Silo Discharge Research Articles

DEM investigation on the dynamic characteristics of wall normal stress and particle flow during silo discharge

Violent fluctuations of wall normal stress and particle velocity during silo discharge have been frequently reported by researchers, yet a detailed particle-scale description of these phenomena is still lacking. In this work, the discharge of granular assembly from a flat-bottomed silo was investigated by performing three-dimensional (3D) Discrete Element Method (DEM) simulations. It is found that arc-shaped rupture surfaces, characterized by the lower solid volume fraction, stronger shear interaction between particles and higher particle rotating velocity, developed continuously in the bottom converging part of flowing zone. And the temporal evolution of particle velocity field in this converging part presented a swinging feature manifested by the alternate appearance of large particle velocity magnitude in the right- and left- side regions. Discrete Fourier transform results showed that the fluctuation of wall normal stress at the transition point contained two dominate frequencies. The lower frequency matched well with the dynamic evolution of the rupture surface joining the transition point, and the higher frequency was nearly the same as the fluctuating frequency of particle vertical velocity. Thus, our DEM simulation results suggested that for the silo discharge considered in this work, the periodic fluctuation of wall normal stress at the transition point was contributed by both the alternating flow pattern mechanism and the free-fall arch mechanism.

Electrostatic discharge mechanisms and dynamic characteristics of different polarity powders in the Silo

Electrostatic discharge in powder silos poses a significant safety hazard due to the risk of fire or explosion. However, the underlying discharge mechanisms are not fully understood. This study aims to improve the comprehension of electrostatic discharge dynamics and mechanisms by introducing a fluid discharge model for charged powder silos. The model validation is carried out by examining discharge patterns and electric field variations. The investigation of the electrostatic field and potential distribution prior to discharge helps to comprehend the discharge conditions. The discharge modes and evolution laws of different polarity powders are explored to reveal disparities in discharge phenomena. These disparities are further scrutinized in relation to electron density, space charge, electric field, and potential. To elucidate the discharge mechanism, the electric field distortion caused by space charge, electron existence time, and migration law are analyzed. The electrostatic and diffusion effects are the main reasons for the discharge differences.

Measurement of acceleration and angular velocity of particles during a 3D silo discharge

Accurate measurement of the three-dimensional (3D) movement of discrete particles is crucial for comprehending complex granular rheology in silos. In this paper, the acceleration and angular velocity of particles in 3D silos are measured by using a spherical detector based on inertial technology and magnetic positioning technology. The acceleration of particles is the largest in the center of silos, which suggest that the resistance generated by friction and extrusion is the smallest. Surprisingly, the angular velocity distribution follows lognormal function except for particles near the outlet. The correlation between acceleration and angular velocity is opposite in different flow regions. It reveals for the first time that the extent to which the resultant force on the particles affects their rotational motion is related to the flow pattern. These results have practical significance for regulating the granular flow pattern and optimizing the structural design of silos.

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.

Flow rate in 2D silo discharge of binary granular mixtures: the role of ordering in monosized systems

A long-standing debate regarding the dynamics of silo discharge revolves around the use of mono-dispersed circular or spherical grains in simplified two-dimensional models. It is well-known that granular systems composed of particles of the same size can generate crystal or quasi-crystal domains with specific structural and dynamic behaviors. Can this ordering affect the flow rate to an extent that monosized systems cannot be good models for realistic materials? In this work, we present simulations of the discharge of two-dimensional silos filled with binary mixtures of circular grains that conserve the same mean particle size. We address the question of how ordering affects the mass flow rate, in particular considering the limit of mono-sized systems. We find that the typical hexagonal order observed does not affect the flow rate significantly. However, the flow rate does exhibit a weak, nonmonotonic dependence on packing bidispersity that correlates with changes in the outpouring speed of grains in the vicinity of the orifice.

The role of particle size and other properties on silo discharge behaviour of chipped wood biomass

To achieve net-zero carbon emissions by 2050, the UK government emphasizes the pivotal role of sustainable bioenergy in electricity, transportation, and heating. However, challenges persist in handling biomass particulate solids in production facilities, leading to economic impacts. This study investigates the flow characteristics of stemwood chips from four tree species using a novel drum chipper. Experimental analyses include bulk density measurements, silo discharge studies, biomass flow property assessments, and wall friction measurements. Comparative analyses are performed using Jenike's procedure for building wedge-shaped silos, with a focus on predicting the critical opening size to prevent arching. Additionally, the paper delves into the creation of statistical models aimed at identifying key factors influencing the flow behaviour during silo discharge. Emphasis is placed on understanding potential discrepancies between theoretical predictions and experimental results concerning critical silo openings for arch-free discharge. The results contribute to understanding the factors influencing the flow behaviour of wood chips, informing silo design considerations. Our findings suggest limitations in applying traditional silo design methods, urging further research for more accurate predictions.

Exploring effect of charge dissipation in charged powder space on electric field distribution in silo

The electric field distribution is important for understanding the underlying mechanism of electrostatic discharge in silos and for developing anti-static strategies. However, the charge dissipation of charged powder usually is not well modeled in the existing electric field simulation, resulting in inaccurate estimation of the electric field in silos. This paper studied the electric field distribution in a cylindrical silo based on the space charge density distribution created by the charged particles. The influences of the normalized filling time, gap distance, and the silo height-to-diameter ratio on the electric field were explored. The results indicate that the area with a larger space charge density shrinks from the heap bottom to the heap surface, leading to a decline in the electric field and suggesting a lower probability of electrostatic discharge at the larger normalized filling time. Continuous decrease of the gap distance when the heap surface is close to the silo ceiling will lead to a sharp increase in the electric field, regardless of the charge dissipation. The silos with a height-to-diameter ratio greater than 2 are demonstrated to have a lower probability of electrostatic discharge and a space higher utilization ratio.

Nonlinear effect of grain elongation on the flow rate in silo discharge.

By means of two-dimensional numerical simulations based on contact dynamics, we present a systematic analysis of the joint effects of grain shape (i.e., grain elongation) and system size on silo discharge for increasing orifice sizes D. Grains are rounded-cap rectangles whose aspect ratio are varied from 1 (disks) to 7. In order to clearly isolate the effect of grain shape, the mass of the grains is keeping constant as well as the condition of the discharge by reintroducing the exiting grains at the top of the silo. In order to quantify the possible size effects, the thickness W of the silos is varied from 7 to 70 grains diameter, while keeping the silos aspect ratio always equal to 2. We find that, as long as size effects are negligible, the flow rate Q increases as a Beverloo-like function with D, also for the most elongated grains. In contrast, the effects of grain elongation on the flow rate depend on orifice size. For small normalized orifice sizes, the flow rate is nearly independent with grain elongation. For intermediate normalized orifice sizes the flow rate first increases with grain elongation up to a maximum value that depends on the normalized size of the orifice and saturates as the grains become more elongated. For larger normalized orifice size, the flow rate is an increasing function of grains' aspect ratio. Velocity profiles and packing fraction profiles close to the orifice turn out to be self-similar for all grain shapes and for the whole range of orifice and system sizes studied. Following the methodology introduced by Janda etal. [Phys. Rev. Lett. 108, 248001 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.248001], we explain the nonlinear variation of Q with grain elongation, and for all orifice sizes, from compensation mechanisms between the velocity and packing fraction measured at the center of the orifice. Finally, an equationto predict the evolution of Q as a function of the aspect ratio of the grains is deduced.

Flow of asymmetric elongated particles

Shear induced orientational ordering of asymmetric elongated particles is investigated experimentally. Corn grains and pegs with one end sharpened are studied using x-ray computed tomography during quasistatic shearing and silo discharge. We show that asymmetries can be detected in the orientational distributions of the particles, which are related to the modulated rotation of the particles during shear flow. Namely, when the particles rotate in a plane that is not horizontal, they spend more time with the sharper (lighter) end pointing up, which can be explained using energetic arguments. We quantify the resulting asymmetry of the orientational distribution in a split bottom Couette cell and in a silo discharge process.

Effect of bevelled silo outlet in the flow rate during discharge

We investigate the effect of a bevelled (or slanted) outlet on the discharge rate of mono-sized spheres from a quasi-two-dimensional silo, using the discrete element method. In contrast to hopper discharges, where the bevelling is across the entire base of the container, we study a bevelled opening that is significantly smaller than the silo width and in which the slanting is limited to half a sphere diameter at the boundary of the outlet. We show that the bevelling increases the flow rate comparably to the inclination in hopper walls. Using Beverloo’s model, we relate this increase in rate to what we define as the ‘effective opening’ of the silo and analyse the velocity profiles associated with the discharges. We show that different openings, having effectively the same discharge rates, give rise to distinctly different internal dynamics in the silo. These results have the potential to aid industrial processes by fine-tuning and improving control of silo discharges, with a minimal impact on silo design, thus significantly reducing production and handling costs.

The influence of flow pattern and hopper angle on static and dynamic pressures in slender silos

This paper focuses on the influence of flow pattern and hopper angle (β) on static and dynamic pressures in slender silos. The results were obtained using a test station developed by the authors, consisting of a silo (6 m high and 0.7 m internal diameter). Maize was used to generate the pressures. Five different hopper angles were tested by filling, static phase followed by complete silo discharge. Results are reported for pressure and flow analyses, and show that hopper angle influences normal pressures on the silo wall; Silos with transition flow do not have a concrete pattern of distribution of thrusts; pressure values are proportional to the variation of the hopper angle; and internal shear zones vary according to hopper angle, completely modifying the behaviour of static and dynamic pressures. Friction pressure and lateral pressure ratio values exceeded those calculated by the Eurocode 1, part 4.

Dynamics of a 2D silo discharge: A competition between structural domains before clogging

Dynamics of a 2D silo discharge: A competition between structural domains before clogging

Why the presence of insert above the outlet can enhance silo discharge: A tentative answer

Why the presence of insert above the outlet can enhance silo discharge: A tentative answer

Spatial–temporal multiscale discrete–continuum simulation of granular flow

Modeling and simulation of granular materials have received great attention in a wide range of scientific and engineering fields. With various discrete or continuum-based methods facing different aspects of the complexity of granular materials, their multi-scale coupling may lead to more effective and efficient methods. In this work, a novel spatial–temporal multiscale method is proposed with spatially overlapped continuum and discrete systems running alternately at different time steps to accelerate the simulation. The continuum system aims at predicting the potential position of each particle, and the discrete system is utilized to provide particle-level information and correct the prediction of the continuum system. The feasibility and accuracy of this method are demonstrated by comparing to typical traditional methods for silo discharge.

Multi-level DEM study on silo discharge behaviors of non-spherical particles

The silo discharge of non-spherical particles has been widely practiced in engineering processes, yet the understanding of multi-level mechanisms during solid transportation is still lacking. In this study, a high-fidelity super-ellipsoid Discrete Element Method (DEM) model is established to investigate the discharge behaviors of non-spherical particles with different size distributions. After the comprehensive model validations, we investigated the effects of particle shape (aspect ratio and particle sharpness) on the particle level discharge behaviors. The discharge rates of the ellipsoid particles used in the current work are larger than the spherical particles due to the larger solid fraction. The discharge rates of the cuboid-like particles are determined by the combined effect of the solid fraction and the contact force. Parcel level data show that the translational movements of the ellipsoid particles are more ordered, which is supported by the global level data. Strong correlations exist between the particle level and parcel level data, especially the ellipsoid particles and the large particles in the polydispersed cases.

Research on the effect of the cone-in-cone insert on the discharge behaviour of conical silo

During silo unloading, the reliability of the silo can be improved by adding inserts with appropriate parameters to improve the flow pattern. In this study, DEM was used to study the influence of the cone-in-cone insert parameters on the silo discharge behaviour. The contact parameters were measured through a series of experiments, and the DEM model was established and verified. Furthermore, the silo unloading process with different insert parameter combinations was simulated, and the flow patterns were characterized by the Mass flow index (MFI). The simulation results revealed that the insert angle β and the height from the silo port H1 significantly affect the flow pattern. To realize the mass flow, H1, H2 and D2 should be reasonably selected under the β > 135°. After adding an insert with the β > 130°, the normal pressure along the silo wall was evenly distributed, and there was less wear.

Effect of particle shape on packing fraction and velocity profiles at outlet of a silo

Many studies on how the particle shape affects the discharge flow mainly focus on discharge rates and avalanche statistics. In this study, the effect of the particle shape on the packing fraction and velocities of particles in the silo discharge flow are investigated by using the discrete element method. The time-averaged packing fraction and velocity profiles through the aperture are systematically measured for superelliptical particles with different blockinesses. Increasing the particle blockiness is found to increase resistance to flow and reduce the flow rate. At an identical outlet size, larger particle blockiness leads to lower velocity and packing fraction at the outlet. The packing fraction profiles display evidently the self-similar feature that can be appropriately adjusted by fractional power law. The velocity profiles for particles with different shapes obey a uniform self-similar law that is in accord with previous experimental results, which is compatible with the hypothesis of free fall arch. To further investigate the origin of flow behaviors, the packing fraction and velocity field in the region above the orifice are computed. Based on these observations, the flow rate of superelliptical particles is calculated and in agreement with the simulated data.

Characterization of powder apparent viscosity: A reapplication of the Poiseuille equation to silo discharges

In the present work, a new method to characterize powder apparent viscosity was proposed by applying the Poiseuille equation to silo discharges. Flow properties of powders were first identified in terms of angle of repose and Hausner ratio, based on which experimental materials were classified as free-flowing and cohesive powders. These powders were further discharged from the pressurized silo, and an exponential increase in the powder discharge rate on the pressure difference was obtained. A model to predict the apparent viscosity was further developed grounded on the Poiseuille equation. The apparent viscosity of both free-flowing and cohesive powders decreases with an increasing flow rate within a specific range, showing the characteristic of shear-thinning. There was a reasonable agreement of powder apparent viscosity characterization between results from the silo discharge and those obtained from the FT4 rheometer, thus, confirming the method's reliability. The new method presented reliable values and had a wide testing range, providing an alternative way to measure powder rheological parameters.

The role of the particle aspect ratio in the discharge of a narrow silo

The time evolution of silo discharge is investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirm the widely known typical behavior with constant discharge rate (except for initial and final transients). For elongated particles with aspect ratios between 2 ⩽ L/d ⩽ 6.1, we find a peculiar flow rate increase for larger orifices before the end of the discharge process. While the flow field is practically homogeneous for spherical grains, it has strong gradients for elongated particles with a fast-flowing region in the middle of the silo surrounded by a stagnant zone. For large enough orifice sizes, the flow rate increase is connected with a suppression of the stagnant zone, resulting in an increase in both the packing fraction and flow velocity near the silo outlet within a certain parameter range.

The effect of obstacles near a silo outlet on the discharge of soft spheres

Soft smooth particles in silo discharge show peculiar characteristics, including, for example, non-permanent clogging and intermittent flow. This paper describes a study of soft, low-frictional hydrogel spheres in a quasi-2D silo. We enforce a more competitive behavior of these spheres during their discharge by placing an obstacle in front of the outlet of the silo. High-speed optical imaging is used to capture the process of discharge. All particles in the field of view are identified and tracked by means of machine learning software using a mask region-based convolutional neural network algorithm. With particle tracking velocimetry, the fields of velocity, egress time, packing fraction, and kinetic stress are analyzed in this study. In pedestrian dynamics, it is known that the placement of an obstacle in front of a narrow gate may reduce the stress near the exit and enable a more efficient egress. The effect is opposite for our soft grains. Placing an obstacle above the orifice always led to a reduction of the flow rates, in some cases even to increased clogging probabilities.