abstract The Discrete Element Model (DEM) is a very promising Modelling strategy for two-phase granular systems. However, owing to a lack of experimental measurements, validation of numerical simulations of two-phase granular systems is still an important issue. In this study, a small two-dimensional gas-fluidized bed was simulated using a Discrete Element Model. The dimensions of the simulated bed were 44 mm × 10 mm × 120 mm and the fluidized particles had a diameter d p = 1.2 mm and density ρ p = 1000 kg/m 3 . The comparison between DEM simulations and experiments are performed on the basis of time-averaged voidage maps. The drag-law of Beetstra et al. [Beetstra, R., van der Hoef, M. A., & Kuipers, J. A. M. (2007b). Drag force of intermediate Reynolds number flow past mono- and bidispersed arrays of spheres . AIChE Journal, 53, 489–501] seems to give the best results. The simulations are fairly insensitive to the coefficient of restitution and the coefficient of friction as long as some route of energy dissipation during particle–particle and particle–wall contact is provided. Changing the boundary condition of the gas phase at the side-walls from zero-slip to full-slip does not affect the simulation results. Care is to be taken that the cell sizes are chosen so that a reasonable number of particles can be found in a fluid cell.
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granular temperature comparison of magnetic resonance measurements with Discrete Element Model simulations
Powder Technology, 2008Co-Authors: Christoph R Muller, Stuart A Scott, Daniel J Holland, A J Sederman, J S Dennis, L F GladdenAbstract:Abstract The Discrete Element Model (DEM) is a very promising Modelling strategy for two-phase granular systems. However, owing to a lack of experimental measurements, validation of numerical simulations of two-phase granular systems is still an important issue. In this study, a small two-dimensional gas-fluidized bed was simulated using a Discrete Element Model. The dimensions of the simulated bed were 44 × 10 × 120 mm and the fluidized particles had a diameter dp = 1.2 mm and density ρp = 1000 kg m− 3. The influence of different drag-force correlations was investigated. Preliminary numerical experiments were also performed to study the effects of (i) the coefficient of restitution and (ii) the Modelled transverse thickness of the two-dimensional bed. Experimental measurements were made using Magnetic Resonance (MR), with the comparisons between DEM simulations and experimental measurements performed on the basis of the time-averaged velocity and granular temperature profiles of the particles. It was found that the DEM simulations of the time-averaged vertical velocity of the particles agreed well with the MR measurements. The drag-force correlation proposed by [R. Beetstra, M.A. van der Hoef and J.A.M. Kuipers, Drag force of intermediate Reynolds number flow past mono- and bidispersed arrays of spheres. AIChE Journal, 53, 489–501 (2007).] showed the best agreement with the experimental data. Fair agreement was found if the granular temperature calculated by the DEM simulations was compared with MR measurements. At lower fluidization velocities and closer to the distributor the DEM simulations under-predicted both the velocity and the granular temperature measurements using MR.
James A Elliott - One of the best experts on this subject based on the ideXlab platform.
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asymptotic limits on tablet coating variability based on cap to band thickness distributions a Discrete Element Model dem study
Chemical Engineering Science, 2017Co-Authors: Chunlei Pei, James A ElliottAbstract:Abstract The uniformity of the coating thickness distribution is an important quality metric in the manufacture of pharmaceutical tablets during the spray coating process. An investigation of the asymptotic limits of coating thickness variability of tablets of different shapes was carried out based on their cap-to-band coating thickness distributions. A theoretical analysis shows that the cap-to-band coating thickness ratio is expected to be equal to the cap-to-band area ratio projected onto the spray direction divided by the actual cap-to-band surface area ratio. When the cap-to-band projected area ratio is larger (or smaller) than the cap-to-band surface area ratio, the mean coating thickness on the cap is larger (or smaller) than that on the band. To verify this, the dynamics of tablets in a rotating pan was Modelled using Discrete Element method (DEM) simulations, while an image analysis technique based on the output of DEM simulations was applied to Model the spray coating process and analyse the cap-to-band coating thickness ratio. A ray-tracing sampling method was further used to obtain the cap-to-band sample ratio. It was also found that a smaller spray angle with respect to the horizontal direction can decrease or even invert the cap-to-band coating thickness ratio, leading to a larger coating thickness on the band than the cap. Nevertheless, an asymptotic value of cap-to-band relative standard deviation can be reached once the cap-to-band coating thickness ratio becomes constant during the coating process. This asymptotic limit is within the range predicted based on the cap-to-band projected area ratio and surface area ratio.
Herman Ramon - One of the best experts on this subject based on the ideXlab platform.
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A Discrete Element Model for simulation of a spinning disc fertilizer spreader I. Single particle simulations
Powder Technology, 2006Co-Authors: P. Van Liedekerke, Engelbert Tijskens, E. Dintwa, J. Anthonis, Herman RamonAbstract:Modeling approaches for centrifugal fertilizer spreaders have so far been based on analytical expressions for single particle trajectories derived in the early 60's. However elegant this approach was, it suffers from several disadvantages, the most important of which is failing to incorporate the interaction between the particles in the flow. This paper is the first in a series aiming at simulating the complete spreading process based on the laws of physics and a physically meaningful Model for the interactions between the particles, c.q. the contact forces. The result is a Model that allows the development of a deeper understanding of the physics underlying the spreading process and provides better predictions. In this paper the Model is presented in detail and a series of simple computer experiments are analysed and compared to theoretical predictions. Also, single particle trajectories from DEM simulations are compared to experimental results. Further, some effects of the Model parameters are analysed. This paper demonstrates that the Model is not only capable of producing realistic simulations, but also provides detailed insight in the physics of the spreading process.
Peter Eberhard - One of the best experts on this subject based on the ideXlab platform.
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A Discrete Element Model and its experimental validation for the prediction of draft forces in cohesive soil
Journal of Terramechanics, 2014Co-Authors: Martin Obermayr, Christos Vrettos, Peter Eberhard, Thomas DäuwelAbstract:Abstract Soil can roughly be classified into cohesionless, cohesive, and cemented soil. In this contribution, a Discrete Element Model for the simulation of cohesive soil is presented. It is based on a Model for cohesionless material with spherical particles, normal repulsive and frictional contacts, as well as rolling resistance with an elastic limit to compensate the excessive particle rolling. The cohesive behavior is Modeled by an additional attractive normal force between particles. The Model is not derived from one of the microscopic origins of cohesion, such as liquid bridges or electrostatic forces. Instead, it is set up in analogy to the macroscopic shear failure characteristics of cohesive soil. The Model is stress history dependent. By that, the amount of cohesion is limited by the pressure that contacting particles have experienced during the course of the simulation. The Discrete Element Model is shown to be scale invariant in the quasi-static regime, i.e. if all lengths of the Model are scaled, the results remain unaffected by the scaling. The Model is applied to a small-scale laboratory test and an excavator digging in natural cohesive soil. The contact parameters are calibrated by simulated triaxial compression tests. A comparison between simulation and measurement shows good qualitative and quantitative agreement.
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a Discrete Element Model predicting the strength of ballast stones
Computers & Structures, 2012Co-Authors: Christian Ergenzinger, Robert Seifried, Peter EberhardAbstract:An extended Discrete Element method (DEM) is used to describe strength and failure properties of ballast stones. For this purpose, a rock material is Modeled using spherical particles bonded by breakable force Elements. An inflation procedure to generate dense sphere packings which is based on a particle's current coordination number is proposed. The particle bonds are enhanced by a progressive failure Model that reproduces the effects of singular stress concentrations near crack tips, which are normally not present in DEM, by successive weakening of bonds. The material Model is investigated in uni- and triaxial compression, where an efficient approach for the simulation of a flexible confining membrane is applied, and calibrated to granite yielding wide agreement in strength and failure properties. Furthermore, a procedure to Model shape and angularity of ballast particles is proposed. Finally, the strength of ballast stones made from bonded particles is subjected to statistical evaluation and compared to published experimental results. Different measures for single particle strength are investigated with respect to the loading state which causes failure.
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A Discrete Element Model to describe failure of strong rock in uniaxial compression
Granular Matter, 2011Co-Authors: Christian Ergenzinger, Robert Seifried, Peter EberhardAbstract:A bonded particle Model is investigated by means of an extended Discrete Element Method with respect to failure of strong rock in uniaxial compression. A coordination number based inflation scheme is presented, which generates isotropic sphere packings that are featuring a higher average coordination number than conventional procedures. A progressive failure Model is proposed, which promotes crack propagation and localization and allows adjusting brittleness of fracture. Failure of the granular solid is discussed in detail. The fracture process is studied in dependence of the introduced failure Models. The influence of particle size and bond strength distributions, particle numbers, particle layering in finite sphere packings and end constraints is addressed. Comparison to published experimental results reveals that many of the observed features of rock failure are reproduced.
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a Discrete Element Model to describe failure of strong rock in uniaxial compression
Granular Matter, 2011Co-Authors: Christian Ergenzinger, Robert Seifried, Peter EberhardAbstract:A bonded particle Model is investigated by means of an extended Discrete Element Method with respect to failure of strong rock in uniaxial compression. A coordination number based inflation scheme is presented, which generates isotropic sphere packings that are featuring a higher average coordination number than conventional procedures. A progressive failure Model is proposed, which promotes crack propagation and localization and allows adjusting brittleness of fracture. Failure of the granular solid is discussed in detail. The fracture process is studied in dependence of the introduced failure Models. The influence of particle size and bond strength distributions, particle numbers, particle layering in finite sphere packings and end constraints is addressed. Comparison to published experimental results reveals that many of the observed features of rock failure are reproduced.
Christopher M Boyce - One of the best experts on this subject based on the ideXlab platform.
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the origin of pressure oscillations in slugging fluidized beds comparison of experimental results from magnetic resonance imaging with a Discrete Element Model
Chemical Engineering Science, 2014Co-Authors: Christopher M Boyce, Stuart A Scott, Daniel J Holland, J F Davidson, J S DennisAbstract:A 3-D cylindrical Discrete Element Model with computational fluid dynamics (DEM-CFD) was used to investigate the origin of pressure oscillations in bubbling and slugging fluidized beds in which only one bubble can erupt at a time. Beds 50. mm in diameter with various settled bed heights and superficial velocities were simulated for direct comparison with experimental results obtained using magnetic resonance imaging (MRI) and a pressure sensor (Muller et al., 2007). The theoretical predictions matched experimental results well for the frequencies of (i) pressure oscillations near the distributor and (ii) bubble eruption at the bed surface. In both Model and experiment it was found that the frequency of pressure oscillations near the distributor matched the frequency of bubble eruption at the top of the bed, rather than the frequency of bubble formation at the distributor. Further simulations showed that pressure oscillations at all positions in the bed originated from the drag force imparted on the fluid by the particles, rather than from a pulse in pressure created upon bubble eruption and subsequently propagated down through the bed, as proposed by Muller et al. (2007). The theory also demonstrated that drag force and thus pressure drop was concentrated in "plugs", viz. regions packed with particles across a horizontal cross section of the bed. These regions were largest in size after bubble eruptions, causing the frequency of pressure oscillations to match that of bubble eruption. © 2014 The Authors.
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novel fluid grid and voidage calculation techniques for a Discrete Element Model of a 3d cylindrical fluidized bed
Computers & Chemical Engineering, 2014Co-Authors: Christopher M Boyce, Stuart A Scott, Daniel J Holland, J S DennisAbstract:Abstract A Discrete Element Model (DEM) combined with computational fluid dynamics (CFD) was developed to Model particle and fluid behaviour in 3D cylindrical fluidized beds. Novel techniques were developed to (1) keep fluid cells, defined in cylindrical coordinates, at a constant volume in order to ensure the conditions for validity of the volume-averaged fluid equations were satisfied and (2) smoothly and accurately measure voidage in arbitrarily shaped fluid cells. The new technique for calculating voidage was more stable than traditional techniques, also examined in the paper, whilst remaining computationally-effective. The Model was validated by quantitative comparison with experimental results from the magnetic resonance imaging of a fluidised bed analysed to give time-averaged particle velocities. Comparisons were also made between theoretical determinations of slug rise velocity in a tall bed. It was concluded that the DEM-CFD Model is able to investigate aspects of the underlying physics of fluidisation not readily investigated by experiment.
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adapting data processing to compare Model and experiment accurately a Discrete Element Model and magnetic resonance measurements of a 3d cylindrical fluidized bed
Industrial & Engineering Chemistry Research, 2013Co-Authors: Christopher M Boyce, Stuart A Scott, Daniel J Holland, J S DennisAbstract:Discrete Element Modeling is being used increasingly to simulate flow in fluidized beds. These Models require complex measurement techniques to provide validation for the approximations inherent in the Model. This paper introduces the idea of Modeling the experiment to ensure that the validation is accurate. Specifically, a 3D, cylindrical gas-fluidized bed was simulated using a Discrete Element Model (DEM) for particle motion coupled with computational fluid dynamics (CFD) to describe the flow of gas. The results for time-averaged, axial velocity during bubbling fluidization were compared with those from magnetic resonance (MR) experiments made on the bed. The DEM-CFD data were postprocessed with various methods to produce time-averaged velocity maps for comparison with the MR results, including a method which closely matched the pulse sequence and data processing procedure used in the MR experiments. The DEM-CFD results processed with the MR-type time-averaging closely matched experimental MR results, validating the DEM-CFD Model. Analysis of different averaging procedures confirmed that MR time-averages of dynamic systems correspond to particle-weighted averaging, rather than frame-weighted averaging, and also demonstrated that the use of Gaussian slices in MR imaging of dynamic systems is valid.