Silo Flow Research Articles

Improved particle tracking velocimetry based on level set segmentation for measuring the velocity field of granular flow

Abstract Using traditional particle tracking velocimetry based on optical flow for measuring areas with large velocity gradient changes may cause oversmoothing, resulting in significant measurement errors. To address this problem, the traditional particle tracking velocimetry method based on an optical flow was improved. The level set segmentation algorithm was used to obtain the boundary contour of the region with large velocity gradient changes, and the non-uniform flow field was divided into regions according to the boundary contour to obtain sub-regions with uniform velocity distribution. The particle tracking velocimetry method based on optical flow was used to measure the granular flow velocity in each sub-region, thus avoiding the problem of granular flow distribution. The simulation results show that the measurement accuracy of this method is approximately 10% higher than that of traditional methods. The method was applied to a velocity measurement experiment on dense granular flow in silos, and the velocity distribution of the granular flow was obtained, verifying the practicality of the method in granular flow fields.

Mass discharge rate of granular flow in eccentric silos with variable side wall friction

There are many studies on the effect of side wall friction on mass discharge rate (MDR), but the physical mechanism is lacking. In this paper, by changing side wall friction of eccentric silos, the relationship between macroscopic MDR and microscopic frictional coefficient was established, and the regulation of MDR up to a range of 40 % was realized. Furthermore, it is revealed that the variation of MDR is caused by the change in geometric structure of free-fall arch (FFA). In addition, the velocity of particles on FFA is the same under different MDR. The reason is that under high frictional coefficient, the gravitational potential energy of particles is more dissipated in the rotational motion caused by resistance. This work explains for the first time the mechanism of the frictional coefficient on MDR, and provides a data reference for improving the theoretical model of MDR.

The dynamic evolution of powder flow and wall normal stress in different flow pattern silos

Major safety accidents involving silos in industrial processes are closely associated with abnormal wall stresses. The effects of different flow patterns and particle velocity distribution on wall stresses are investigated during silo discharge. The existence of particle velocity retardation layer near the wall boundary in the mass flow was observed by tracer particles and high-speed camera. The silo discharge undergoes a transformation from funnel flow to mass flow at a hopper half angle of 30°. As the outlet diameter increases, the time point of the flow pattern transformation becomes more and more later. The evolution of particle velocity in the central of the funnel flow is related to the outlet velocity wave propagation and the V-shaped surface expansion. The relationship between stress fluctuations and velocity characteristics is established. The flow channel simultaneously expands upwards as the velocity wave propagates and generates a periodic fan-shaped velocity wave at the top. The formation of stress concentration zone at the bin/hopper transition was observed.

DEM modeling of the dilute-to-dense transition of granular flow in silos

The dilute-to-dense transition of granular flow in silo has been investigated by researchers, yet a detailed particle-scale description of this transition is still lacking. In this study, three-dimensional (3D) discrete element method (DEM) simulations are performed to investigate the dynamics of particles during this transition process through adjusting the inflow rate at the upper inlet. Based on Voronoi tessellation, the concept of caged particles is introduced to characterize the microscopic packing details of particles inside the silo. The analyses of probability density functions (PDFs) of vertical velocity and local packing fraction of particle in the proximity of outlet suggest that the dilute flow regime can be divided into two sub-stages (stage I and stage II). And the transition from stage I to stage II is manifested by the formation of persistent connection between the left and right clusters made of caged particles. It is found that for the dilute-to-dense transition process under fixed inflow rate condition, there exist two well defined critical packing heights that scale with the outlet size. Dynamic buildup and collapse of contact force network astride the whole outlet take place when the packing height of particle assembly at the centerline is in between these two critical heights. The simulation results thus suggest that the formation of a persistent contact force network spanning the whole outlet signals the transition of flow state from the dilute to dense.

Flow and clogging in a horizontal silo with a rotary obstacle

The external perturbation applied to a silo and the placement of an immobile obstacle before an exit are two common and effective ways to diminish clogging in the hopper/silo flow. Here, we incorporate the local perturbation into a fixed obstacle, and experimentally explore the effects of a rotary obstacle on clogging and the flowing characteristics in the horizontal silo flow driven by a conveyor belt. Even with a low spin rate, the total blocking probability that a particle constructs a stable blocking arch with its neighbors significantly drops. Correspondingly, the average flow rate of the particles through the exit abruptly rises, at least 1 order of magnitude better than that with an immobile obstacle and approaching the flow rate of continuous flow. The rotation enhances the breakage of clogging arches, which is responsible for improving the flowability in the horizontal silo. In addition, there always exists an optimal obstacle position at which the total blocking probability is minimal and the average flow rate peaks, regardless of the spin rate. Finally, clogging is relieved with the increase of the driving velocity of the conveyor belt, showing a “fast is fast” effect that is opposite to the “fast is slow” effect in other systems such as crowd evacuation and gravity-driven hoppers.

Silo flow and clogging in the presence of an obstacle

In a simple silo flow, the presence of an obstacle has been found to suppress clogging, but little systematic work has been done to test how general this is. By systematically varying the size and position of an obstacle, we find that generally an obstacle does suppress clogging, but in some cases may enhance clogging. By examining micro/mesoscale features of the flow we find that, even when comparing scenarios where clogging is suppressed, the reasons for the suppression may be different -- the intruder introduces both geometric and dynamic effects which compete differently.

Translation and rotation of particles in different flow pattern areas of a silo

The present paper reports the results obtained for translational and rotational velocity profiles of spherical particles for the mixed flow in a conical silo. The discrete element method (DEM) based on Hertz-Mindlin (no slip) with RVD rolling friction contact model is used for simulations. Opposite correlations are found between translational and rotational velocities in different flow areas of the silo. In particular, the abrasion caused by rotation is dominant in the funnel flow area. In addition, increase of the mass flow rate of silo can effectively reduce the abrasion induced by rotation. This highlights that understanding of dynamic characteristics of particles is helpful for optimization of silos and reduction of granular material abrasion.

Experimental analysis of wheat-wall friction and grain flow in a steel silo with corrugated walls

Frictional behaviour of wheat on corrugated steel walls, pressure distribution, and grain flow were investigated using a silo model with a square cross section. Corrugated steel sheets were attached to both side walls of the model silo, while clear polymethyl methacrylate was used for the front and rear walls to allow observation of the flow. In addition, a truncated steel pyramid was used for the hopper and a square outlet with sides varying from 0.02 m to 0.06 m. A test panel was placed at one of the side walls in the place of a cut off fragment of the wall to measure lateral and frictional wall forces. The tests were conducted using winter wheat, and two filling methods (central and distributed) were considered to analyse their possible influence on bulk density or shear behaviour during silo discharge. It was found for the distributed filling that the grain-wall friction coefficient during discharge was nearly 20% larger than that for the central filling. This coefficient was found to increase slightly with the increasing size of the outlet, a trend that was not observed for the central filling. The experimentally determined pressures and coefficient of wheat-wall friction were compared with the recommendations of Eurocode standard EN 1991-4. Differences between the experimental and theoretical results demonstrate that the formulae proposed by the Eurocode do not take into account some important issues associated with friction between the grain and the corrugated silo wall. • A silo with corrugated steel walls was constructed. • A test panel was placed at the side wall to measure lateral and vertical forces. • A side length of an square outlet in truncated pyramid hopper was varied. • No effect of the filling method on the mass flow rate of material was found. • Larger wheat-wall friction coefficient for distributed filling was obtained.

Measurement of granular temperature and velocity profile of granular flow in silos

In this paper we report experimental results on the granular temperature and velocity profile of glass beads in steady-state flow inside a flat-bottomed silo. We found that the dynamics of the granular system during the discharge is stable on the mesoscopic scale. In particular, the ordered directional movement of particles can occur near the orifice. In addition, the correlation between the granular temperature distribution and velocity vector field is opposite in the radial and axial directions. According to the kinetic model, the main cause for the reversal of spatial correlation is that the intensity of the particle diffusion decrease with the decrease of silo height. This highlights a surprising and unrecognized role that the diffusion motion of particles plays in energy dissipation for hopper flow.

Relationship between mass discharge rate and granular temperature of silo flow with variance of outlets

A lower granular temperature zone near the orifice in silos was found by speckle visibility spectroscopy technology. Surprisingly, the values of granular temperature in this area are inversely proportional to the mass discharge rate (MDR). This finding that links the macroscopic MDR with mesoscopic dynamics temperature of the particle system, is obtained by looking at the effect that different outlet sizes of silos on MDR and granular temperature field. Apart from evidencing the relevance of a parameter that has been traditionally overlooked in silos, this relationship supposes a benchmark with which to explore the influence of small orifice sizes on MDR. Among these, we found that the frequency of intermittent clogging and their dependence on the size of the outlet.

Effects of vibrations on tilted silo discharge

Introducing vibrations is a common way to improve the granular flow in silos to avoid clogging. In this study, a three-dimensional silo submitted to vertical vibration was built. The effect of vibration on the discharge of tilted silo was studied. The results showed that, the discharge rate did not depend on the particle depth but decreased with the increase of the outlet tilt angle, regardless if the silo was vibrated or not. Discharge rate showed non-monotonic relationship with vibration frequency at high amplitude, while increased with increasing frequency at low amplitude. The contour plots of the influence of vibration and tilt angle on the discharge were given. Based on the analysis of variance (ANOVA), the effect of the tilt angle on the discharge was higher than the vibration amplitude and frequency.

Micro-modelling of shear localization during quasi-static confined granular flow in silos using DEM

The paper deals with the quasi-static confined flow of cohesionless sand in a plane strain model silo with parallel walls and a slowly movable bottom. Numerical modelling was carried out by the discrete element method (DEM) using spheres with contact moments to approximately capture a non-uniform shape of sand particles. Different initial void ratios of sand and silo wall roughness grades were employed. Regular triangular grooves (asperities) with the same inclination and a different height were used to describe the varying wall surface topography. The emphasis was on the formation and evolution of both wall and internal shear zones during sand flow. DEM simulation outcomes were compared with corresponding model experiments and theoretical solutions. In addition, the particle displacements, particle rotations, normal contact forces, void ratios and wall stresses were evaluated. The numerical findings enhance the understanding of shear localization at the grain level during confined flow in silos and its strong impact on the magnitude, distribution and oscillation of wall stresses.

Granular size segregation in silos with and without inserts.

The storage of granular materials is a critical process in industry, which has driven research into flow in silos. Varying material properties, such as particle size, can cause segregation of mixtures. This work seeks to elucidate the effects of size differences and determine how using a flow-correcting insert mitigates segregation during silo discharge. A rotating table was used to collect mustard seeds discharged from a three-dimensional (3D)-printed silo. This was loaded with bidisperse mixtures of varying proportions. A 3D-printed biconical insert was suspended near the hopper exit to assess its effect on the flow. Samples were analysed to determine the mass fractions of small particle species. The experiments without the insert resulted in patterns consistent with segregation. Introducing the insert into the silo eliminated the observed segregation during discharge. Discrete element method simulations of silo discharge were performed with and without the insert. These results mirrored the physical experiment and, when complimented with coarse graining analysis, explained the effect of the insert. Most of the segregation occurs at the grain-air free surface and is driven by large velocity gradients. In the silo with an insert, the velocity gradient at the free surface is greatly reduced, hence, so is the degree of segregation.

ANÁLISE DA DESCARGA EM SILOS BIDIMENSIONAIS COM INSERTOS POR MEIO DE FLUIDODINÂMICA COMPUTACIONAL (CFD)

O manuseio de materiais granulares está muito presente na indústria, compondo metade dos produtos da indústria química, sendo geralmente armazenadas em silos. Os silos apresentam desafios quanto à descarga, podendo apresentar zonas mortas que prejudicam o armazenamento. Para compreender o escoamento de partículas nos silos, o estudo realizado visou a obtenção, a partir de simulações por fluidodinâmica computacional (CFD), das vazões mássicas em silos com diferentes configurações de insertos: no insert, BINSERT®, BINSERT 6cm e conical insert, comparando os resultados com valores experimentais da literatura. Foi verificada a influência da altura de partículas, pressão atmosférica na saída do silo e dos coeficientes de restituição e especularidade. Foi realizada também uma análise do fluxo de partículas a partir de perfis de velocidade. Os resultados sem modificações de parâmetros foram satisfatórios para os silos de configuração no insert, enquanto modificações foram necessárias para as demais configurações. Observou-se alteração insignificante nas vazões em relação a altura e a pressão atmosférica. O aumento do coeficiente de especularidade resultou em menores vazões mássicas, e o mesmo foi observado para o coeficiente de restituição. Constatou-se, assim, que entre os parâmetros analisados, os coeficientes podem ser ajustados para se obter convergência satisfatória. O fluxo de partículas se aproximou ao mass flow apenas nas configurações BINSERT 6cm e conical insert, as quais se dão como boas adições estruturais para o silo.

Particle flow rate in silos under rotational shear.

Very recently, To etal. have experimentally explored granular flow in a cylindrical silo, with a bottom wall that rotates horizontally with respect to the lateral wall [Phys. Rev. E 100, 012906 (2019)10.1103/PhysRevE.100.012906]. Here we numerically reproduce their experimental findings, in particular, the peculiar behavior of the mass flow rate Q as a function of the frequency of rotation f. Namely, we find that for small outlet diameters D the flow rate increased with f, while for larger D a nonmonotonic behavior is confirmed. Furthermore, using a coarse-graining technique, we compute the macroscopic density, momentum, and the stress tensor fields. These results show conclusively that changes in the discharge process are directly related to changes in the flow pattern from funnel flow to mass flow. Moreover, by decomposing the mass flux (linear momentum field) at the orifice into two main factors, macroscopic velocity and density fields, we obtain that the nonmonotonic behavior of the linear momentum is caused by density changes rather than by changes in the macroscopic velocity. In addition, by analyzing the spatial distribution of the kinetic stress, we find that for small orifices increasing rotational shear enhances the mean kinetic pressure 〈p^{k}〉 and the system dilatancy. This reduces the stability of the arches, and, consequently, the volumetric flow rate increases monotonically. For large orifices, however, we detected that 〈p^{k}〉 changes nonmonotonically, which might explain the nonmonotonic behavior of Q when varying the rotational shear.

Modelling of shear zones during quasi-static granular silo flow using material point method (MPM)

The paper focuses on confined silo flow of cohesionless sand. The problem considered is a quasi-static flow in a plane strain model silo with parallel walls simulated with the material point method (MPM). The simulation used a non-local hypoplastic constitutive model. Initially, the paper validated the implemented numerical approach with basic element tests and a plane strain compression test. The subsequent MPM calculations for a model silo were performed with different initial void ratios of sand and silo wall roughness. The flow simulations took also into account the different both location and width of the outlet. The emphasis was on the evolution of both shear zones (wall and internal curvilinear shear zones) and their impact on wall stresses/forces during flow. The numerical findings enhance the understanding of shear localization in granular materials during confined controlled flow in silos and its immense effect on the magnitude and distribution of wall pressures.

Interactive Timeline Approach for Contextual Spatio-Temporal ECT Data Investigation.

This paper presents a novel approach to a complex process of electrical capacitance tomography (ECT) measurement data analysis. ECT is frequently employed for non-invasive monitoring of industrial process phenomena. Proposed methodology is based on the premeditated integration of the spatial and temporal relations inherent in the measurement records into the workflow of the analysis procedure. We propose a concept of interactive timeline that enables arranging data visualization according to the user’s current focus along the process of analysis. We evaluated the proposed method using a prototype system in a task-based user study conducted with a group of domain experts. The evaluation is based on gravitational silo flow measurement datasets. Proposed prototype system enables diverse data manipulation in a more natural way allowing the user to switch back and forth between space and time domains along the data analysis trail. Experiments with the prototype system showed that the accuracy and completion times have significantly improved in comparison to the performance measured in the baseline condition. Additionally, the participants reported decreased physical load with improved efficiency measured with NASA task load index. Finally, a short discussion coupled with directions for the future of interactive spatio-temporal ECT measurement data analysis conclude the paper.

A hybrid particle-geometric scaling approach for elasto-plastic adhesive DEM contact models

The computation time of Discrete Element Method (DEM) simulations increases exponentially when particle size is reduced or the number of particles increased. This critical challenge limits the use of DEM simulation for industrial applications, such as powder flow in silos. Scaling techniques can offer a solution to reduce computation time. In this paper, we have developed a hybrid particle-geometric scaling approach with a focus on Elasto-Plastic Adhesive contact models. It established relationships between particle scaling factors and DEM contact input parameters. The isolated effects of varying particle size and geometric dimensions on bulk properties were also evaluated using uniaxial consolidation, static angle of repose, and ring shear tests. This paper shows how the particle scaling can be applied together with geometric scaling to incorporate two important aspects of bulk materials, their Elasto-Plastic behaviour and their cohesive forces.

Multifractal Intermittency in Granular Flow through Bottlenecks.

We experimentally analyze the intermittent nature of granular silo flow when the discharge is controlled by an extracting belt at the bottom. We discover the existence of four different scenarios. For low extraction rates, the system is characterized by an on-off intermittency. When the extraction rate is increased the structure functions of the grains velocity increments, calculated for different lag times, reveal the emergence of multifractal intermittency. Finally, for very high extraction rates that approach the purely gravitational discharge, we observe that the dynamics become dependent on the outlet size. For large orifices the behavior is monofractal, whereas for small ones, the fluctuations of the velocity increments deviate from Gaussianity even for very large time lags.

Influence of external pressure on granular flow in a cylindrical silo based on discrete element method

Granular flow exists widely in the fields of natural environments and industrial production. Despite the fact that external driving forces are recognized to have a significant effect on the dynamic of granular systems, a large number of experimental measurements and numerical simulations have been contributed to the study of granular flow considering gravity as the sole driving force. In this work, the granular flow driven by external pressure is simulated by GPU-based DEM (discrete element method). To examine the validity of the DEM model, the corresponding experimental results for spherical granular materials and the traditional Beverloo equation are compared. Furthermore, the effects of gravity acceleration, orifice diameter, and particle friction on the flow rate under different external pressures are studied. Finally, the distribution of the particle velocity and the normal contact force between particles are analyzed to demonstrate the influence of external pressure on the macroscopic discharge rate.