POWDERS AND GRAINS 2009: PROCEEDINGS OF THE 6TH INTERNATIONAL CONFERENCE ON MICROMECHANICS OF GRANULAR MEDIA

Surface wave acoustics of granular packing under gravity
View Description Hide DescriptionDue to the non‐linearity of Hertzian contacts, the speed of sound in granular matter increases with pressure. For a packing under gravity and in the presence of a free surface, bulk acoustic waves cannot propagate due to the inherent refraction toward the surface (the mirage effect). Thus, only modes corresponding to surface waves (Raleigh‐Hertz modes) are able to propagate the acoustic signal. First, based on a non‐linear elasticity model, we describe the main features associated to these surface waves. We show that under gravity, a granular packing is from the acoustic propagation point of view an index gradient waveguide that selects modes of two distinct families i.e. the sagittal and transverse waves localized in the vicinity of the free surface. A striking feature of these surface waves is the multi‐modal propagation: for both transverse and sagittal waves, we show the existence of a infinite but discrete series of propagating modes. In each case, we determine the mode shape and and the corresponding dispersion relation. In the case of a finite size system, a geometric waveguide is superimposed to the index gradient wave guide. In this later case, the dispersion relations are modified by the appearance of a cut‐off frequency that scales with depth. The second part is devoted to an experimental study of surface waves propagating in a granular packing confined in a long channel. This set‐up allows to tune a monomodal emission by taking advantage of the geometric waveguide features combined with properly designed emitters. For both sagittal and transverses waves, we were able to isolate a single mode (the fundamental one) and to plot the dispersion relation. This measurements agree well with the Hertzian scaling law as predicted by meanfield models. Furthermore, it allows us to determine quantitatively relations on the elastic moduli. However, we observe that our data yield a shear modulus abnormally weak when compared to several meanfield predictions.

Elements of an Improved Model of Debris‐flow Motion
View Description Hide DescriptionA new depth‐averaged model of debris‐flow motion describes simultaneous evolution of flow velocity and depth, solid and fluid volume fractions, and pore‐fluid pressure. Non‐hydrostatic pore‐fluid pressure is produced by dilatancy, a state‐dependent property that links the depth‐averaged shear rate and volumetric strain rate of the granular phase. Pore‐pressure changes caused by shearing allow the model to exhibit rate‐dependent flow resistance, despite the fact that the basal shear traction involves only rate‐independent Coulomb friction. An analytical solution of simplified model equations shows that the onset of downslope motion can be accelerated or retarded by pore‐pressure change, contingent on whether dilatancy is positive or negative. A different analytical solution shows that such effects will likely be muted if downslope motion continues long enough, because dilatancy then evolves toward zero, and volume fractions and pore pressure concurrently evolve toward steady states.

Critical State‐based Geo‐micromechanics on Granular Flow
View Description Hide DescriptionFlow behaviors of dry granular materials on a slope were examined by way of model tests using PIV image analysis and a discrete element method in two dimensions. The simulation results replicate the tendencies shown in the model test results. The relationships between macro and micro behaviors on typical phenomena such as the velocity distribution in depth stratification and uplift of large particles changing grading including inverse grading in the flow were examined on the basis of the numerical results, focusing on the stress chain formed by a shear deformation mechanism in granular materials. The distribution of the averaged coordination number was found to correspond to the distribution of velocity, indicating three layers of structure. This stratification could be explained by state parameter, which is a relative variable among the void ratio, the mean normal stress and critical state in geomechanics. Moreover, the stress chains formed from the riverbed concentrated the larger particles and pushed them upwards towards the flow surface.

Isolated movement zone for interacting draw points in underground mining: dilation effects
View Description Hide DescriptionThe success of the underground mining procedure is based on a good characterization of the rock flow inside the cave. The Kinematic model, used in the granular flow description in hoppers, has been applied to describe experimental situations aimed at sketching the mine operation. The local packing changes occurring at early stages of rock motion that affect the loosening zone is taken into account by introducing a dilation front propagating upward. The dynamics of the dilation front limits the extent of the flow generated by the drawing process allowing the assessment of the loosening zone. Based on the linear character of the kinematic model, it is shown that loosening zone resulting from simultaneous and alternating extractions differ significantly depending on the separation distance between the drawpoints as has been observed experimentally. Based on these finding a brief discussion on optimization of drawpoints distance is given.

Flow and jamming of sheared granular media
View Description Hide DescriptionGranular materials, such as athermal suspensions, can either be jammed and rigid, or yield and flow. Recent experiments on granular suspensions in a annular shear cell (vibrated and/or sheared) show a hysteretic freezing and melting transition [1, 2]: a crystallised state is found, which can be melted by sufficient shear. The question is open on what are the mechanisms underlying these phenomena and which are the control parameters. Via Molecular Dynamics simulations, we study the rheology of vibrated and sheared granular materials [3]. In particular, we aim to understand the nature of a critical line separating crystallised and melted states and the “jammed” region in the phase diagram, as well as the connections with thermal glass formers and colloidal suspensions.

Force and fabric states in granular media
View Description Hide DescriptionThe plastic flow of granular materials reflects to a large extent the constraints imposed by steric exclusions and mechanical equilibrium at the particle scale. An accurate formulation of these local constraints is the key to a statistical mechanical approach but requires a rich set of state parameters. We show that the constraints can be taken into account in a simple way with a reduced set of anisotropy parameters akin to the lowest‐order description of the contact and force networks. We then introduce a model of kinematic jamming defined as a state of saturation in the evolution of the contact network. This model correctly predicts the accessible geometrical states as well as the evolution of the system to a kinematically jammed state. We also show that a harmonic decomposition of shear stress as a function of the anisotropy parameters and phase factors representing the loading history leads to the “fragile” character of force networks.

Granular Thermodynamics
View Description Hide DescriptionWe present experimental evidence for a strong analogy between quasi‐2D uniform non‐equilibrium steady states (NESS) of excited granular materials and equilibrium thermodynamics. Under isochoric conditions we find that the structure of granular NESS, as measured by the radial distribution function, the bond order parameter, and the distribution of Voronoi cells, is the same as that found in equilibrium simulations of hard disks. Three distinct states are found corresponding to a gas, a dense gas, and a crystal. The dynamics of the dense gas is characterized by sub‐diffusive behavior on intermediate time scales (caging). Under isobaric conditions we find a sharp first‐order phase transition characterized by a discontinuous change in density and granular temperature as a function of excitation strength. The transition shows rate dependent hysteresis but is completely reversible if the excitation strength changes quasi‐statically. All of these behaviors are analogous to equilibrium thermodynamics. The one difference is the velocity distributions, which are well described by in the range where v is one component of the velocity, T is the granular temperature, is a Maxwell‐Boltzmann and is a second order Sonine polynomial. The single adjustable parameter, is a function of the filling fraction, but not T. For as observed in many other experiments.

Thermomicromechanics of dense granular materials
View Description Hide DescriptionA new approach is proposed for the development of a class of elastoplastic thermomicromechanical constitutive laws for dense, cohesionless granular media. The resulting constitutive law is expressed in terms of particle scale properties. Micromechanical relations for the internal variables, tied to nonaffine deformation and their evolution laws, are derived from a structural mechanics analysis of a particular mesoscopic event: confined, elastoplastic buckling of a force chain. The capabilities of the constitutive law to reproduce several defining aspects of material behavior, viz. strain‐softening under dilatation, noncoaxiality, and the evolution of shear bands, are briefly discussed.

Granular Flows in Split Bottom Geometries
View Description Hide DescriptionThere is a simple and general experimental protocol to generate slow granular flows that exhibit wide shear zones, qualitatively different from the narrow shear bands usually observed. The essence is to drive the granular medium not from the sidewalls, but to split the bottom of the container that supports the grains in two parts that slide past each other. Here we discuss main features of granular flows in so‐called split bottom geometries. We focuss on reviewing the results for dry, slow granular flow. New developments, on faster flows, on flows of particles submerged in fluids, and on flows accompanied by weak vibration will be briefly outlined—these new developments will be discussed in more details in the accompanying talk at P&G 2009.

2d Granular Gas in Knudsen Regime and in Microgravity Excited by Vibration: Velocity and Position Distributions
View Description Hide DescriptionDynamics of quasi‐2d dissipative granular gas is studied in microgravity condition (of the order of ) in the limit of Knudsen regime. The gas, made of 4 spheres, is confined in a square cell enforced to follow linear sinusoidal vibration in ten different vibration modes. The trajectory of one of the particles is tracked and reconstructed from the 2‐hour video data. From statistical analysis, we find that (i) loss due to wall friction is small, (ii) trajectory looks ergodic in space, and (iii) distribution ρ(v) of speed follows an exponential distribution, i.e. with being a characteristic velocity along a direction parallel (y) or perpendicular (x) to vibration direction. This law deviates strongly from the Boltzmann distribution of speed in molecular gas. Comparisons of this result with previous measurements in earth environment, and what was found in 3d cell [1] performed in environment are given.

The Flow Of Granular Matter Under Reduced‐Gravity Conditions
View Description Hide DescriptionTo gain a better understanding of the surfaces of planets and small bodies in the solar system, the flow behavior of granular material for various gravity levels is of utmost interest. We performed a set of reduced‐gravity measurements to analyze the flow behavior of granular matter with a quasi‐2D hourglass under coarse‐vacuum conditions and with a tilting avalanche box. We used the Bremen drop tower and a small centrifuge to achieve residual‐gravity levels between 0.01 and 0.3 Both experiments were carried out with basalt and glass grains as well as with two kinds of ordinary sand. For the hourglass experiments, the volume flow through the orifice, the repose and friction angles, and the flow behavior of the particles close to the surface were determined. In the avalanche‐box experiment, we measured the duration of the avalanche, the maximum slope angle as well as the width of the avalanche as a function of the gravity level.

Modelling Of The Electrochemical Conduction Of Different Types Of Partially Sintered Fuel Cell Electrodes By Discrete Simulations
View Description Hide DescriptionComposite electrodes for Solid Oxide Fuel Cells (SOFC) are generally obtained through partial sintering of a mixture of ionic and electronic conducting powders. Enhancing the electrochemical performances of SOFC electrodes requires the multiplication of so‐called Triple Phase Boundary points (where the gas and the ionic and electronic conducting materials meet), some residual porosity, and the percolation of the two particle networks (ionic and electronic). Also, the poor intrinsic conductivity of ionic particles requires the reinforcement of the ionic network. Thus, the optimization of the electrode micro structure is a complex task and must take into account the particulate nature of the partially sintered material of the electrode. We propose to model the electrode material as a 3D packing of spheres, which sintering is simulated by the Discrete Element Method (DEM). This allows the generation of a realistic numerical microstructure for which the geometric features of each contact is known. Typically we generate electrodes with 40 000 spherical particles and residual porosity of 25%. For the determination of the electrochemical performance, the packing is sandwiched between a current collector and an electrolyte. The packing is then replaced by a network of electronic, ionic and electrochemical resistances, and the effective conductivity of the electrode is calculated. Our simulations allow the importance of percolation effects to be demonstrated. We also compute the effective conductivity of composition graded electrodes and compare them to non‐graded composite electrodes. We show that only slightly graded electrodes can compete with non‐graded composite electrodes. In any case, the simulations show that due to percolation problems, one should not expect large gains in terms of electrochemical performance when grading electrodes. Instead, we propose a new and more effective microstructural architecture for which the electronic network percolation is imposed.

Bonding Strength by Methane Hydrate Formed among Sand Particles
View Description Hide DescriptionThe mechanical properties of methane hydrate‐bearing sand were investigated by low temperature and high confining pressure triaxial testing apparatus in the present study. The specimens were prepared by infiltrating the methane gas into partially saturated sand specimen under the given temperature and stress condition which is compatible with the phase equilibrium condition for the stability of methane hydrate. The tests were firstly performed to investigate the effect of temperature on the shear behaviour of the specimen. Then the effect of backpressure was investigated. The strength of methane hydrate bearing sand increased as the temperature decreased and the back pressure increased. The bonding strength due to methane hydrate was dependent on methane hydrate saturation, temperature and back pressure but independent of effective stress. Dissociation tests of methane hydrate were also performed by applying the temperature to the specimen at the various initial stress conditions. The marked development of shear and volumetric strains were observed due to dissociation of the methane hydrate in the specimen corresponding to the initial stress conditions.

Simulation of the Fuel Reactor of a Coal‐Fired Chemical Looping Combustor
View Description Hide DescriptionResponsible carbon management (CM) will be required for the future utilization of coal for power generation. separation is the more costly component of CM, not sequestration. Most methods of capture require a costly process of gas separation to obtain a ‐rich gas stream. However, recently a process termed Chemical Looping Combustion (CLC) has been proposed, in which an oxygen‐carrier is used to provide the oxygen for combustion. This process quite naturally generates a separate exhaust gas stream containing mainly and but requires two reaction vessels, an Air Reactor (AR) and a Fuel Reactor (FR). The carrier (M for metal, the usual carrier) is oxidized in the AR. This highly exothermic process provides heat for power generation. The oxidized carrier (MO) is separated from this hot, vitiated air stream and transported to the FR where it oxidizes the hydrocarbon fuel, yielding an exhaust gas stream of mainly and This process is usually slightly endothermic so that the carrier must also transport the necessary heat of reaction. The reduced carrier (M) is then returned to the air reactor for regeneration, hence the term “looping.” The net chemical reaction and energy release is identical to that of conventional combustion of the fuel. However, separation is easily achieved, the only operational penalty being the slight pressure losses required to circulate the carrier. CLC requires many unit operations involving gas‐solid or granular flow. To utilize coal in the fuel reactor, in either a moving bed or bubbling fluidized bed, the granular flow is especially critical. The solid coal fuel must be heated by the recycled metal oxide, driving off moisture and volatile material. The remaining char must be gasified by (or ), which is recycled from the product stream. The gaseous product of these reactions must then contact the MO before leaving the bed to obtain complete conversion to and Further, the reduced M particles must be removed from the bed and returned to the air reactor without any accompanying unburned fuel. This paper presents a simulation of the gas‐particle granular flow, with heat transfer and chemical reactions, in the FR. Accurate simulation of the segregation processes, depending on particle density and size differences between the carrier and the fuel, allows the design of a reactor with the desired behavior.

The Quantitative Mineralogy of Granular Materials
View Description Hide DescriptionKnowledge of the mineralogy of granular materials can be critical to understanding the behavior of these materials. Particles in which minerals impact process development include concentrates, flotation and leach feed materials (mining industry), cuttings (oil and gas industry), soils, dust, and precipitates (environmental science), and lunar soils and simulants (planetary research). In each of these cases, the grain size, aspect ratio, angularity, hardness, porosity and mineral association are key components of the behavior of the material and are directly related to its mineralogy.

Quasi‐static Compaction of Polyhedra by the Discrete Element Method
View Description Hide DescriptionMetal hydrides have tremendous potential to meet on‐board hydrogen storage requirements for fuel cell vehicles as set by the US DoE. Cyclic strain caused by addition and depletion of hydrogen in metal hydride beds results in brittle fracture and subsequent formation of micron‐sized, faceted particles. These beds inhibit hydride formation because of poor inter‐particle heat conduction that increases the bed’s temperature during exothermic hydriding reactions. This work involves the development of a model for generating loose configurations of metal hydride powder and for assessing the commensurate quasi‐static loading characteristics. Particles in the powder are modeled by regular tetrahedra and cubes. An energy‐based elastic contact mechanics model for particles of general shape is utilized. The numerical methods utilized to determine quasi‐static equilibrium are described and exercised with particular emphasis on issues of stability and computational efficiency. Triaxial strain is applied to simulate evolution of the solid fraction, coordination number, force network connectivity, and internal pressure as consolidation occurs in the absence of interparticle friction. These modeling elements form the mechanical basis of a model that will ultimately predict the thermo‐mechanical behavior of metal hydride powders and compacts.

Alternating Field Electronanofluidization
View Description Hide DescriptionThe use of fluidized beds to remove submicron particles from gases has been investigated since 1949. High efficiency removal was achieved in the 1970’s by imposing an electric field on a fluidized bed of semi‐insulating granules that were able to collect the charged pollutant entrained in the fluidizing gas. In spite of their extended use nowadays, the collection efficiency of electrofluidized beds (EFB) is still hindered by gas bypassing associated to gas bubbling and the consequent requirement of too high gas flow and pressure drop. In this paper we report on the electromechanical behavior of an EFB of insulating nanoparticles. When fluidized by gas, these nanoparticles form extremely porous light agglomerates of size of the order of hundreds of microns that allow for a highly expanded nonbubbling fluidized state at reduced gas flow. It is found that fluidization uniformity and bed expansion are additionally enhanced by an imposed AC electric field for field oscillation frequencies of several tens of hertzs and field strengths of the order of 1 kV/cm. For oscillation frequencies of the order of hertzs, or smaller, bed expansion is hindered due to electrophoretic deposition of the agglomerates onto the vessel walls, whereas for oscillation frequencies of the order of kilohertzs, or larger, electrophoresis is nullified and bed expansion is not affected. According to a proposed model, the size of nanoparticle agglomerates stems from the balance between shear, which depends on field strength, and van der Waals forces. The optimum field strength for enhancing bed expansion produces an electric force on the agglomerates similar to their weight force, while the oscillation velocity of the agglomerates is similar to the gas velocity.

Numerical Model for Ultra‐fine Particles in the Absence and Presence of Gravity
View Description Hide DescriptionLength scales of particles and their surrounding medium strongly determines the nature of their interactions with one another and their responses to external fields. We are interested in systems of ultra‐fine particles (0.1–1.0 micron) such as volcanic ash, soot from forest fires, solid aerosols, or fine powders for pharmaceutical inhalation applications. We have a developed a numerical model which captures the dominant physical interactions which control the behavior of these systems. The adhesive interactions between the particles use the Derjaguin‐Muller‐Toporov (DMT) adhesion theory along with the van der Waals attraction. The elastic restoring forces are modeled by the Hertz’s contact model, and require details of material properties such as the Young’s modulus and Poisson ratio. Commencing with a three dimensional gas of ultra‐fine particles, the absence of gravity does not produce any noticeable clustering. The presence of gravity initially generates a large population of clusters with small number of particles, as the particles settle. The initial population of small clusters or single particles which have settled decrease with time as more particles, or clusters, agglomerate with one another. Our final results show clusters containing 10 to 100 particles, with a larger population of small clusters. We present details of the model, and some preliminary results which demonstrate the influence of the particle surface properties on the clustering dynamics of these systems, in the absence and presence of gravity (M. Dutt, J. A. Elliott, et al. in press).

Discrete Simulation of the Consolidation of Nano‐sized Aggregated Powders
View Description Hide DescriptionThe present work describes, using Discrete Element Simulations (DEM), the consolidation of aggregated ceramic powders made of submicronic primary particles. We first investigate the crushing behavior of a single aggregate made of primary particles (or crystallites). Two types of aggregates with: a) strongly bonded (by solid necks) and b) weakly bonded (by adhesive forces) particles are investigated. In both cases spherical aggregates consisting of approximately 500 spherical primary particles are used. Series of one hundred crushing tests are carried out on these two types of aggregates with particle sizes of 10, 100 and 1000 nm. The crushing behavior of the aggregates is well captured by a Weibull distribution function. In both aggregate types, the smaller particle size aggregates exhibit larger Weibull modulus and strength. Stress strain curves showed strongly bonded aggregate to be brittle, while weakly bonded aggregates to be ductile in nature. We also investigate the packing evolution of several aggregates during uniaxial compaction in a periodic simulation box. These simulations show that below a certain particle size, adhesive forces play an important role all along the compaction stage even under large compressive stresses.