Bulk Granular Material Research Articles

Mathematical model, numerical simulation and optimization of rotating valve feeder in animal feed production

The processes of transportation of bulk materials from silos and hoppers are significant in various industrial applications because of their influences on material characteristics and working parameters of the production process. In this paper, a rotating valve feeder, with eight vanes was investigated for transport action of bulk materials, such as wheat, maize and rice, which were ground, using the sieve sizes of 1, 3 and 5 mm. The rotating valve feeders under investigation have proven to be useful in transportation processes despite their construction simplicity. All investigations were done experimentally and numerically, using coupled Discrete Element Method (DEM) and Computational Fluid Dynamics calculation (CFD). The influences of different types of bulk materials and its particle size, on the performances of the rotating valve feeder during material transport were explored. The artificial neural network was developed (in the form of a multi-layer perceptron model) in order to optimize the granular flow of the bulk material, showing the high prediction capability of bulk density, dosing time and granular material flow, with the coefficient of determination equal to 0.999 during the training period. The decreasing of the sieve opening diameter caused the decrease in bulk density of the ground material, but statistically significant only for rice, as seen from the experiments and the results of the neural network model. The 5 mm sieve ensured the material with the highest flowability, significantly increasing the granular flow and decreasing the dosing time. The granular particles were modelled as the spheres in the DEM/CFD simulation, with a small-sized triangular surfaces. The DEM/CFD prediction of the mass transport for rice, wheat and maize was quite adequate, obtaining the coefficient of determination being 0.997; 0.998 and 0.849, respectively.

Optimal strengthening of particle-loaded liquid foams.

Foams made of complex fluids such as particle suspensions have a great potential for the development of advanced aerated materials. In this paper, we study the rheological behavior of liquid foams loaded with granular suspensions. We focus on the effect of small particles, i.e., particle-to-bubble size ratio smaller than 0.1, and we measure the complex modulus as a function of particle size and particle volume fraction. With respect to previous work, the results highlight a new elastic regime characterized by unequaled modulus values as well as independence of size ratio. A careful investigation of the material microstructure reveals that particles organize through the network between the gas bubbles and form a granular skeleton structure with tightly packed particles. The latter is proven to be responsible for the reported new elastic regime. Rheological probing performed by strain sweep reveals a two-step yielding of the material: The first one occurs at small strain and is clearly attributed to yielding of the granular skeleton; the second one corresponds to the yielding of the bubble assembly, as observed for particle-free foams. Moreover, the elastic modulus measured at small strain is quantitatively described by models for solid foams in assuming that the granular skeleton possesses a bulk elastic modulus of order 100 kPa. Additional rheology experiments performed on the bulk granular material indicate that this surprisingly high value can be understood as soon as the magnitude of the confinement pressure exerted by foam bubbles on packed grains is considered.

Strong enhancement of bulk superconductivity by engineered nanogranularity

It is now well established, both theoretically and experimentally, that very small changes in the size of isolated nanograins lead to substantial nonmonotonic variations, and sometimes enhancement, of the mean-field spectroscopic gap of conventional superconductors. A natural question to ask, of broad relevance for the theory and applications of superconductivity, is whether these size effects can also enhance the critical temperature of a bulk granular material composed of such nanograins. Here we answer this question affirmatively. We combine mean-field, semiclassical, and percolation techniques to show that engineered nanoscale granularity in conventional superconductors can enhance the critical temperature by up to a few times compared to the nongranular bulk limit. This prediction is valid for three-dimensional and also quasi-two-dimensional samples, provided the thickness is much larger than the grain size. Our model takes into account an experimentally realistic distribution of grain sizes in the array, charging effects, tunnelling by quasiparticles, and limitations related to the proliferation of thermal fluctuations for sufficiently small grains.

Vibration-induced phenomena in bulk granular materials

Vibration is one of the mechanisms affecting bulk granular materials behaviour in transportation and material separation efficiency in pharmaceutical, chemical, food, mineral processing and other industries. Understanding fundamentals of vibration and their influence on bulk materials handling is gaining an increasing importance in the economies of scale. However, at present there appears to be no accurate and sufficient description of various phenomena observed in a granular material affected by vibration. This paper presents a concise review of physical phenomena observed in bulk granular materials affected by vibration, taken here as mechanical oscillations of relatively low amplitude and relatively high frequency. This includes three main characteristic regimes of vibration defined as vibro-fluidisation, vibro-compaction and vibro-boiling. The analysis of other factors influencing bulk granular material behaviour under vibration including air resistance, wave effects and particle oscillations decay are also given. A new characteristic regime of vibration is introduced, defined as vibro-hovering. Description of main characteristic vibration-induced bulk granular material states combined with the analysis of other contributing factors result in a more complete classification of vibration phenomena and an improved understanding of vibration-induced bulk material behaviour. Analytical conclusions are supported by the results of experimental studies, with significant potential for improvement in material separation efficiency indicated. Prerequisite vibrational parameters for effective bulk granular material flow promotion are discussed with a specific focus on physical separation processes and optimum ranges of vibrational parameters for separation and flow promotion of dry particulate matter are recommended.

Surface roughness effects in granular matter: Influence on angle of repose and the absence of segregation

We investigate the effect of nanoscale variations in the surface roughness of individual particles on macroscale granular flow characteristics. Experiments were conducted in circular rotating tumblers with smooth and rough 2 and 3 mm steel particles. The smooth beads had a rms surface roughness of approximately 30 to 60 nm; rough beads had a surface roughness of approximately 240 to 350 nm. The dynamic angle of repose for rough particles increased by 10 degrees to 25 degrees over that of smooth particles over a wide range of rotation speeds. Even though surface roughness affects the angle of repose, we were unable to detect any segregation of bidisperse mixtures of rough and smooth particles in the radial direction in two-dimensional (2D) tumblers. Furthermore, no axial banding segregation occurred in 3D tumblers, both cylindrical and spherical. For mixtures of smooth and rough particles, the angle of repose increased monotonically with increasing concentration of rough particles. Particle dynamics simulations verified that the dependence of the angle of repose on the concentration of rough particles can be directly related to the coefficient of friction of the particles. Simulations over a broad range of friction parameters failed to induce segregation solely from differences in the angle of repose. These results indicate that nanoscale surface roughness can affect the flowability and angle of repose of granular matter without driving demixing of the bulk granular material.

Discrete element simulation of the behavior of bulk granular material during truck braking

PurposeTo simulate the dynamic feature of bulk granular material (such as agricultural products) during sudden braking of a truck by applying discrete element method.Design/methodology/approachThe bulk granular material was modeled by the discrete element approach, in which the spherical elements were used to represent the granular particles; the interaction between two in‐adhesive particles was modeled by Hertz for normal interaction, and by Mindlin and Deresiewicz for tangential interaction; the interaction between two particles with adhesion was modeled by the JKR theory for normal interaction, and by Thornton's theory for the tangential interaction. Different initial conditions (braking speeds/accelerations) were considered. The dynamic system was numerically solved by the central difference based explicit time integration, and the dynamic impact forces were recorded to further analysis.FindingsThe computation predicted that the resultant dynamic force acting upon the front wall behaves in four stages, i.e. increasing, plateau, sharp increasing and drop with damped fluctuation. It was observed that, the shorter the breaking time is, the faster the force reaches its peak, and the greater the peak value is. The phenomenon was in good agreement with physical principals and common knowledge.Research limitations/implicationsIt is an application of the discrete element method and, therefore, no important contribution is made to advance the methodology.Practical implicationsThe proposed modeling approach may serve as a useful tool for advanced design of trucks.Originality/valueThis paper is the first to apply the advanced discrete element method to the problem concerned.

Analysis of Forced Flow of Granular Materials in Vertical Pipes without and with Air Permeation

The problems associated with grain elevation and conveying under forced flow in vertical pipes are discussed. Based on experimental results, a theory is presented to describe forced flow with varying degrees of air permeation up to and just beyond the fluidization point. The theory takes into account the boundary and internal frictional properties, the degree of consolidation of the bulk granular material, and the stress fields that occur during forced flow. The force to elevate grain in a vertical tube is shown to be composed of two components, one to overcome Coulomb friction and initiate motion, and the other a time-dependent component that depends on the stiffness and damping characteristics of the granular material. The Coulomb friction component increases approximately exponentially with column height due to the positive feedback effect of the shear stresses at the pipe wall opposing the motion. Air permeation is shown to significantly reduce this component of the conveying force by reducing both the internal friction and the apparent bulk specific weight, the latter being the actual bulk specific weight less the air pressure gradient. Air permeation has a very significant effect on reducing both the bulk stiffness and, particularly, the damping characteristics, thereby reducing the time-dependent component of the conveying force.

Microscopic and macroscopic effects of surface lubricant films in granular shear

Experiments were conducted to investigate the link between particle-particle interaction forces and the bulk properties of granular shear using an idealized system of near-spherical, monosized glass beads. The atomic force microscopy colloidal probe technique was employed to investigate the adhesion and friction between a single bead and a second glass surface, while the annular shear cell was used to measure the shear properties of the bulk granular material. A covalently bound monomolecular film of aliphatic chains was introduced to alter the tribological interactions between the particles. The atomic force microscope was used to measure the reduction in the particle-particle surface forces resulting from the addition of the boundary lubricant, while the shear cell showed that the effect of the lubricant film was to reduce the coefficient of internal friction and the dilation during shear. This is an experimental study to provide quantitative data linking particle-particle interaction forces and the shear properties of a granular body.

Josephson behaviour of variable thickness bridges in textured YBa2Cu3O7−δ

Abstract The i - V characteristics of textured yttrium barium cuprate have been measured using variable thickness bridges (VTBs) fabricated from bulk granular material. The results show the characteristic DC Josephson effect and, using microwave irradiation, the presence of integral and sub-integral Shapiro steps in the characteristics. In addition a strong field dependence of the critical Josephson current is also found. The V - H variation at various junction currents shows a well developed periodic structure with features that may be associated with flux trapping in itergranular regions. These features, combined with the temperature dependence of the Josephson current support the view that we are seeing the behaviour of a small number of Josephson junctions in these VTBs. Comparable measurements on non-aligned material do not show these effects, probably because of the wide range of intergranular misorientation across the bridge.

Josephson behaviour of microbridges in textured YBa2Cu3O7−δ

The i-V characteristics of textured yttrium barium cuprate have been measured using variable thickness bridges (VTB) prepared from bulk granular material. The results show the presence of integral and sub-integral Shapiro steps in the characteristics in addition to a strong field dependence of the critical Josephson current. The V-H variation at various junction currents show a well developed periodic structure in addition to features which may be associated with flux trapping in intergranular regions. These features, combined with the Josephson current support the view that we are seeing the behaviour ofȧsingle Josephson junction in these VTB's. Comparabale measurements on non-aligned material do not show these effects, probably because of the wide range of intergranular misorientation across the bridge.