scholarly journals Effect of particle surface friction on nonlocal constitutive behavior of flowing granular media

2014 ◽  
Vol 1 (2) ◽  
pp. 169-176 ◽  
Author(s):  
Ken Kamrin ◽  
Georg Koval
Keyword(s):  
Granular Media ◽  
Particle Surface ◽  
Surface Friction ◽  
Constitutive Behavior

Particle Scale Heat Transfer Analysis in Rotary Kiln

2012 ◽  
Author(s):  
Amit Ravindra Amritkar ◽  
Danesh Tafti ◽  
Surya Deb
Keyword(s):  
Heat Transfer ◽  
Rotary Kiln ◽  
Granular Media ◽  
Particle Surface ◽  
Convection Heat Transfer ◽  
Heat Transfer Analysis ◽  
Convection Heat ◽  
Bed Temperature ◽  
Rotary Kilns ◽  
Pass Through

Rotary furnaces have multiple applications including calcination, pyrolysis, carburization, drying, etc. Heat transfer through granular media in rotary kilns is a complex phenomenon and plays an important role in the thermal efficiency of rotary furnaces. Thorough mixing of particles in a rotary kiln determines the bed temperature uniformity. Hence it is essential to understand the particle scale heat transfer modes through which the granular media temperature changes. In this study, numerical simulations are performed using coupled Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) to analyze heat transfer in a non-reacting rotary kiln. The microscopic models of particle-particle, particle-fluid, particle-surface and fluid-surface heat transfer are used in the analysis. The heat transfer simulations are validated against experimental data. The effect of particle cascading on the bed temperature is measured and contributions from various modes of particle scale heat transfer mechanisms are reported. Particles are heated near the rotary kiln walls by convection heat transfer as they pass through the thermal boundary layer of the heated fluid. These particles are transported to the center of the kiln where they transfer heat to the cooler particles in the core of the kiln and back to the cooler fluid at the center of the kiln. It is found that 90% of the heat transferred to particles from the kiln walls is a result of convection heat transfer, whereas only 10% of the total heat transfer is due to conduction from the kiln walls.

Granular shear flows at the elastic limit

2002 ◽  
Vol 465 ◽  
pp. 261-291 ◽  
Author(s):  
CHARLES S. CAMPBELL
Keyword(s):  
Stress Reduction ◽  
Particle Surface ◽  
Surface Friction ◽  
Internal Force ◽  
Force Chains ◽  
Rheological Parameter ◽  
Nonlinear Behaviour ◽  
Static Regime ◽  
Computer Simulation Studies ◽  
Inertial Regime

This paper describes computer simulation studies of granular materials under dense conditions where particles are in persistent contact with their neighbours and the elasticity of the material becomes an important rheological parameter. There are two regimes at this limit, one for which the stresses scale with both elastic and inertial properties (called the elastic–inertial regime), and a non-inertial quasi-static regime in which the stresses scale purely elastically (elastic–quasi-static). In these elastic regimes, the forces are generated by internal force chains. Reducing the concentration slightly causes a transition from an elastic to a purely inertial behaviour. This transition occurs so abruptly that a 2% concentration reduction can be accompanied by nearly three orders of magnitude of stress reduction. This indicates that granular flows near this limit are prone to instabilities such as those commonly observed in shear cells. Unexpectedly, there is no path between inertial (rapid) flow and quasi-static flow by varying the shear rate at a fixed concentration; only by reducing the concentration can one cause a transition from quasi-static to inertial flow. The solid concentrations at which this transition occurs as well as the magnitude of the stresses in the elastic regimes are strong functions of the particle surface friction, because the surface friction strongly affects the strength of the force chains. A parametric analysis of the elastic regime generated flowmaps showing the various regimes that might be realized in practice. Many common materials such as sand require such large shear rates to reach the elastic–inertial regime that it is unattainable for all practical purposes; such materials will demonstrate either an elastic–quasi-static behaviour or a pure inertial behaviour depending on the concentration – with many orders of magnitude of stress change between them. Finally, the effects of nonlinear contacts are investigated and an appropriate scaling is proposed that accounts for the nonlinear behaviour in the elastic–quasi-static regime.

Clusters in dense-inertial granular flows

2011 ◽  
Vol 687 ◽  
pp. 341-359 ◽  
Author(s):  
Charles S. Campbell
Keyword(s):  
Granular Flow ◽  
Significant Contribution ◽  
Cluster Formation ◽  
Particle Surface ◽  
Surface Friction ◽  
Momentum Transport ◽  
Cluster Development ◽  
Hard Sphere Models ◽  
Rate Limiting ◽  
Inertial Regime

AbstractIn the dense-inertial regime of granular flow, the stresses scale inertially, but the flow is dominated by clusters of particles. This paper describes observations of cluster development in this regime. Clusters were seen to form for both elastic and inelastic reasons: elastic when the shear rate pushes the particles together faster than the contacts can elastically disperse them, and inelastic as large energy dissipation leads to cluster formation. Furthermore, large particle surface friction leads to cluster formation both for structural reasons, because it generates stronger clusters, and for energetic reasons, as friction dissipates energy. However, the most intriguing result of this work is that clusters appear to have little effect on the rheology of the dense inertial regime, which suggests that one can model the dense inertial regime with entirely collisional hard sphere models, and not have to worry about the complexities of modelling clusters. But at the same time it presents a physical puzzle, as one would normally expect the rheology to be strongly dependent on microstructural features such as clusters, particularly as they present an elastic pathway for internal momentum transport. There is no completely satisfying explanation for why the clusters can be ignored, but two possibilities suggest themselves. Because the clusters are short-lived, it is possible that they do not survive long enough to make a significant contribution to the momentum transport. And it is also possible for the granular temperature that governs transport between clusters to act as a rate-limiting bottleneck that is in overall control of the momentum transport rate.

UNDERSTANDING FORCE CHAINS IN DENSE GRANULAR MATERIALS

2010 ◽  
Vol 24 (29) ◽  
pp. 5743-5759 ◽  
Author(s):  
QICHENG SUN ◽  
FENG JIN ◽  
JIANGUO LIU ◽  
GUOHUA ZHANG
Keyword(s):  
Particle Surface ◽  
Length Distribution ◽  
Surface Friction ◽  
Contact Forces ◽  
Force Chains ◽  
Compression Tests ◽  
Physical Processes ◽  
Dimensionless Numbers ◽  
Chain Length Distribution ◽  
Insight Into

When a load is applied to a dense granular material, the stress is largely transmitted by relatively rigid, heavily stressed chains of particles forming a sparse network of larger contact forces. Force chains act as the key determinant of mechanical properties such as stability, elasticity and flowability. To understand the structure and evolution of force chains, related physical processes and three corresponding characteristic time scales are analyzed in this study. We also propose three dimensionless numbers for the measurement of the relative importance of force chains. To solely study the effect of particle surface friction on force chains, uniaxial compression tests of 11,000 equal-sized particles in 2D were numerically simulated using the discrete element method. By proposing three conditions to define a force chain, the chain length distribution is found in the form of a power law. The exponent of 1.744 is independent of the surface friction. Although these results were obtained from partially crystallized jammed packings, they provide new insight into the physical processes and the structure of force chains, and thus will be helpful in the interpretation of force chains in other dense granular systems.

HORIZONS FOR DESIGN OF FILLED RUBBER INFORMED BY MOLECULAR DYNAMICS SIMULATION

2017 ◽  
Vol 90 (2) ◽  
pp. 238-263 ◽  
Author(s):  
Scott M. Smith ◽  
David S. Simmons
Keyword(s):  
Molecular Dynamics ◽  
Granular Media ◽  
Mechanical Response ◽  
Particle Surface ◽  
Dynamics Simulation ◽  
Particle Surface Area ◽  
Mechanical Reinforcement ◽  
Microscopic Mechanism ◽  
Filler Structure ◽  
Rubber Filler

ABSTRACT Fillers such as carbon black provide a long-standing and essential strategy for the mechanical reinforcement of rubber in tires and other load-bearing applications. Despite their technological importance, however, the microscopic mechanism of this reinforcement remains a matter of considerable debate. A predictive understanding of filler-based reinforcement could catalyze the design of new rubber-filler composites with enhanced performance. Molecular dynamics simulations of rubber mechanical response in the presence of structured fillers offer a new strategy for resolving the origins of filler-based reinforcement and guiding filler design. Results of for ideal rubber-filler dispersions over a range of filler structures suggest that neither hydrodynamic effects nor non-deformable “bound rubber domains” are necessary to achieve high reinforcement. Moreover, simulations show that particle surface area is a poor predictor of reinforcement. Instead, simulated reinforcement correlates strongly with filler structure, with more rarified filler structure predicting much greater reinforcement at fixed loading. Simulation results are consistent with a scenario in which reinforcement at industrially relevant loadings is dominated by formation of a jammed network of filler particles, suggesting that reinforced rubber can be understood as a superposition of two materials: a rubbery solid, and a jammed granular solid. This perspective points to an opportunity to improve filler-reinforced rubber design by leveraging concepts and expertise developed over many decades in the fields of jamming and granular media.

Advances in ultramicrotomy for physical sciences applications

1993 ◽  
Vol 51 ◽  
pp. 710-711
Author(s):  
G. McMahon ◽  
T. Malis
Keyword(s):  
Particle Surface ◽  
Zeolite Catalysts ◽  
Physical Sciences ◽  
Pull Out ◽  
Large Particles ◽  
Novel Applications ◽  
Metal Wires ◽  
Frequent Problem ◽  
Specific Orientation ◽  
Diamond Knife

As with all techniques which are relatively new and therefore underutilized, diamond knife sectioning in the physical sciences continues to see both developments of the technique and novel applications.Technique Developments Development of specific orientation/embedding procedures for small pieces of awkward shape is exemplified by the work of Bradley et al on large, rather fragile particles of nuclear waste glass. At the same time, the frequent problem of pullout with large particles can be reduced by roughening of the particle surface, and a proven methodology using a commercial coupling agent developed for glasses has been utilized with good results on large zeolite catalysts. The same principle (using acid etches) should work for ceramic fibres or metal wires which may only partially pull out but result in unacceptably thick sections. Researchers from the life sciences continue to develop aspects of embedding media which may be applicable to certain cases in the physical sciences.

Structural Studies on the Eukaryotic Ribosome

1990 ◽  
Vol 48 (1) ◽  
pp. 256-257
Author(s):  
Adriana Verschoor ◽  
Ronald Milligan ◽  
Suman Srivastava ◽  
Joachim Frank
Keyword(s):  
Chick Embryo ◽  
3D Reconstruction ◽  
Single Particle ◽  
Mass Density ◽  
Particle Surface ◽  
Uranyl Acetate ◽  
Electron Microscopic ◽  
Eukaryotic Ribosome ◽  
Negative Stain ◽  
Reconstruction Methods

We have studied the eukaryotic ribosome from two vertebrate species (rabbit reticulocyte and chick embryo ribosomes) in several different electron microscopic preparations (Fig. 1a-d), and we have applied image processing methods to two of the types of images. Reticulocyte ribosomes were examined in both negative stain (0.5% uranyl acetate, in a double-carbon preparation) and frozen hydrated preparation as single-particle specimens. In addition, chick embryo ribosomes in tetrameric and crystalline assemblies in frozen hydrated preparation have been examined. 2D averaging, multivariate statistical analysis, and classification methods have been applied to the negatively stained single-particle micrographs and the frozen hydrated tetramer micrographs to obtain statistically well defined projection images of the ribosome (Fig. 2a,c). 3D reconstruction methods, the random conical reconstruction scheme and weighted back projection, were applied to the negative-stain data, and several closely related reconstructions were obtained. The principal 3D reconstruction (Fig. 2b), which has a resolution of 3.7 nm according to the differential phase residual criterion, can be compared to the images of individual ribosomes in a 2D tetramer average (Fig. 2c) at a similar resolution, and a good agreement of the general morphology and of many of the characteristic features is seen.Both data sets show the ribosome in roughly the same ’view’ or orientation, with respect to the adsorptive surface in the electron microscopic preparation, as judged by the agreement in both the projected form and the distribution of characteristic density features. The negative-stain reconstruction reveals details of the ribosome morphology; the 2D frozen-hydrated average provides projection information on the native mass-density distribution within the structure. The 40S subunit appears to have an elongate core of higher density, while the 60S subunit shows a more complex pattern of dense features, comprising a rather globular core, locally extending close to the particle surface.

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