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Granular Flow (granular + flow)
Selected AbstractsProbability Density Function (PDF) Simulation of Turbulent Reactive Gas-Solid Flow in a RiserCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2009S. N. P. Vegendla Abstract A hybrid Lagrangian-Eulerian methodology is developed for the numerical simulation of turbulent reactive gas-solid flow. The SO2 -NOx Adsorption Process (SNAP) in a riser reactor with dilute gas-solid flow is taken as a test case. A three-dimensional time-dependent simulation is performed. By using the transported composition PDF method [1], modeling of the mean chemical source term and mass transfer terms in the gas-solid flow model equations is no longer needed. A notional particle-based Monte-Carlo algorithm is used to solve the transported composition PDF equations. A Finite-Volume technique is used to calculate the hydrodynamic fields from the Reynolds Averaged Navier Stokes (RANS) equations combined with the k -, turbulence model for the gas phase and the Kinetic Theory of Granular Flow (KTGF) for the solid phase [2]. The newly developed hybrid solution technique is tested with the SNAP chemistry that has a total of 13 scalars (i.e., 5 gas phase components and 8 solid phase species) for which the composition fields of the reactive species are calculated. A good agreement between simulated and experimental gas-outlet composition of a demonstration unit is obtained. [source] Wet Snow Avalanche Deposits in the French Alps: Structure and SedimentologyGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1-2 2001Vincent Jomelli We analyse the morphology and sedimentology of 25 dirty snow avalanche deposits in the French Alps. The deposits typically have either a snow-ball structure or a massive structure with sliding planes. The snow balls show a longitudinal and a vertical sorting that reflects a sieve effect, similar to that observed in other rapid inertial granular flows. The massive type results from snow compaction when the avalanche is channelled by a gully or when it reaches the distal part of the scree. Velocity decrease and compaction limit the deformation to a zone at the base of the snow mass and cause the formation of distinctive sliding planes. These appear as smooth recrystallised surfaces due to local melt from frictional heating. The flow can be assimilated to a frictional granular flow. No systematic variation of size and shape of the rock debris has been observed along the profiles in both types of deposit. The distribution of rock debris and its fabric suggest that the clasts are transported passively and do not undergo any sorting during displacement. Snow melt after avalanching causes a redistribution of rock debris particularly when the snow thickness is important. This redistribution does not generate new sedimentological characteristics such as enhanced sorting or fabric. [source] Fault rock analysis of the northern part of the Chelungpu Fault and its relation to earthquake faulting of the 1999 Chi-Chi earthquake, TaiwanISLAND ARC, Issue 1 2005Kohtaro UjiieArticle first published online: 3 MAR 200 Abstract The 1999 Chi-Chi earthquake in Taiwan (Mw = 7.6) produced a surface rupture along the north,south-striking Chelungpu thrust fault with pure dip-slip (east side up) and left lateral strike-slip displacements. Near-field strong-motion data for the northern part of the fault illustrate a distinct lack of the high-frequency seismic radiation associated with a large slip (10,15 m) and a rapid slip velocity (2,4 m/s), suggesting a smooth seismic slip associated with low dynamic frictional resistance on the fault. A drillhole was constructed at shallow depths in the possible fault zones of the northern part of the Chelungpu Fault, which may have slipped during the 1999 earthquake. One of the zones consists of a 20-cm-thick, unconsolidated fault breccia with a chaotic texture lacking both discrete slip surfaces (e.g. Riedel shears) and grain crushing. Other possible fault zones are marked by the narrow (less than a few centimeters) gouge zone in which clayey material intrudes into the damaged zone outside of the gouge zone. These characteristic fault rock textures suggest that the slip mechanisms at shallow levels during the earthquake involved either granular flow of initially unconsolidated material or slip localization under elevated pore pressure along the narrow clayey gouge zone. Because both mechanisms lead to low dynamic frictional resistance on the fault, the rapid seismic slip in the deep portions of the fault (i.e. the source region of strong-motion radiation) could have been accommodated by frictionless slip on the shallow portions of the fault. The combination of strong-motion data and fault rock analysis suggests that smooth slip associated with low dynamic friction occurred on both the deep and shallow portions of the fault, resulting in a large slip between the source region and the surface in the northern region. [source] The effect of mixer properties and fill level on granular flow in a bladed mixerAICHE JOURNAL, Issue 2 2010Brenda Remy Abstract The discrete element method was used to study the effect of mixer properties and fill level on the granular flow of monodisperse, cohesionless spheres in a bladed mixer. For fill levels just covering the span of the blades, a three-dimensional (3-D) recirculation zone develops in front of the blades, which promotes vertical and radial mixing. Increasing fill level reduces the size of the recirculation zone, decreases bed dilation and hinders particle diffusivities. However, above a critical fill level, the behavior of the particles within the span of the blade is found to be invariant of fill level. At low-fill levels, the pressure within the particle bed varies linearly with bed height and can be approximated by hydrostatics. At higher fill levels, a constant pressure region develops within the span of the blades due to the angled pitch of the blades. Cylinder wall friction is shown to significantly influence granular behavior in bladed mixers. At low-wall friction, the 3-D recirculation zone observed for high-wall friction conditions does not develop. High-wall friction leads to an increase in convective and diffusive particle mixing. Shear stresses are shown to be a function of wall friction. Blade position along the vertical axis is shown to influence flow patterns, granular temperature and stress. The effect of increasing the mixer diameter at a constant particle diameter was also studied. When the mixer diameter is larger than a critical size such that wall effects are minimized, the observed granular behavior follows simple scaling relations. Particle velocities and diffusivities scale linearly with mixer size and blade speed. Normal and shear stress profiles are found to scale linearly with the total weight of the particle bed. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Discrete element simulation of free flowing grains in a four-bladed mixerAICHE JOURNAL, Issue 8 2009Brenda Remy Abstract Numerical simulations of granular flow in a cylindrical vessel agitated by a four-blade impeller were performed using the discrete element method. Velocity, density, and stress profiles within the mixer displayed a periodic behavior with a fluctuation frequency equal to that of the blade rotation. Blade orientation was found to affect flow patterns and mixing kinetics. For an obtuse blade pitch orientation, a three-dimensional recirculation zone develops in-front of the blade due to formation of heaps where the blades are present. This flow pattern promotes vertical and radial mixing. No recirculation zone was observed when the blade orientation was changed to an acute blade pitch. The system's frictional characteristics are shown to strongly influence the granular behavior within the mixer. At low friction coefficients, the 3-D recirculation in front of the obtuse blade is not present reducing convective mixing. Higher friction coefficients lead to an increase in granular temperature which is associated with an increase in diffusive mixing. Normal and shear stresses were found to vary with mixer height with maximum values near the bottom plate. Additionally, a strong dependence between the magnitude of the shear stresses and the friction coefficient of the particles was found. The stress tensor characteristics indicate that the granular flow in our simulations occurs in the quasi-static regime. At the same time, the averaged pressure was found to vary linearly with bed height and could be predicted by a simple hydrostatic approximation. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Origin of migmatites by deformation-enhanced melt infiltration of orthogneiss: a new model based on quantitative microstructural analysisJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2008P. HASALOVÁ Abstract A detailed field study reveals a gradual transition from high-grade solid-state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation-parallel bodies of nebulitic migmatite within the eastern part of the Gföhl Unit (Moldanubian domain, Bohemian Massif). The orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well-equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high-energy like,like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole-rock volume and changes completely its macro- and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt-enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains. [source] Extension of PIV for measuring granular temperature field in dense fluidized bedsAICHE JOURNAL, Issue 1 2007W. Dijkhuizen Abstract In this work a particle image velocimetry (PIV) technique has been extended to enable the simultaneous measurement of the instantaneous velocity and granular temperature fields. The PIV algorithm has been specifically optimized for dense granular systems and has been thoroughly tested with artificially generated images. The new PIV technique has been successfully applied to a fluidized bed at incipient fluidization conditions in which a single bubble is injected by a jet and to a freely bubbling fluidized bed. The instantaneous spatial distribution of the solids-phase velocity and granular temperature that can be measured with this new technique can be used to validate CFD models for dense granular systems, such as multifluid continuum models using the KTGF (kinetic theory of granular flow) to describe the internal momentum transport in the particulate phase. © 2006 American Institute of Chemical Engineers AIChE J, 2007 [source] On the electrostatic equilibrium of granular flow in pneumatic conveying systemsAICHE JOURNAL, Issue 11 2006Jun Yao Abstract An analytical methodology involving the concept of "electrostatic equilibrium" is developed for granular flow in pneumatic conveying systems. The methodology can be used for estimation of the electrostatic field distribution at various sections of the system and explanation of the mechanisms involved for various electrostatic phenomena observed. For all cases conducted in the conveying system, there was a "charging time" required for the system to reach the state of "electrostatic equilibrium." Experiments conducted at different sections of the system showed that the time required increased in the order: horizontal pipe, vertical pipe, and pipe bend. Through a physical analysis, it is deduced that electrostatic equilibrium is related to the granules' behavior and local flow characteristics. In general, a longer time duration taken to reach equilibrium corresponds to a process with more complicated granular flow patterns. In the electrostatic equilibrium state, the field distribution shows the highest electrostatic field strength near the pipe wall, and this field strength degrades from the pipe wall to the pipe center. At various pipe sections, the highest strength occurs at the bend, in accord with observations that electric sparking first occurs at that location within the entire pneumatic conveying system. In the vertical pipe, granular distribution was measured using electrical capacitance tomography (ECT), and granular velocities were cross-referenced with those using particle image velocimetry (PIV). The electrostatic force at low air flow rates is found to be the primary cause for granules sticking to the pipe wall and results in the formation of the half-ring or ring structure. The state of electrostatic equilibrium is physically influenced by several elements in conveying systems. In a cyclic conveying system, a new pipe (or low humidity or no antistatic agent) tends to expedite the process to reach electrostatic equilibrium and attain high magnitude of electrostatic current at the state. In a non-cyclic horizontal conveying system, a thin film (pipe) is found to prolong the process duration to reach equilibrium, while the case with charged film (pipe) takes shorter duration to do so. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] Characterization of granular flow of wet solids in a bladed mixerAICHE JOURNAL, Issue 8 2006Azzeddine Lekhal Abstract In this study, we measure instantaneous, average, and fluctuating velocity fields at exposed surfaces for dry and wet grains in a vertical cylindrical mixer, agitated by four pitched blades. When the material is dry, the free surface of the granular bed deforms, rising where the blades are present, and falling between blade passes. Although average velocities are predominantly azimuthal, instantaneous velocities tracked in time reveal three-dimensional particle circulations, including significant periods of particle motion in the opposite direction to that of the blades, indicative of bed penetration. When moisture is added to the solid particles, the flow dynamics change from a regime dominated by the motion of individual grains to a regime controlled by the motion of small clumps that form as a result of the cohesive forces. This transition is characterized by a reduced particle,particle collision frequency and exhibits a sharp decrease in the granular temperature at the free surface. This transition is also characterized by an increase in bed porosity, which is attributed to increased cohesiveness arising from liquid bridges. A Fourier transform analysis conducted on the tangential component of the velocities (dominant flow) shows that a group of high frequencies exceeding the blade rotation frequency become significant with added moisture. These are characteristics of the large number of wet agglomerates flowing between successive blade passes. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] A fluid mechanical model for granular flow in silosPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Caroline Leppert Granular materials may display both solid and fluid like behaviour. For low densities and high strain rates as in avalanches or during the discharge of silos the behaviour is mainly governed by interparticle collisions. On the other hand, frictional contacts characterise the solid state which is represented within the framework of plasticity theory. A fluid like constitutive model describes granular materials when subjected to large deformations and high strain rates. It bases upon a modified viscoplastic model that is valid for both yielded and unyielded regions. The central idea is the distinction between fluid and solid regions by means of comparing actual shear stress and Coulomb yield stress. The application to the simultion of the discharge of silos shows the feasibility of the chosen method. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Wet Snow Avalanche Deposits in the French Alps: Structure and SedimentologyGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1-2 2001Vincent Jomelli We analyse the morphology and sedimentology of 25 dirty snow avalanche deposits in the French Alps. The deposits typically have either a snow-ball structure or a massive structure with sliding planes. The snow balls show a longitudinal and a vertical sorting that reflects a sieve effect, similar to that observed in other rapid inertial granular flows. The massive type results from snow compaction when the avalanche is channelled by a gully or when it reaches the distal part of the scree. Velocity decrease and compaction limit the deformation to a zone at the base of the snow mass and cause the formation of distinctive sliding planes. These appear as smooth recrystallised surfaces due to local melt from frictional heating. The flow can be assimilated to a frictional granular flow. No systematic variation of size and shape of the rock debris has been observed along the profiles in both types of deposit. The distribution of rock debris and its fabric suggest that the clasts are transported passively and do not undergo any sorting during displacement. Snow melt after avalanching causes a redistribution of rock debris particularly when the snow thickness is important. This redistribution does not generate new sedimentological characteristics such as enhanced sorting or fabric. [source] Rheological characteristics of solid,fluid transition in dry granular dense flows: A thermodynamically consistent constitutive model with a pressure-ratio order parameterINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 9 2010Chung Fang Abstract Dry granular flows are characterized as quasi-static, dense and collisional states by the interactions among the grains, which is indexed macroscopically by an internal variable, called the order parameter defined as the square root of the static pressure to the total pressure. The solid,fluid state transition is regarded as a second-order phase transition process, and is described by a kinematic evolution of the order parameter. The thermodynamic analysis, based on the Müller,Liu entropy principle, is employed to deduce the equilibrium responses of the constitutive equations, while the dynamic responses are postulated on the basis of a quasi-linear and the second-order Ginzburg,Landau phase transition theories. The obtained model is applied to study the rheological characteristics of a dry granular dense flow between two infinite parallel plates, of which the results are compared with those from DEM simulations to estimate the model validity. The present study provides a general framework for the theoretical justifications on the thermodynamic consistencies of order-parameter-based constitutive models, and can be extended to flows in quasi-static or collisional states. Copyright © 2009 John Wiley & Sons, Ltd. [source] Shearing flows of a dry granular material,hypoplastic constitutive theory and numerical simulationsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2006Chung Fang Abstract In the present study, the Goodman,Cowin theory is extended to incorporate plastic features to construct an elasto-visco-plastic constitutive model for flowing dry granular materials. A thermodynamic analysis, based on the Müller,Liu entropy principle, is performed to derive the equilibrium expressions of the constitutive variables. Non-equilibrium responses are proposed by use of a quasi-linear theory, in particular a hypoplastic-type relation is introduced to model the internal friction and plastic effects. It is illustrated that the Goodman,Cowin theory can appropriately be extended to include frictional effects into the evolution equation of the volume fraction (i.e. the so-called balance of equilibrated force) and the equilibrium expression of the Cauchy stress tensor. The implemented model is applied to investigate conventional steady isothermal granular flows with incompressible grains, namely simple plane shear, inclined gravity-driven and vertical channel-flows, respectively. Numerical results show that the hypoplastic effect plays a significant role in the behaviour of a flowing granular material. The obtained profiles of the velocity and the volume fraction with hypoplastic features are usually sharper and the shear-thinning effect is more significant than that without such plastic effects. This points at the possible wide applicability of the present model in the fields of granular materials and soil mechanics. In addition, the present paper also provides a framework for a possible extension of the hypoplastic theories which can be further undertaken. Copyright © 2006 John Wiley & Sons, Ltd. [source] Flow of particles suspended in a sheared viscous fluid: Effects of finite inertia and inelastic collisionsAICHE JOURNAL, Issue 10 2010Micheline Abbas Abstract We investigate in this article the macroscopic behavior of sheared suspensions of spherical particles. The effects of the fluid inertia, the Brownian diffusion, and the gravity are neglected. We highlight the influence of the solid-phase inertia on the macroscopic behavior of the suspension, considering moderate to high Stokes numbers. Typically, this study is concerned with solid particles O (100 ,m) suspended in a gas with a concentration varying from 5% to 30%. A hard-sphere collision model (with elastic or inelasic rebounds) coupled with the particle Lagrangian tracking is used to simulate the suspension dynamics in an unbounded periodic domain. We first consider the behavior of the suspension with perfect elastic collisions. The suspension properties reveal a strong dependence on the particle inertia and concentration. Increasing the Stokes number from 1 to 10 induces an enhancement of the particle agitation by three orders of magnitude and an evolution of the probability density function of the fluctuating velocity from a highly peaked (close to the Dirac function) to a Maxwellian shape. This sharp transition in the velocity distribution function is related to the time scale which controls the overall dynamics of the suspension flow. The particle relaxation (resp. collision) time scale dominates the particulate phase behavior in the weakly (resp. highly) agitated suspensions. The numerical results are compared with the prediction of two statistical models based on the kinetic theory for granular flows adapted to moderately inertial regimes. The suspensions have a Newtonian behavior when they are highly agitated similarly to rapid granular flows. However, the stress tensors are highly anisotropic in weakly agitated suspensions as a difference of normal stresses arises. Finally, we discuss the effect of energy dissipation due to inelastic collisions on the statistical quantities. We also tested the influence of a simple modeling of local hydrodynamic interactions during the collision by using a restitution coefficient which depends on the local impact velocities. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] Wet granulation in a twin-screw extruder: Implications of screw designJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 4 2010M.R. Thompson Abstract Wet granulation in twin-screw extrusion machinery is an attractive technology for the continuous processing of pharmaceuticals. The performance of this machinery is integrally tied to its screw design yet little fundamental knowledge exists in this emerging field for granulation to intelligently create, troubleshoot, and scale-up such processes. This study endeavored to systematically examine the influence of different commercially available screw elements on the flow behavior and granulation mechanics of lactose monohydrate saturated at low concentration (5,12%, w/w) with an aqueous polyvinyl-pyrrolidone binder. The results of the work showed that current screw elements could be successfully incorporated into designs for wet granulation, to tailor the particle size as well as particle shape of an agglomerate product. Conveying elements for cohesive granular flows were shown to perform similar to their use in polymer processing, as effective transport units with low specific mechanical energy input. The conveying zones provided little significant change to the particle size or shape, though the degree of channel fill in these sections had a significant influence on the more energy-intensive mixing elements studied. The standard mixing elements for this machine, kneading blocks and comb mixers, were found to be effective for generating coarser particles, though their mechanisms of granulation differed significantly. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99: 2090,2103, 2010 [source] Non-homogeneous Navier,Stokes systems with order-parameter-dependent stressesMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 13 2010Helmut Abels Abstract We consider the Navier,Stokes system with variable density and variable viscosity coupled to a transport equation for an order-parameter c. Moreover, an extra stress depending on c and ,c, which describes surface tension like effects, is included in the Navier,Stokes system. Such a system arises, e.g. for certain models of granular flows and as a diffuse interface model for a two-phase flow of viscous incompressible fluids. The so-called density-dependent Navier,Stokes system is also a special case of our system. We prove short-time existence of strong solution in Lq -Sobolev spaces with q>d. We consider the case of a bounded domain and an asymptotically flat layer with a combination of a Dirichlet boundary condition and a free surface boundary condition. The result is based on a maximal regularity result for the linearized system. Copyright © 2010 John Wiley & Sons, Ltd. [source] | ||||||||||||||||