Gisle G. Enstad

Numerical Calculations of Wall Pressures and Stresses in Steel Cylindrical Silos with Concentric and Eccentric Hoppers

This article presents finite element (FE) calculations of stresses in a silo structure caused by the loads exerted by the discharging material. Two different conical hoppers were studied, concentric and eccentric withdrawal. The eccentricity was introduced by the change of the outlet position towards one side of the hopper, and the influence of this modification is presented. The simulation of silo flow with the common Drucker-Prager model for bulk solids was performed in order to obtain the wall pressure distribution. All FE calculations are carried out with aid of the commercial software ABAQUS. Significant differences are found in the hopper-wall stresses between the concentric and eccentric cases. Strong bending actions appear in the eccentric hopper, and they are influenced by stiffness of the hopper/cylinder junction.

Effect of Passive Inserts on the Granular Flow from Silos Using Numerical Solutions

Experimental investigations indicate that placing a passive insert in a silo is a method for influencing the discharging flow pattern. These inserts have consisted of an inverted cone, a cone-in-cone, and a double cone. However, providing unequivocal guidelines on where those inserts should be placed for an optimum effect has not been possible experimentally. A numerical approach was therefore developed to predict material flow in the presence of such inserts in silos. Simulation results showed that all these inserts could make a funnel-flow silo perform in mass flow under certain circumstances if positioned correctly. The inserts should be installed at higher levels close to the transition rather than at lower positions close to the outlet, especially with the cone-in-cone insert and the inverted inserts; the maximum diameter of the inverted cone and the double cone should, however, be below the transition of the silo. Among the three inserts investigated, the double cone appeared to be the best, although even with this insert mass flow could not always be obtained if the hopper had an inclination angle as large as 45°. In such a situation, more simulations revealed that the extension of the lower part of the double cone had the potential to improve the flow pattern in the hopper. This potential could be utilized by a combination of extending the lower cone of the double cone insert and reducing the friction between the material and the hopper wall. To ensure improvement, further simulations illustrated that a ratio of 2:7 between the maximum diameter of the insert and the diameter of the silo was also crucial for the best effect.

Segregation of Construction Materials in Silos. Part 2: Identification of Relevant Segregation Mechanisms

The segregation of commercial dry mineral-based construction products and a mixture resembling these has been investigated in silos of different scale, and the results are presented in two parts. This work encompasses the second part, where the relevant segregation mechanisms for the handling of construction materials in silos have been identified through experiments in small scale (0.5 m3). The radial distribution of the particles at filling was determined by sampling from the upper layers of the heap and the influence of different initial conditions on the segregation at discharge was investigated, the overall discharge flow pattern being identified with tracer particles. The results imply that the quality of the mixture is mainly determined by the segregation induced at filling and by the discharge flow pattern, that is, no segregation mechanisms are active to a significant degree at discharge. The findings from the experiments in the small silo were confirmed both in intermediate (20 m3) and large (70 m3) scale. Based on these results, it is suggested that segregation of commercial construction materials handled in silos is caused by a combination of embedding, fluidization, and air-current effects.

Segregation of Construction Materials in Silos. Part 1: Experimental Findings on Different Scales

The segregation of commercial dry mineral-based construction products and a mixture resembling these has been investigated in silos of different scale, and the results are presented in two parts. This work encompasses the first part, where the segregation of four different construction products at discharge from a 70 m3 silo is followed at a production plant in Finland. In addition, the segregation of two other construction products at discharge from two different 20 m3 silos is determined. Finally, the segregation of a three-component mixture resembling real construction products is investigated in a small (0.5 m3) silo. The results show that a similar segregation pattern, that is, increase of fine material at the end of discharge, is obtained regardless of scale. Although the degree of segregation depends on silo size and material properties, the results of this work show that segregation of commercial construction products in large scale can very well be investigated with idealized mixtures in small scale with the advantage of more controlled experimental conditions and ease of generating quantitative data at reduced cost.

Experimental investigation of load exerted on a double-cone insert and effect of the insert on pressure along walls of a large-scale axisymmetrical silo

Applications of flow aid devices such as inserts improve the silo discharging mode. Considerable effort has been made in finding the best configurations between inserts and silos to achieve optimal functional results. In the present investigation, experiments were carried out to measure the loads imposed on a double-cone insert by particulate solids (free-flowing sand) when it was fitted within an axisymmetrical large-scale silo. Concentric filling/discharging was implemented to carry out such measurements. Pressures along the walls of the silo were also measured. Analyses of the measurement results showed that: (1) the loads on the insert were rather stable in total, but appeared to be asymmetrical at the end of filling; (2) a sudden increase of the loads on the insert was observed at the transition from filling to discharging, but this increase lasted only for a short moment; and (3) the loads on the insert decreased slowly but remained rather high for a large part of the discharge. Effects of the double-cone insert on the pressures along the silo walls were discussed.

Experimental and Calculated Loads on “Cone in Cone” Installations

This study focused on the installation of small hopper inserts designed for mass flow. An FEM-“Silo-Flow-Program-System” was used for the calculations. Using full-scale silo experiments, the loads on the “cone in cone” inserts were measured as a function of the installed positions. The results obtained from all experiments and in the numerical simulation showed a rise of the vertical loads on the hopper inserts as the height position increased. The comparisons between experiment and calculation with the more complex material law variant show relatively good agreement, above all for the lower to middle height positions. The load increases as a function of the height position are reproduced well by the numerical simulations. The summarizing representation of the experimental built-in loads for the three hopper inserts shows an approximately linear dependence of the vertical load on the height position for emptying.

Changes of Dextrose Particles with Pneumatic Conveying: Analysis of Size and Shape

When conveying particulate materials, changes in size and shape of individual particles can be observed. These changes can have a great impact on the bulk powder and affect its flow properties in the pipeline. Changes can be wanted or unwanted depending on the whole process chain and final use of the powder. In this investigation, dextrose monohydrate particles were pneumatically conveyed repetitively in a pilot plant–scale rig, and the size and shape of the particles were characterized by a semiautomatic image analysis method. This characterization was done qualitatively by observing micrographs and quantitatively for each individual particle (a total of 16,120 dextrose crystals) by using two statistical diameters and two shape factors. The effect of the changes in shape and size of the particles on the mass flow rate in the system was studied. It was concluded that the mass flow rate of the pneumatic conveying tests was not affected considerably by the changes of the dextrose particles.

FE Approach to the Sensitivity of Load on a Hopper to the Filling Method and Process With Granular Material

An analysis (ABAQUS) has been carried out to investigate the development of loads exerted on the wall of a hopper when it is being filled with granular material in a method of either distributed-filling or concentric-filling. A finite element program was created to model the hopper, the granular material and the interaction between them. To simulate a filling process, a partition technique was applied to the granular material to divide it into layers; the meshes, the gravity loading and the interaction were all suspended first. They were then reactivated in a designated sequence layer by layer. Such an operation of reactivation was regarded as an approach to a process of filling. There were two geometries designed for the granular material, the distributed-filling or concentric-filling methods were also thought to be obtained. The sensitivity of loads to the method of filling and to the process of filling was addressed.Copyright