Showing papers in "Granular Matter in 2001"

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

Abstract: 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.

A segregation mechanism in a vertically shaken bed

Abstract: Metal disks of different size and density were placed at the bottom of a bed of monodisperse granu- lar material. The system was vibrated sinusoidally in the vertical direction. It was observed that, if the angular ac- celeration of the shaking was slightly greater than that of gravity, the metal disks rose to the top of the bed. This result has been known for over sixty years, but a basic understanding of the mechanism responsible for the rise of the disks is still a subject of debate. Our experiments and theoretical model show that the ascent speed of the disk is proportional to the square root of the disk densi- ty, approximately proportional to the disk size, and is a function of the disk's depth in the bed. We also investi- gated the speed of ascent of the disk as a function of the shaking frequency, fs. We found that the effective friction or drag coefficient, , between the disk and the granular bed, is proportional to a functional form of the frequency: ∝ (fs − fc) −4 , where fc is the critical shaking frequency for the disk to start moving through the bed. We discuss how such a dependency may arise.

Forces in piles of granular material: an analytic and 3D DEM study

Abstract: We investigate the stress distribution at the base of a conical sandpile using both analytic calculations and a three dimensional discrete element code. In particular, we study how a minimum in the normal stress can occur under the highest part of the sandpile. It is found that piles composed of particles with the same size do not show a minimum in the normal stress. A stress minimum is only observed when the piles are composed of particles with different sizes, where the particles are size segregated in an ordered, symmetric, circular fashion, around the central axis of the sandpile. If a pile is composed of particles with different sizes, where the particles are randomly distributed throughout the pile, then no stress dip is observed. These results suggest that the stress dip is due to ordered, force contacts between equiheight particles which direct stress to the outer parts of the pile.

Isostaticity in granular matter

Abstract: Recent work (PRL V81, 1998, p. 1634) has proven that cohesionless granular packings are isostatic in the limit of low applied pressure, and suggested that this property is responsible for the peculiar static behavior of granular materials. On disordered isostatic systems, stress–stress response functions are power-law distributed, with a cutoff that grows exponentially with distance. If the external pressure is increased sufficiently, excess contacts are created, the packing becomes hyperstatic, and the medium behaves as a normal elastic continuum. I discuss here these ideas briefly, and also report on numerical results (mainly stress distributions) from a simple two-dimensional model of packings which captures the essential physics of stress transmission in granulates under varying load conditions.

Nonlinear acoustics in granular assemblies

Abstract: We present a brief review of the problem of acoustic propagation in granular media and discuss recent progress on nonlinear acoustics in granular media The presentation emphasizes the solitary wave like properties of impulse propagation in granular media at vanishingly small loading conditions and discusses the possible spectroscopic applications of nonlinear impulse acoustics in the detection of buried inclusions

FE-studies on rapid flow of bulk solids in silos

Abstract: This paper contains results of numerical modelling of the onset of silo flow for granular material in a model silo with convergent walls. The calculations were performed with a finite element method based on a polar elasto-plastic constitutive relation by Muhlhaus. It differs from the conventional theory of plasticity by the presence of Cosserat rotations and couple stresses using a mean grain diameter as a characteristic length. The characteristic length causes that numerical results do not depend upon the mesh discretisation. The model tests on rapid silo flow of glass beads performed by Renner in a glass hopper with a large wall inclination from the bottom were numerically simulated. The FE-calculations were performed for plane strain by taking into account inertial forces and linear viscous damping. A satisfactory agreement between numerical and experimental results was obtained. In addition, the FE-calculations were performed for very rough walls. Advantages and limitations of a continuum approach for simulations of rapid silo flow were outlined.

Granular packing: influence of different parameters on its stability

Abstract: A packing of equal glass beads is placed on a box with a rough bottom. The box is slowly inclined till an avalanche begins at a critical angle. The avalanche dynamics decreases the surface slope until a second critical angle is reached. In previous works [1–3], the stability of the packing was found to be affected by the number of layers, the packing length and the surrounding humidity.

Stable Ni/Al2O3 catalysts for methane dry reforming

Abstract: To obtain insight into the effect of metal-support interactions in the CO2 reforming of CH4, a series of alumina-supported nickel catalysts were prepared by calcination of the catalyst precursors in air at different temperatures. The increase in the intensity of Ni-Al2O3 interactions with the calcination temperature was found to be unfavorable to the reduction of the catalyst. However, the catalyst with strong Ni-Al2O3 interactions suppressed carbon deposition effectively, which can be attributed to the formation of spinel, NiAl2O4, after calcination. When the reaction was carried out at temperatures higher than 973 K, all the catalysts tended to exhibit equilibrium activity.

Structures in deformed granular packings

Abstract: The deformation of granular packings typically produces shear planes. We study the internal structure of such a shear plane and investigate the segregation and fragmentation of the grains. The most important question is to explain the finite width of the shear planes and we will give an argument to estimate this width.

Crater formation on a three dimensional granular heap

Abstract: We present an experimental investigation of crater formation on a three dimensional granular heap due to the impact of particles having a high initial energy. The granular medium used is composed of macroscopic lead beads of diameter 2 mm. The particles are dropped from a height of up to 5 m above the heap. Experimental results show that impacting particles must have a minimum energy to create a crater on top of the heap. The depth of the crater created first increases linearly for a given range of initial energies of particles and then tends to saturate for higher energies. We give qualitative explanations for the experimental results.

A surface instability of granules under vibration in partitioned containers

Abstract: This paper describes a surface instability of vibrated granules contained in rectangular vessels. The vessels are partitioned in two equal sections by a vertical plate whose lower end is placed at a certain distance from the vessel base. The free surfaces of granules in two sections, initially at an equal level, start to move when submitted to vertical vibrations, and stabilize at different levels. Measurements are made of the pressure at the bottom of each of the partitioned beds, which reveals the underlying mechanism of the surface instability. Computer simulations are performed to show microscopic structures of this surface instability.

Granular matter in a silo

Abstract: A dynamic model for slow accumulation of granular matter, with standing and rolling layers, is adapted to the geometry of a silo with vertical walls and bounded cross section. For sources constant in time the typical behavior of a rising level of matter with constant surface profile is described by a similarity solution. This solution can be characterized by a nonlinear boundary value problem. For the case of a circular silo with central point source and the corresponding 1D problem of an interval with central point source the profiles of the standing and rolling layers are explicitly computed. The computed shapes agree qualitatively with experimental observations and seem better justified than results based on the assumption of constant speed of the rolling layer.

The dynamics of granular flow in an hourglass

Abstract: We present experimental investigations of flow in an hourglass with a slowly narrowing elongated stem. The primary concern is the interaction between grains and air. For large grains the flow is steady. For smaller grains we find a relaxation oscillation (ticking) due to the counterflow of air, as previously reported by Wu et al. [Phys. Rev. Lett. 71, 1363 (1993)]. In addition, we find that the air/grain interface in the stem is either stationary or propagating depending on the average grain diameter. In particular, a propagating interface results in power-law relaxation, as opposed to exponential relaxation for a stationary interface. We present a simple model to explain this effect. We also investigate the long-time properties of the relaxation flow and find, contrary to expectations, that the relaxation time scale is remarkably constant. Finally, we subject the system to transverse vibrations of maximum acceleration Γ. Contrary to results for non-ticking flows, the average flow rate increases with Γ. Also, the relaxation period becomes shorter, probably due to the larger effective permeability induced by the vibrations.

Surface fluctuations and the inertia effect in sandpiles

Abstract: We report both an experiment and numerical simulations on superficial fluctuations of a slowly driven granular system: a box filled with a certain number of layers of glass beads is tilted very slowly up to the maximum angle of stability where an avalanche is produced. The avalanche decreases the slope of the free surface of the packing until a second critical angle is reached: the angle of repose. During the build up period many rearrangements occur on the free surface. The distribution function for the observed mass fluctuations follows a power-law behavior. We reproduce this results (rearrangements and avalanche) with a cellular automata model which takes into account the transfer of momentum to layers below. It is the purpose of this work to study the transition between static and flowing states in a very slowly driven granular system. We will focus on the very reach behavior of the surface rearrangements leading up to a large slide.

Characterization and modelling of oil reservoirs and groundwater aquifers: application of Wavelet Transformations

Abstract: We describe wavelet transformations (WTs) and their basic properties, and discuss their application to analysis of complex and noisy data. These transformations are a powerful tool for data and image analysis by (1) providing a flexible spatial- and/or time-scale window which narrows when one focusses on small-scale features and widens when one wishes to analyze large scale features, and (2) providing the capability for analyzing special characteristics of a set of data or images around specific points. We discuss application of WTs to developing a unified and highly efficient method for characterization and simulation of porous media problems, from pore to field scale. The method uses WTs for data mining and characterization, scale-up of the geological (fine-scale) model of porous media, and interpretation of their transient flow properties. The data that can be treated by WTs include the direct data, such as various well logs and the permeability distributions, and the indirect data, such as seismic signals, and other characteristics of porous media. Wavelet transformations denoise the data, uncover their special features, and discover the structure of their distribution. They also provide an efficient method for processing seismic data, which are typically in huge volumes. Wavelet transformations can also analyze time-dependent flow data, such as long-term production data for oil reservoirs, as well as time-dependent seismic data (i.e., repeated measurements over regular time intervals). Therefore, one has, for the first time, a unified approach to characterization and modelling of large-scale porous media and extraction of information from their flow properties, from pore to field scale. The computational cost of the method can be orders of magnitude less than those of the most efficient methods currently available.

Spatial memories and correlation functions in the theory of stress distribution in granular materials

Abstract: The problem of calculating stress distributions in granular materials is addressed via a formalism involving spatial memories. The stochastic origin of the memories is explained and the connection of the memories to correlation functions in the granular system are clarified in several ways: via stochastic considerations, through effective medium arguments, and by generalization of existing constitutive relations. It is indicated how to unify existing theories in the literature with the help of the memory formalism and how to apply the theory to compaction in dies to explain observed oscillations in the stress.

Modifications in the microstructure of alumina porous materials by hydrothermal treatment

Abstract: Thin films of microporous γ-alumina were prepared by hydrothermal treatment of boehmite which was obtained by a sol-gel process. Films of boehmite hydrothermally treated at different temperatures and times were studied by X-ray diffraction, Nitrogen Adsorption, DTA/TG Thermal Analysis and Scanning Electron Microscopy. The results showed that a well-crystallized boehmite with an anisotropic particle growth mainly in the (002) plane was obtained. The modifications introduced into the boehmite affected the microstructure, such as surface area, pore morphology, pore volume, pore size distribution and thermal stability of the corresponding alumina.

Granular mixing in a Galton board

Abstract: Diffusional mixing of a granular system differs from that of fluids systems in many respects, being the most important, the lack of particulate motion equivalent to molecular diffusion; i.e. there is no relative movement of particles without an energy input.

Simulations for dynamics of granular mixtures in a rotating drum

Abstract: 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.

The physics of evaporation-induced assembly of sol-gel materials

Abstract: Remarkable materials ordered at the nanoscale emerge when a sol-gel solution becomes co-organized with a surfactant. At sufficiently high concentration, the surfactant forms crystalline or liquid-crystalline arrays of micelles in the presence of the sol-gel, and as gelation proceeds the arrays become locked into the gel. Recent experiments demonstrate that the degree of order in the resulting mesoporous ceramic phase can be enhanced and controlled by continuous dip coating in which the solution, initially dilute, evolves through the critical micelle concentration by steady-state evaporation. The long-range order and microstructural orientation in these films suggest that the propagation of a critical-micelle-concentration transition front, with large physico-chemical gradients, promotes oriented self assembly of surfactant aggregates. This “steep-gradient” view is supported by results from unsteady evaporation of aerosols of similar solutions, in which internally well-ordered but complex particles are formed.

A molecular dynamics approach in simulations of fluid-solid particles flow

Abstract: In the paper a discrete system of particles carried by fluid is considered in a planar motion. The volumetric density of particles is assumed to be small enough such that they can be treated within the framework of a molecular dynamics model. The fluid is then considered as a carrier of particles. The Landau-Lifshitz concept of turbulence is used to describe the fluctuating part of fluid velocity. This approach is applied to simulate different regimes (laminar and turbulent) and various states of particle motion (moving bed, heterogeneous flow, and homogeneous flow) using only two parameters, which have to be determined experimentally. These two parameters, found for a particular pipe and for a particular velocity from a simple experiment, then can be used for other pipe diameters and different velocities. The computer simulations performed for the flow of particles in pipes at different flow velocities and different pipe diameters agree favorably with experimental observations of the type of flow and critical velocities identifying transitions from one type to another.

Proper stability in porous media based in a natural clay

Abstract: In this work, the synthesis and characterisation of two aluminium pillared clays is introduced and their thermal, hydrothermal and chemical stability is studied. The source material was a Spanish clay with a high percentage of montmorillonite, called SerBca. This clay was pillared using two different methods with the same relationship metal/clay. Some differences in the characteristics were found, but both showed thermal and hydrothermal stability until 700 °C and chemical stability between pH values from 1.5 to 11. These characteristics make those materials appropriate to be used in processes such as heterogeneous catalysis, where those conditions are required.

Exact time-dependent solutions for anomalous diffusion with absorption

Abstract: Recently, analytical solutions of a nonlinear Fokker–Planck equation describing anomalous diffusion with an external linear force were found using a non extensive thermostatistical Ansatz. We have extended these solutions to the case when an homogeneous absorption process is also present. Some peculiar aspects of the interrelation between the deterministic force, the nonlinear diffusion and the absorption process are discussed.

Density difference driven instabilities in porous media

Abstract: In the present work, we analyze density driven instabilities in miscible fluids in a random packing of monosized glass spheres (porous medium). The aim of this work is to detect heterogeneities in flow velocity using tracer dispersion because of the sensitivity of this technique. The influence of non-Newtonian rheological properties on the displacement front between two fluids of different densities is particularly studied. We compared the width of the displaced fronts in the case of Newtonian fluids with the same density contrast.

NMR study of the slow dynamics and local structure in lyotropic micellar liquid crystals

Abstract: The first experimental evidence of director fluctuations in a micellar lyotropic liquid crystal, studied by 1H spin-lattice relaxation rate, is reported. The system is constituted by the ternary mixture: potassium laurate/1-decanol/water. With the aim to test the slow dynamics, the experiment has been performed over a broad range of Larmor frequencies (2 × 103− 6.6 × 106 Hz), using fast field-cycling NMR relaxometry. The results evidence that in the nematic and isotropic mesophases and the poliphasyc region, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range ( 105 Hz), two relaxation mechanisms are assigned: i) molecular reorientation by translational diffusion on the micellar surface, and ii) molecular exchange between the micelle and the bulk.

Catalytic behaviour Ni/γ-Al2O3 microporous catalysts in the methane dry-reforming reaction

Abstract: Two series of Ni/γ-Al2O3 catalysts with Ni content between 3.5 and 14 were prepared. In the first series, a γ-alumina support previously prepared was impregnated with Ni(NO3)2·6H2O (Merk) using an incipient wetness method and in the second, the Ni salt was incorporated into the precursor solution during the γ-alumina preparation. All the samples were characterised by XRD, DTA/TG, Nitrogen adsorption, CO-chemisorption, and tested by methane dry-reforming reaction experiments.

Simulation of granular compacts in two dimensions

Abstract: Simulations of granular packings in 2-D by throwing disks in a rectangular die are performed. Different size distributions as bimodal, uniform and gaussian are used. Once the array of particles is done, a relaxation process is carried on using a large-amplitude, low-frequency vertical shaking. This relaxation is performed a number N of times. Then, we measure the density of the package, contact distribution, coordination number distribution, entropy and also the disks size distribution vs. height. The dependence of all these magnitudes on the number N of “shakings” used to relax the packing and on the size distribution parameters are explored and discussed.

Asymmetric ceramic microporous membranes

Abstract: Ceramic composite asymmetric microporous membranes were prepared for Micro- and Ultra-Filtration. The characterisation of their microporous separative layers was made by the nitrogen adsorption method, gas permeability measurements and SEM observations. This was connected with the characterisation of the corresponding composite substrates, as regards their morphology, structure and permeability. Certain anomalies, related to the intrinsic gas permeability of those separative layers, were observed in connection with the texture of their surface. These rough surfaces might play an important role in filtration or in gas separation processes. In order to evaluate a surface correction factor further studies are envisaged.

Determination of the pore size distribution of correlated mesoporous networks

Abstract: In the present work we study how the adsorption desorption hysteresis loop of a mesoporous disordered medium represented by a 3-dimensional Dual Site-Bond Model (DSBM) is affected by percolation. Site and bond distributions are assumed to be gaussians. The behavior of the threshold pressure for the evaporation processe suggests a method to determine the site and bond distributions from experimental adsorption-desorption hysteresis curves. Traditional methods developed for non-correlated networks are tested and evaluated against our simulation results showing the discrepancy mainly for highly correlated networks. Results of the prediction capability of our method are shown.

Efficiency of membranes for isotopic separation: effect of cracks

Abstract: In order to evaluate the separation factor for the isotopic separation of Uranium in the conventional enrichment process by gaseous diffusion through membranes, it is fundamental to control the size of the pore radius. The efficiency of the separation process depends critically on this value. The existence of cracks may affect the perfomance of the membrane or even disable it. In the particular case of Uranium enrichment, in order to increase the separation factor, it is necessary to have pores of a very small radius making even more critical the presence of cracks. Here we discuss the influence of cracks on the separation process, deriving the corresponding expressions for the separation factor (α) by considering its effect on the separation effectiveness (S) and the permeabi lity (H).