Flow Of Granular Materials Research Articles

Gas flow and heat transfer in granular energy-releasing materials: Novel computational model and important features

In this paper, the process of flowing the coolant gas through granular or porous media with heat-generating sources is considered. Such process takes place in various ruinous objects with energy release sources formed after natural or man-made disasters, e.g., in destroyed power unit of a nuclear power plant like exploded reactor No. 4 of the Chernobyl NPP. Also, this process is typical for some technologies, e.g., for innovative pebble-bed nuclear reactors. The aim of this work is, firstly, to develop a tool for numerical simulation of 3D processes in porous media with energy release sources, which will allow one to get the proper results quickly, and, secondly, to study some important features of these processes. A computational model and OpenFOAM solver have been developed in the work to simulate the fluid flow and heat transfer in granular or porous energy-releasing materials. The proposed mathematical model is based on the assumption of interacting interpenetrating continua, and the developed numerical method is based on the PIMPLE algorithm adapted for the system of governing equations. The novel OpenFOAM solver has been tested on 1D and 2D problems, which were previously solved by known algorithms based on the finite difference method. The new solver is faster by 1–3 orders of magnitude (depending on the number of cells in the mesh) and makes it possible to carry out calculations of 2D and 3D processes without parallelization within reasonable time, in contrast to the previously used numerical methods. The 3D gas flows through porous energy-releasing object have been studied, and some important features of these processes have been revealed. It has been shown that the temperature dependence of the gas dynamic viscosity leads not only to the fact that the gas tends to flow around the hot zones, but also to an additional increase in temperature in the heated area, caused by a drop of the density and velocity of the gas due to increasing the flow resistance force in this region. It has been revealed that if the total intensity of the energy release in the porous object is known a priori, but the relative location of the energy release sources is unknown, it is impossible to predict the maximum temperature of the object.

Confined sheared flows of hard and soft granular materials: Some challenges in tribology and fault mechanics

This paper belongs to a Focus Series dedicated to Challenges in Granular Matter. As such, it does not report any new finding, but rather offers an overview of the state of the art and of current and future challenges for the granular physics community. The focus is placed on the case of confined sheared flows of granular samples with particular complexities, such as soft, angular, or cemented grains, with applications in tribology and fault mechanics. We first explain why granular science may help to improve our understanding of dry friction in these two applications, and illustrate some recently unveiled aspects regarding the complexity of the problem. We then describe a few avenues for future research on the topic, focusing on the dialogue between simulations and experiments, on scale-related challenges, and on some pivotal computational aspects.

The Nonlinear Dynamic Behavior of a Particle on a Vibrating Screen Based on the Elastoplastic Contact Model

The dynamic response of particles is closely related to screening efficiency. To study the dynamic response of particles, the dynamic equations of a particle on a screening surface are established based on the elastoplastic contact model of spherical particles and are solved for the coal particle. Then, the trajectories of the particles are given with different falling heights and particle radii. The completely different trajectories with slight changes in the falling height and particle radius indicate strong nonlinearity. Second, the nonlinear dynamic behavior under different amplitudes and frequencies is discussed, and the route of transition from quasiperiodic motion to chaotic motion is revealed. Finally, we discuss the average speed along the screening surface considering the frequency, amplitude, friction coefficient, inclination angle, and vibration direction angle. In addition, the convergence conditions of particle motion are proposed, and they are only affected by the inclination angle and friction angle. The results show that in the normal direction of the vibrating screen, the particle motion is quasiperiodic at low frequencies. With increasing frequency, the motion of the particle becomes chaotic, and its Poincaré map becomes petal-shaped. In addition, the number of petals increases at the mutation of the bifurcation diagram. The increase in frequency, amplitude and inclination angle and the decrease in friction coefficient lead to an increase in particle speed along the screen surface. In addition, the particle speed reaches a maximum when the vibration direction angle is 65°. This work provides a theoretical basis for controlling the thickness of granular material flow on a vibrating screen and selecting screening process parameters.

Gravity-driven granular flows in pipes: teaching experimental skills in the context of granular flows

Granular flows appear frequently in the natural world and in civil engineering applications. These flows can exhibit features which are surprising and counter-intuitive and are often used to test the limits of the classical continuum approximation for modelling of fluid flows. An important sub-class of the granular flows are the gravity-driven granular flows, which include the granular column collapse and the flow of granular material down through a vertical pipe. In this article, quantitative analysis is performed of the flow using video analysis software. The utility and relevance of the experiment for development of experimental skills in physics students and modelling of unexpected phenomena is discussed.

Stick-Slip Dynamics in a Granular Material With Varying Grain Angularity

Experiments, simulations, and theoretical treatments of granular materials typically feature circular or elliptical grains. However, grains found in natural systems often have flat faces that introduce local rotational constraints; these rotational constraints have been shown to affect, for example, the jamming transition, discontinuous shear thickening, and ordered states in colloids and thermalized grains. In this work, we experimentally investigate the effects of grain angularity on stick-slip dynamics. A weighted slider is pulled by a spring over a gravity-packed granular bed composed of polygonal grains with varying angularity. We find that packings of triangular or square grains have higher shear strengths than packings of pentagons, hexagons, heptagons, or disks. Additionally, as the number of sides increases, sticking periods, during which the slider remains motionless while the spring force on it increases, become shorter on average, with the material yielding at smaller applied stresses. Lastly, we find that dilation of the medium during sticking periods tends to be larger for grains with higher angularity, in part because of the presence of stilt-like columnar structures that prop the slider up. We report on measurements of the pulling force on the slider, particle dynamics during slip events, and properties of force-bearing contact networks identified via photoelasticity. Our findings indicate that high angularity of grains (pentagons, squares, triangles) leads to differences in grain-scale flow and macroscopic stick-slip dynamics of bulk granular materials. Our experiments also indicate a continuous change in dynamics with decreasing angularity as the circular grain limit is approached.

Scale-up of granular material flow in an agitated filter dryer

A challenge in agitated drying is to take information from the laboratory scale and use it to scale up the process to the manufacturing scale. In this study, we focus on scale-up of the granular flow in an agitated dryer by measuring the impeller torque which can be related to the particle shear stress. The shear stress can in turn be related to particle breakage. The impeller torque was independent of rotation rate and increased linearly with material bed height, and the particle size had an impact on the torque. The bulk friction coefficient and material cohesiveness were computed from a linear correlation between normal stresses and shear stresses in the particle bed. Torque predictions from the bulk friction coefficients exhibited good agreement with torque measurements. This work serves as an effective way to predict the granular flow on a larger scale using data from the laboratory scale.

On the Fidelity of Computational Models for the Flow of Milled Loblolly Pine: A Benchmark Study on Continuum-Mechanics Models and Discrete-Particle Models

The upstream of bioenergy industry has suffered from unreliable operations of granular biomass feedstocks in handling equipment. Computational modeling, including continuum-mechanics models and discrete-particle models, offers insightful understandings and predictive capabilities on the flow of milled biomass and can assist equipment design and optimization. This paper presents a benchmark study on the fidelity of the continuum and discrete modeling approaches for predicting granular biomass flow. We first introduce the constitutive law of the continuum-mechanics model and the contact law of the coarse-grained discrete-particle model, with model parameters calibrated against laboratory characterization tests of the milled loblolly pine. Three classical granular material flow systems (i.e., a lab-scale rotating drum, a pilot-scale hopper, and a full-scale inclined plane) are then simulated using the two models with the same initial and boundary conditions as the physical experiments. The close agreement of the numerical predictions with the experimental measurements on the hopper mass flow rate, the hopper critical outlet width, the material stopping thickness on the inclined plane, and the dynamic angle of repose, clearly indicates that the two methods can capture the critical flow behavior of granular biomass. The qualitative comparison shows that the continuum-mechanics model outperforms in parameterization of materials and wall friction, and large-scale systems, while the discrete-particle model is more preferred for discontinuous flow systems at smaller scales. Industry stakeholders can use these findings as guidance for choosing appropriate numerical tools to model biomass material flow in part of the optimization of material handling equipment in biorefineries.

Measuring powder flow properties in a rotating drum

The rotating drum geometry is widely used to analyze the flow of powders and granular materials in low consolidation conditions. Many different physical quantities can be measured using image analysis algorithms. This variety of methods makes it difficult to compare the results obtained with different set-ups and methods. In this paper, the set of physical quantities (Dynamic angle of repose α, Cohesive index CI, Roughness index RI, Aeration AE and Thixotropy index TI) measured by GranuDrum instrument from the images using dedicated algorithms are presented in details. Moreover, the results obtained with a set of typical powders are analyzed to illustrate the interpretation of the different measurements. For the whole set of parameters, a strong dependence to the drum rotating speed is shown by the measurement results, highlighting the importance of the measurement at different flowing speeds. Moreover, time-dependent or shear-dependent properties are evidenced.

Reduction mechanism of the normal stress exerted on silo wall with chutes during eccentric discharge

This paper studied the reduction mechanism of wall normal stress exerted on silo wall with chutes during eccentric discharge utilizing the experiment and analytical methods. The result shows that the chute leads to form a new flow chamber and drives the original flow chamber to move to the center of the silo. The double flow chamber makes the granular materials flow out orderly from the discharge port, which weakens the interaction force when the granular materials are rearranged at the variable cross section of the flow chamber. The granular materials outside the chutes enter the new flow chamber through the gap between the chutes, which reduces the interaction force of the particles in the original flow chamber and effectively weakens the density of cluster strength chains. Instantaneous arch foot loses the support of silo wall, resulting in reducing the wall normal stress exerted on silo wall with chutes.

Effect of Particles Shape on the Hydraulic Conductivity of Stokesian Flow in Granular Materials

Effect of Particles Shape on the Hydraulic Conductivity of Stokesian Flow in Granular Materials

Mechanisms and Kinetics of Separation of Impurities of Large and Small Particles in Gravity Flow of Granular Material

Mechanisms and Kinetics of Separation of Impurities of Large and Small Particles in Gravity Flow of Granular Material

An instrument for in situ characterization of powder spreading dynamics in powder-bed-based additive manufacturing processes.

In powder-bed-based metal additive manufacturing (AM), the visualization and analysis of the powder spreading process are critical for understanding the powder spreading dynamics and mechanisms. Unfortunately, the high spreading speeds, the small size of the powder, and the opacity of the materials present a great challenge for directly observing the powder spreading behavior. Here, we report a compact and flexible powder spreading system for in situ characterization of the dynamics of the powders during the spreading process by high-speed x-ray imaging. The system enables the tracing of individual powder movement within the narrow gap between the recoater and the substrate at variable spreading speeds from 17 to 322 mm/s. The instrument and method reported here provide a powerful tool for studying powder spreading physics in AM processes and for investigating the physics of granular material flow behavior in a confined environment.

Comparison of the compressible class of models and non-local models with the discrete element method for steady fully developed flow of cohesionless granular materials through a vertical channel

The flow of granular materials through a vertical channel is examined using the discrete element method (DEM) and the recent continuum models of Henann & Kamrin (Proc. Natl Acad. Sci. USA, vol. 110, 2013, pp. 6730–6735), Barker et al. (Proc. R. Soc. Lond. A, vol. 473, 2017, p. 20160846), Schaeffer et al. (J. Fluid Mech., vol. 874, 2019, pp. 926–951) and Dsouza & Nott (J. Fluid Mech., vol. 888, 2020, p. R3). The channel is bounded by walls separated by a distance $2 \, W$ in the $x$ -direction. For the DEM, periodic boundary conditions are used in the $z$ - and $y$ - (vertical) directions with no exit at the bottom of the channel. The governing equations reduce to ordinary differential equations in the $x$ -direction. There is a plug layer near the centre and a shear layer near the wall, as observed in experiments. There is a decrease in the solids fraction $\phi$ in the shear layer, except for the models of Barker et al. and Henann & Kamrin. A modification of the latter gives more realistic $\phi$ profiles. The thickness of the shear layer depends on $2\,W$ and the bulk solids fraction $\bar {\phi }$ . For all the models, solutions could not be obtained for some parameter values. An example is the negative fluidity in the model of Henann & Kamrin. The model of Dsouza & Nott predicts much higher normal stresses, possibly because of large contributions from the non-local terms. None of the models specify a complete set of boundary conditions (b.c.). The DEM results suggest that the slip velocity and the wall friction b.c. lead to a slip length and an angle of wall friction that are independent of $2\,W$ . The models are based on extensions of the equations for slow, rate-independent flow. A model that includes collisional effects, such as kinetic theory, should be combined with the present models. A preliminary analysis of the kinetic theory model of Berzi et al. (J. Fluid Mech., vol. 885, 2020, p. A27), shows that it may have undesirable feature.

Structural Inhomogeneity and Effects of Separation by Size and Density in Gravity Flow of Granular Materials

Structural Inhomogeneity and Effects of Separation by Size and Density in Gravity Flow of Granular Materials

A generalized model for dense axisymmetric grains flow with orientational diffusion

The flow of non-spherical grains is strongly affected by the orientation of the grains, as observed in experiments and simulations. An existing model that predicts the orientation of grains as a function of the flow field and shape of the grains, is limited to homogenous orientational fields where the interaction of the grains with their surroundings is negligible. To address this limitation, we generalized the model to account for inhomogeneous orientational fields in the form of spatial non-convective orientational flux. The orientational flux accounts for the interactions of grains with their surroundings and the boundaries. The proposed model is used to study the influence of orientational diffusion and boundary conditions on the flow of dense granular material down an incline. It is shown that the boundary conditions can play a significant role in the orientation of grains in such flows.

Flow and arrest in stressed granular materials.

Flowing granular materials often abruptly arrest if not driven by sufficient applied stresses. Such abrupt cessation of motion can be economically expensive in industrial materials handling and processing, and is significantly consequential in intermittent geophysical phenomena such as landslides and earthquakes. Using discrete element simulations, we calculate states of steady flow and arrest for granular materials under the conditions of constant applied pressure and shear stress, which are also most relevant in practice. Here the material can dilate or compact, and flow or arrest, in response to the applied stress. Our simulations highlight that under external stress, the intrinsic response of granular materials is characterized by uniquely-defined steady states of flow or arrest, which are highly sensitive to interparticle friction. While the flowing states can be equivalently characterized by volume fraction, coordination number or internal stress ratio, to characterize the states of shear arrest, one needs to also consider the structural anisotropy in the contact network. We highlight the role of dilation in the flow-arrest transition, and discuss our findings in the context of rheological transitions in granular materials.

Heat Transfer and Gas Flow in Granular Energy-Releasing Materials: Novel Computational Model and Important Features

Heat Transfer and Gas Flow in Granular Energy-Releasing Materials: Novel Computational Model and Important Features

Ispitivanje uticaja veličine čestica na mehaničko ponašanje granularnog materijala metodom diskretnih elemenata

Granular materials are used in various industries, including pharmaceutical and agriculture, where the material properties of elements have an important impact on their flow behavior. Numerical codes based on the Discrete Element Method (DEM) are decisive for describing the flow of granular material. The DEM could investigate granular materials' macro and micro-mechanical shear behaviors. The commercial software EDEM® based on the DEM was utilized for this purpose. A gravitational disposition for the geometrical arrangement model has been performed in this study to model different particle sizes for a direct, simple shear test (DSST). The results indicated that referring to the size index (SI), a positive correlation occurred with the shear strength, dilation, volumetric strain, a negative correlation with the average particle angular velocity, and a neutral correlation with the coordination number (CN).

Evolution of separation effects under variable conditions of gravity flows of granular materials

Evolution of separation effects under variable conditions of gravity flows of granular materials

Gravity Governs Shear Localization in Confined Dense Granular Flows.

The prediction of flow profiles of slowly sheared granular materials is a major geophysical and industrial challenge. Understanding the role of gravity is particularly important for future planetary exploration in varying gravitational environments. Using the principle of minimization of energy dissipation, and combining experiments and variational analysis, we disentangle the contributions of the gravitational acceleration, confining pressure, and layer thickness on shear strain localization induced by moving fault boundaries at the bottom of a granular layer. The flow profile is independent of the gravity for geometries with a free top surface. However, under a confining pressure or if the sheared layer withstands the weight of the upper layers, increasing gravity promotes the transition from closed shear zones buried in the bulk to open ones that intersect the top surface. We show that the center position and width of the shear zone and the axial angular velocity at the top surface follow universal scaling laws when properly scaled by the gravity, applied pressure, and layer thickness. Our finding that the flow profiles lie on a universal master curve opens the possibility to predict the quasistatic shear flow of granular materials in varying gravitational environments.