Numerical Investigation of Crater Phenomena in a Particle Stream Impact onto a Granular Bed

This paper presents an experimental and numerical study of the impact of a particle stream onto a particle bed using a 2D slot model. The numerical simulation is performed by means of the discrete element method (DEM). The results show that the DEM simulation can reproduce the experimental results well under comparative conditions. The dynamics in the formation of a crater is then analyzed in terms of velocity field, force structure, bottom stress distribution and energy exchange based on the DEM results. It is shown that as a result of impact by the falling particles, the particles in the top central region of the particle bed have relatively large velocities and contact forces. The velocities and forces propagate into the bed, and reach the bottom of the base layer quickly. They then continue to propagate leftwards and rightwards to create a crater. During the impact process, most of the energy from the falling particles is dissipated due to the inelastic collision and frictional contacts between particles, and only a small amount of the energy contributes to the formation of the crater. The crater size is shown to be affected by the discharging rate, discharging height and materials properties, and be related to the ratio of the input energy from the falling stream to the inertial energy from the original packing.     

Production of Metallic Foams Having Open Porosity Using a Powder Metallurgy Approach

A technique has been recently developed to produce foamed metallic structures from dry powder blends containing a metallic powder, a polymeric binder, and a foaming agent. The blend is molded and heat-treated to foam and consolidate the material. The final properties may be tailored by varying the sintering temperature. Microstructure, chemical composition, and properties of nickel (Ni) foams sintered at different temperature are presented and discussed. The resulting material has an open cell microstructure with three levels of porosity. This structure leads to materials having low density (∼ 90% porosity) and high specific surface area. The specific surface area is reduced and the mechanical strength is increased when the sintering temperature increases.     

Porosity Distribution in Monodisperse and Polydisperse Fixed Beds and Its Impact on the Fluid Flow

This work is devoted to the numerical study of the porosity distribution and gas flow within randomly packed fixed beds comprising polydisperse spherical particles with Rosin–Rammler particle size distribution in a cylindrical container. The fixed bed is numerically generated using gravity-forced sedimentation modeled utilizing the discrete element method. The radial porosity distribution of monodisperse fixed beds was validated against published experimental data and good agreement was achieved. The diameter ratio of smallest to largest particle was varied from 1:2 to 1:5 and then 1:10. The simulation revealed overall porosities of 0.38 for the monodisperse bed and 0.345 and 0.33 for polydisperse beds with ratios of 1:2 and 1:10, respectively. In the second part, the fluid flow within the generated fixed beds was examined using a numerical solution to the incompressible Navier–Stokes equations in the Brinkman–Forcheimer formulation. An analysis of the results showed that in the case of a monodisperse fixed bed and low Reynolds numbers (Re) the pressure drop predicted numerically is close to the values calculated using Ergun's relation. The increase inRe leads to the deviation between the numerical and analytical predictions. This effect is because of channeling due to the sinusoidal distribution of the void fraction close to the wall.     

An approach to simulate the motion of spherical and non-spherical fuel particles in combustion chambers

The objective of this paper is to identify a numerical method to simulate motion of a packed or fluidized bed of fuel particles in combustion chambers, such as a grate furnace and a rotary kiln. Therefore, the various numerical methods applied in the areas of granular matter and molecular dynamics were reviewed extensively. As a result, a time driven approach was found to be suited for the numerical simulation of particle motion in combustion chambers. Furthermore, this method can also be employed to moving boundaries which are required for the present application e.g. travelling grate. The method works in a Lagrangian frame of reference, which uses the position and orientation of particles as independent variables. These are obtained by time integration of the three-dimensional dynamics equations derived from the classical Newtonian approach for each particle. This includes the keeping track of all forces and momentums acting on each particle at every time step. Viscoelastic contact forces include normal and tangential components with viscoelastic models for energy dissipation and friction. The particle shapes are approximated by spheres and ellipsoids with a varying size and ratio of the semi-axis accounting for the variety of particle geometries in a combustion chamber. For these shapes the overlap of particles during contact is expressed by a polynomial of 4th order in the two-dimensional case and a polynomial of 6th order in the three-dimensional case. A new algorithm to detect two-dimensional elliptical particle contact with sufficient accuracy was developed. It is based on a sequence of coordinate transformations and has demonstrated its reliability in numerous applications. Finally, the method was applied to simulate the motion of spherical and elliptical particles in a rectangular enclosure, on a travelling grate, and in a rotary kiln. Received: 16 November 2001     

Application of a Compaction Simulator to the Design of a High-Dose Tablet Formulation. Part I

The compaction properties of an investigational drug are studied by the use of a compaction simulator. The effects of punch velocity over the range of 30-640 mm^-1 on the compaction properties of the pure drug and a variety of formulas incorporating a high dose of the active compound have been investigated. The data were analyzed by applying the Heckel equation. The pure drug was found to have a high yield pressure at a relatively low punch velocity of 31 mm^-1. As the punch velocity was increased there was a decrease in crushing strength, primarily as a result of increasing yield pressure. These findings indicate that the pure drug predominantly consolidated by fragmentation and elastic deformation, with a slow plastically deforming component. The information obtained on the consolidation mechanism of the pure drug and, subsequently, on model formulas were instrumental in the design and selection of a robust formula and granulation process. The advantages of conducting dosage form design and characterization studies during the early phase of tablet formulation using means such as a compaction simulator are emphasized in this investigation.     

Application of a Compaction Simulator to the Design of a High-Dose Tablet Formulation. Part I

Abstract

The compaction properties of an investigational drug are studied by the use of a compaction simulator. The effects of punch velocity over the range of 30-640 mm^?1 on the compaction properties of the pure drug and a variety of formulas incorporating a high dose of the active compound have been investigated. The data were analyzed by applying the Heckel equation. The pure drug was found to have a high yield pressure at a relatively low punch velocity of 31 mm^?1. As the punch velocity was increased there was a decrease in crushing strength, primarily as a result of increasing yield pressure. These findings indicate that the pure drug predominantly consolidated by fragmentation and elastic deformation, with a slow plastically deforming component. The information obtained on the consolidation mechanism of the pure drug and, subsequently, on model formulas were instrumental in the design and selection of a robust formula and granulation process. The advantages of conducting dosage form design and characterization studies during the early phase of tablet formulation using means such as a compaction simulator are emphasized in this investigation.     

Discharge Characteristics of Polydisperse Powders through Conical Hoppers. Part 2: Predictions for Coarse,Granular, Free Flowing Powders

Abstract

The tribo/contact electrification theory of materials is envisaged from the chemical Lewis acid-base concept and an attempt has been made to find a correlation between these two phenomena. The donor-acceptor/acid-base properties of apatite, calcite, feldspar, quartz and wollastonite mineral samples have been determined by electrokinetic studies in water and in different organic solvents having different donicity values. From the results, the donicity values for mineral samples were defined and the position of Fermi level in the forbidden gap energy was estimated. The charge acquired by a single mineral particle of varying sizes was determined after contacting with plate and cyclone type tribochargers made up of different materials. The work functions of the mineral samples were estimated.

The results showed a good correlation between the physical tribo-electrification process and the chemical acceptor-donor property of the minerals. The results also demonstrate the feasibility of arranging the minerals into triboelectric series and thereby to predict their triboelectric charge characteristics and their separation behavior in an electric field with reference to a particular tribocharger medium.     

Discharge Characteristics of Polydisperse Powders through Conical Hoppers. Part 1: Predictions for Fine, Granular, Free Flowing Powders

Discharge characteristics of fine polydisperse granular powders of equal-solid density and near-spherical particle shape through a conical hopper were investigated by measuring solid discharge rates of powders. Effects of orifice size of hopper and size distribution of powders on discharge rate were determined by means of experiments conducted for six different sizes of hopper orifice and three different powder types under gravity flow conditions. A new effective mean diameter characterizing polydisperse powders is first introduced and determined from the particle size versus weight fraction distribution of a powder as the size corresponding to 50% cumulative weight fraction. This effective mean diameter was efficiently used in two modified forms of the Beverloo equation to predict discharge rates of polydisperse powders through hopper orifices.     

Discharge Characteristics of Polydisperse Powders through Conical Hoppers. Part 2: Predictions for Coarse, Granular, Free Flowing Powders

Discharge characteristics of coarse polydisperse granular powders through a conical hopper are experimentally investigated. The average discharge rates of four different powder types are systematically measured for seven different diameters of hopper orifice under gravity flow conditions. Each powder type is a mixture of some sub-sizes (smaller than 3 mm) of same powder material. Effects of orifice diameter of hopper and size distribution of polydisperse powders on discharge characteristics are experimentally determined. The measured discharge data are compared with discharge values predicted by using modified forms of the well-known Beverloo correlation. The volume-moment-mean diameter, d_VM, and the 50% weight fraction diameter, d₅₀, cited in available literature are both checked to characterize the coarse polydisperse powders and used throughout the predictions. Comparisons implied that both d_VM and d₅₀ diameters can be successfully used to characterize polydisperse test powders, and discharge rate predictions are in good agreement with experimental data, with mean deviations lower than ±4.04%.     

Simulations for dynamics of granular mixtures in a rotating drum

We present a simple model and carry out simulations to investigate the dynamics of mixtures of granular material within a rotating drum. On the basis of the commonly held belief (supported by considerable experimental evidence) that segregation is due to motion of particles on the active layer, the bulk playing little or no role, we introduce a 2d lattice gas model which takes into account the rotational frequency, frictional forces, and the gravitational field, and represents segregation tendencies via activated effective grain-grain interactions. Our results include the onset of segregation perpendicular to the drum axis, the appearance and subsequent coarsening of bands and peculiarities of the effects of periodic modulation of the drum. Observed effects such as the segregation of rougher (smoother) particles into the bellies (necks) of the modulation are reproduced by our simulation. Received: 30 March 2000     

Analysis of a triangulation based approach for specimen generation for discrete element simulations

Discrete element methods are emerging as useful numerical analysis tools for engineers concerned with granular materials such as soil, food grains, or pharmaceutical powders. Obviously, the first step in a discrete element simulation is the generation of the geometry of the system of interest. The system geometry is defined by the boundary conditions as well as the shape characteristics (including size) and initial coordinates of the particles in the system. While a variety of specimen generation methods for particulate materials have been developed, there is no uniform agreement on the optimum specimen generation approach. This paper proposes a new triangulation based approach that can easily be implemented in two or three dimensions. The concept of this approach (in two dimensions) is to triangulate a system of points within the domain of interest, creating a mesh of triangles. Then the particles are inserted as the incircles of these triangles. Extension to three dimensions using a mesh of tetrahedra and inserting the inspheres is relatively trivial. The major advantages of this approach include the relative simplicity of the algorithm and the small computational cost associated with the preparation of an initial particle assembly. The sensitivity of the characteristics of the particulate material that is generated to the topology of the triangular mesh used is explored. The approach is compared with other currently used methods in both two and three dimensions. These comparisons indicate that while this approach can successfully generate relatively dense two-dimensional particle assemblies, the three- dimensional implementation is less effective at generating dense systems than other available approaches. The research presented in this paper made use of software developed by other researchers. For the two-dimensional study the program Triangle developed by Jonathan Shewchuk was used. The three-dimensional analysis used the Geompack++ program developed by Barry Joe as well as an implementation of the Jodrey and Tory (1985) algorithm by Monika Bargiel and Jacek Moscinski called NSCP3D.     

Continuum Models for the Thermomechanical Behavior of Discontinuously Reinforced Materials

Continuum micromechanical models have become important tools for understanding the thermomechanical behavior of composite materials. This work presents the most important continuum‐level approaches for modeling the thermomechanical behavior of discontinuously reinforced composites. Analytical and numerical models are covered, special emphasis being put on multi‐inclusion unit cell methods. The fields of application of the different models are discussed and selected applications are demonstrated.     

A note on the velocity of granular flow down a bumpy inclined plane

The velocity distribution of granular flow down a bumpy inclined plane is theoretically studied. The characteristic length scale of local transient cluster plays an important role in determining the flow rheology. After discussing the factors influencing the cluster size, we reproduce all observed velocity distributions successfully.This research was supported by the National Key Basic Research and Development Foundation of the Ministry of Science and Technology of China No. G2000048702.     

Effect of the cooling rate in the corrosion behavior of a hot worked Ti-6Al-4V extra-low interstitial alloy

This work discusses the effect of the cooling rate during a forging process on the microstructure and corrosion behavior of a Ti–6Al–4V extra-low interstitial (ELI) alloy, which is commonly used as biomaterial. The samples were hot forged at two different temperatures, both of them within the dual phase field (α + β) and a constant strain rate of 4 × 10⁻³ s⁻¹ was employed during the tests. The samples were cooled in three different cooling media (water, air and clay) and the microstructure was analyzed using scanning electron microscopy (SEM). The corrosion resistance was determined by cyclic polarization tests in Ringer’s solution at 37 °C. Comparison between the results obtained for forged and commercial samples allowed to establish some correlations between cooling rate, microstructure and corrosion resistance. It was found that the clay as a cooling medium is a good candidate to obtain a proper microstructure and properties for biomedical applications, eliminating the requirement of subsequent heat treatment and reducing costs.     

Granular segregation studies for the development of a radar-based three-dimensional sensing system

The behavior of dense granular materials is difficult to measure in three-dimensions due to the opacity of the materials. We present a new radar-based sensing system that has the capability of measuring three-dimensional particle movement throughout the bulk of high solids fraction granular systems. A key component of the new system involves retroreflectors imbedded in objects resembling the particles in the bulk granular systems. These embedded retroreflectors may be used as tracers in systems comprised of relatively large particles. However, in systems of smaller particles the most versatile use of this new sensing system requires an understanding of the details of relative particle movement based on particle size and other particle properties. Towards this, we present new ongoing experimental and computational results toward building a versatile sensing system for high solids fraction granular systems. We then comment on additional research needed on the behavior of the components in granular mixtures for a fully versatile sensing system.     

Premises for Statistic Control of a Tribocharging Process for Granular Materials

This article aims to identify the appropriate sampling duration for a tribocharging process on a vibratory feeder device in order to compute the capability indexes and set up a statistical control procedure. The outcome of the process is evaluated as the ratio between the charge and the mass of the granules that exit the tribocharging device during a given laps of time. A virtual instrument developed in LabWiew was used in conjunction with a Faraday cage connected to an electrometer and with an electronic scale, to simultaneously measure the charge and the mass of tribocharged granular plastics, for fixed sampling durations.     

The stress response of a semi-infinite micropolar granular material subject to a concentrated force normal to the boundary

We consider the problem of a two dimensional semi-infinite granular material subject to a concentrated or point force normal to the boundary. This boundary value problem was originally solved for a classical elastic material by Flamant in 1892 and, hence, is also known as the Flamant problem (Johnson [8]). In this paper, the granular material is considered as an elastic micropolar or Cosserat continuum and is represented by a particular form of the general constitutive law derived in Walsh and Tordesillas [29]. The stress distribution predicted by the model is in good agreement with experimental data for small strains. In particular, two important features that are captured by the proposed model are: (i) the presence of tensile stress response regions, and (ii) the dependence of the stresses on the microstructural properties, i.e. the particles normal, tangential and rotational stiffness constants. The proposed analysis utilizes two new stress functions, similar to Airys stress functions in classical elastic theory.The support of the US Army Research Office through a grant to AT (Grant No. DAAD19-02-1-0216) and the Melbourne Research and Development Grant scheme is gratefully acknowledged. We thank our reviewers for their useful suggestions and insightful comments.