Silo music and silo quake: granular flow-induced vibration

Acceleration and sound measurements during granular discharge from silos are used to show that silo music is a sound resonance produced by silo quake. In tall and narrow silos, the latter is produced by stick-slip friction between the wall and the granular material. For the discharge rates studied, the occurrence of flow pulsations is determined primarily by the surface properties of the granular material and the silo wall. The measurements show that the pulsating motion of the granular material drives the oscillatory motion of the silo.     

Silo music — Mechanism of dynamic flow and structure interaction

This papers deals with the strong dynamic effects (called silo music) appearing during confined granular flow in the cylindrical silos. Silo experiments with dry cohesionless sand during gravitational outflow were performed in a cylindrical perspex model silo. During tests the wall accelerations and acoustic signals were recorded and the mode shapes of the silo structure were determined. In addition, experiments were performed with additional modifications of the silo structure and silo flow. A novel simple mechanism of the origin of silo music was proposed.     

Determination of bulk solid concentration changes during granular flow in a model silo with ECT sensors

Experiments in a cylindrical model silo were carried out with different initial densities of sand and silo wall roughness. Solid concentration changes during granular flow in the model silo were measured with electrical capacitance tomography (ECT) sensors. During silo flow, strong dynamic effects connected with booming sound occurred. Local one-dimensional (1D) and cross-sectional 2D evolutions of solid concentrations in dry sand during silo discharge were observed. The 1D phenomena were estimated from the raw data and the 2D phenomena were obtained from the reconstructed data by solving an inverse problem with a linear back projection algorithm.     

Arch-Free flow in aerated silo discharge of cohesive powders

Arching can occur during silo discharge of cohesive powders. In general this happens when the outlet size is not wide enough. Flow aid devices, such as aeration pads, are commonly used in the industry to achieve proper flow of cohesive materials. However, no design criteria are presently available for such kind of devices and, in particular, for the intensity of aeration to be used to avoid arching. Aim of this paper is the evaluation of the limiting aeration condition to produce the collapse of established arches and the minimum aeration rate necessary for no arching discharge flow. Experimental tests are carried out in an aerated flat bottom silo. The measured quantities are the aeration rate at arch collapse and the arch size. Powder permeability is characterized by fluidization experiments. A simplified model is proposed to assess on the prevailing physical phenomena and predictively evaluate the minimum aeration rate to determine no arching discharge flow.     

Numerical study on the behavior of funnel flow silos with and without inserts through a continuum hydrodynamic approach

In this paper the results from simulations performed using a hydrodynamic model proposed by Artoni et al. [Chem. Eng. Sci. 64 (2009a) 4040–4050] have been compared with published data of an extensive experimental investigation carried out at the Tel-Tek Research Institute in Porsgrunn, Norway. The experiments collected several data and observations on the wall stresses and the flow patterns observed during discharge of a full-scale funnel flow silo with and without inserts. The comparison between simulation and experiments showed the ability of the model to capture quantitatively the main features of both the flow and of the wall stress profiles when flow corrective inserts are put in the hopper of the silo in order to convert the discharge regime to a mass flow regime. Moreover information such as the stresses on the internals, which are difficult or impossible to get experimentally, have been collected from the simulations and discussed.     

DEM simulations and experiments of pebble flow with monosized spheres

Pebble flow in the core of a pebble bed high-temperature reactor (HTR) is a typical granular flow with a huge number of monosized spherical pebbles, where the kinematics of the pebbles needs to be predicted more accurately. In this study the discharge of monosized glass beads from a semi-cylindrical silo has been investigated using the Discrete Element Method (DEM), in which the particle-particle interaction model is based on contact mechanics with measured friction coefficients for precisely predicting the flow behaviour. Particle flow experiments were carried out with a transparent semi-cylindrical silo. Two cases were considered: one is for tracking the trajectories of identified particles with 11,500 glass beads, and the other is for the ‘double-zone’ flow for estimating the mixing evolution with 10,460 glass beads. Comparisons between the experimental and DEM results show good agreement, which indicates the applicability of DEM for predicting the pebble flow in a HTR.     

Validation and experimental calibration of 3D discrete element models for the simulation of the discharge flow in silos

The aim of the present work was to develop 3D discrete element models capable of simulating the observed flow of glass beads (simple glass spheres) and maize grains (represented as a combination of spheres) during their discharge from a small model silo. A preliminary model for each material was constructed based on values for variables measured in the laboratory or taken from the literature. The ability of the models to predict the flow of these materials was then tested by comparing their results with observed discharge flows. Three variables were recorded for this: the mean bulk density at the end of the filling phase, the discharge rate and the flow pattern. The comparison of the results for the last of these variables required the discharge process be filmed using a high speed camera in order to more easily recognise the details of the flow. The preliminary model for the glass beads made very reasonable predictions, but that for the maize grains required calibration. This involved modifying the values of the friction properties of the material until a model capable of making acceptable predictions was obtained. The results obtained highlighted the influence of friction properties on the characteristics of the discharge flow. Finally, some of the numerical results provided by the models were analysed in order to describe the flow characteristics and the behaviour of the discharge rate in more detail.     

The simulation and experimental study of granular materials discharged from a silo with the placement of inserts

The simulation method of DEM and experimental method are used to investigate the flow pattern of the filling and discharging process for two-dimensional plane silos. Two kinds of inserts (conical insert and BINSERT®) are used in the silo to change the flow fields of the silo. The placement of inserts improves the flow behaviors of funnel flow type to mass flow type during discharging. The wall normal stresses are influenced by the change of the flow type. The effects of using differently shaped inserts on the flow pattern and wall stress are analyzed in this study. The controlling parameters include the silo half-angle, the orifice width, the shape of the insert and the properties of the granular materials. The simulation and experimental results are in good agreement.     

Application of particle image velocimetry (PIV) for deformation measurement during granular silo flow

The paper presents results of deformation measurements in dry cohesionless sand during free flow in small rectangular model silos using a non-invasive, indirect method called particle image velocimetry (PIV). It is an optical technique for measuring surface deformations from successive digital images. Laboratory model tests were performed with a mass and funnel flow silo to investigate the kinematics of the flowing sand. The measurements were carried out for granular flow in model silos without inserts and in a funnel flow silo equipped with three different types of inserts: cone-in-cone, inverted cone and double cone. The effect of the initial sand density and roughness of silo walls on the volumetric and deviatoric strain in sand was investigated. The accuracy of measurements was discussed. Advantages and disadvantages of PIV were outlined. The results were qualitatively compared with sand displacements obtained with colored sand layers and with the aid of X-rays (initially dense sand).     

Application of ECT to solid concentration measurements during granular flow in a rectangular model silo

The paper presents results of concentration changes in cohesionless sand during dynamic mass flow in a rectangular model silo composed of a bin and hopper. Electrical capacitance tomography (ECT) was used. Sensors were located outside the silo along both the periphery and height. Local horizontal one-dimensional and cross-sectional two-dimensional evolutions of solid concentrations in dry sand during silo discharge were determined. The first ones were estimated from the raw data and the latter were obtained with the aid of the reconstructed data using a Linear Back Projection algorithm (LBP) to solve an inverse problem. Experiments in a model silo were carried out with two different initial sand densities and wall roughness grades. The measured results with ECT were compared with corresponding ones obtained with a Particle Image Velocimetry (PIV) method.     

Austragorgane und Austraghilfen

Feeders and Discharge Aids. Feeders and discharge aids have an influence on the flow behaviour of bulk solids in silos. Together with the geometric form of the silo, they are responsible for the operating conditions of a silo. Therefore, feeders and discharge aids have to be chosen and installed under consideration of the flow properties of the bulk solid to be stored. The necessary information is obtained by using the silo design method due to Jenike. This method yields the geometrical data of the hopper (wall steepness, outlet diameter) necessary for a troublefree operation (no doming and ratholing; mass flow; no segregation; uniform flow). Furthermore, the design of feeders and discharge aids plays an important role in determining their function. For example, stagnant zones will build up and cause flow problems on protruding edges and on walls which are not steep enough. For the correct function of the hopper/feeder combination it is important to make sure that the feeder fully activates the hopper outlet and stagnant zones cannot build up. This can be ensured by an appropriate feeder design (increasing capacity in direction of conveying). The driving power of a feeder has to be calculated also on the basis of the flow properties of the bulk solid to be stored.     

Experimental and theoretical investigations of silo music

This paper deals with strong dynamic effects connected with booming sound (called silo music) appearing during confined granular flow in silos. Novel experimental data have been gained through Electrical Capacitance Tomography (ECT) and an extraction of operational deflection shapes. A dynamic plane strain FE model of the granular material and oscillating wall was presented. The numerical results support the key role of dynamic coupling between silo wall oscillations and dynamics of the flowing material induced by a change of a flow direction at the transition zone between cylindrical and converging flow.     

Verfahrenstechnische und statische Aspekte der Siloauslegung

Process Engineering Aspects and Statics of Silo Design. Design of silos for flow is described, starting from the flow profiles developing during discharge. The respective flow profile depends on the flow properties of the bulk solids and on the silo geometry. Based on measured flow properties the determination of the maximum permissible hopper wall inclination to achieve mass flow and of the minimum dimension of the outlet to prevent arching and ratholing is demonstrated, taking into account time consolidation aspects. This yields the size of a feeder and the position of discharge aids. The flow properties are required additionally for structural design of silos. Flow of bulk solids in silos, especially the development of asymmetric flow channels, has a significant influence on the pressure distribution.     

Pressure measurements in steel silos with eccentric hoppers

It is known that concentric filling and discharge are the most recommended solutions in silo design. However, different reasons such as costs, space problems and the client's specific requirements have a significant influence on the final design. Therefore, engineers sometimes need to make use of eccentric filling and discharge solutions. These designs are usually related to phenomena such as unfavorable switches in the flow regime, associated with extreme unsymmetrical pressure distributions around the silo wall, oscillations of the load, etc., which may cause failures or instabilities of silo structures, involving then high costs for the industry and, in the worst cases, accidents where human life may be endangered. Understanding about these phenomena associated with hopper eccentricities is still needed.This research work deals with pressure measurements on three steel silos with different hopper eccentricities. The results not only confirm, at a qualitative level, phenomena that have also been described in previous research, but also provide new knowledge concerning non-symmetric loads in wheat and maize silos with conical hoppers of different eccentricities. Non-symmetric components of the pressures and shifts in flow regimes have been studied and discussed in relation to load assessment for the design of silos.     

Shape dependence of resistance force exerted on an obstacle placed in a gravity‐driven granular silo flow

Resistance force exerted on an obstacle in a gravity‐driven slow granular silo flow is studied by experiments and numerical simulations. In a two‐dimensional granular silo, an obstacle is placed just above the exit. Then, steady discharge flow is made and its flow rate can be controlled by the width of exit and the position of obstacle. During the discharge of particles, flow rate and resistance force exerting on the obstacle are measured. Using the obtained data, a dimensionless number characterizing the force balance in granular flow is defined by the relation between the discharge flow rate and resistance‐force decreasing rate. The dimensionless number is independent of flow rate. Rather, we find the weak shape dependence of the dimensionless number. This tendency is a unique feature for the resistance force in granular silo flow. It characterizes the effective flow width interacting with the obstacle in granular silo flow. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3849–3856, 2018     

Flow Channel Boundaries in Silos

The critical slip planes at the silo filling state are compared with the flow channel boundary during silo discharge for semi‐mass flows. The static critical slip planes are determined by using the dynamic programming method based on the stress field of granular solids stored in silos at the filling state. The flow channel boundary is estimated through the finite element analysis of the silo discharge. The results indicate that the critical slip line lies above the flow channel boundary. This characteristic can be attributed to the changeover of major principal stress directions of granular solids from the silo filling to the silo discharge. The analysis demonstrates that the silo wall friction tends to lift up the critical slip plane and flow boundary. A simple correlation is developed between the positions of critical slip planes and flow boundaries and is experimentally verified.     

料仓流型及改善贮料流动性分析

本文介绍了各种料仓流型,并详细分析了影响料仓流型的因素,尤其是装料形式、改流体对料仓流型的影响,为料仓设计、选择流型和改善散体流动措施提供了参考。     

粉体料仓的设计

介绍了料仓设计的基本方法, 包括流型选择、流型设计以及卸料口尺寸的确定方法等, 最后给出一个应用实例。     

Flow pattern measurement in a full scale silo containing iron ore

The flow pattern in a silo is important because it affects both the recovery of solids and the pressures on the silo wall during discharge. Wherever mass flow is not achieved, the boundary of the flow channel has significant implications for both the functional and structural design of the silo. Many techniques have been used for the study of flow patterns in model silos, but most cannot be used at full scale, and very few quality measurements at full scale have ever been made. This paper outlines a full scale experimental study in which the patterns of solids flow and the flow channel boundaries are reliably quantified.The full scale silo was specially designed, constructed and instrumented to exhibit funnel flow and to make observations of the solids flow pattern and the silo wall pressures. It had three outlets: one concentric, one fully eccentric and one in between. Three materials were used: iron ore pellets, slag fines and crushed basalt. This paper describes experiments involving iron ore pellets. The silo was seeded with radio frequency tags whose residence times were measured by detecting them on exit during discharge. The residence time data were studied to deduce the discharge flow pattern. This paper presents the results of three different flow pattern interpretation techniques: the best of them (mass-time relationships) is shown to give a very clear identification of the solids flow pattern and the flow channel boundary.     

Simulation of Temperature Distribution in Stored Wheat without Aeration

A neural-deterministic simulation model applied for calculating distributions of temperatures and moisture content in a bed of wheat stored in a steel silo without aeration is presented in the article. The model consists of differential equations of heat and moisture transfer, initial and boundary conditions, and three artificial neural networks used during simulated ambient air conditions. Experiments and computer simulations were carried out in order to determine temperature fields in wheat grain stored in a steel silo for two months. The computer simulations were carried using MATLAB and FEMLAB software. The difference between measured and simulated temperature in grain near the silo wall at a height of 2.5 m from the bottom was less than 3.0°C. On the basis of the analysis performed it was concluded that the temperature distributions obtained with the model were consistent with the measured results obtained for grain stored in a steel silo without aeration.