Free-flowing Materials Research Articles

Forms of flow regions in free-flowing materials during discharge

Forms of flow regions in free-flowing materials during discharge

Periodic strain-gauge feeders

The present stage of development of the technology of batch feeding of free-flowing materials (FM) is associated with wide application of resistance strain-gauge power transducers (RSPT) as converters of load into electrical signal and of secondary digital type instrumentation [i, 2]. Periodic strain-gauge feeders, which allow FM batching from several tens of grams to several hundred kilograms with a precision of 0.2-0.3%, have been developed and produced for the past twenty years at the Red Army Scientific-Research Institute of Mechanization. Three models of such feeders are described in this paper. The VD-0.3 volumetric-gravimetric strain-gauge feeder is designed for small batches of various powdery FM when the bulk density of the material varies within • of the normal value. In order to stall the effect of these variations on the batching precision, an iterative algorithm of its operation was chosen for the development of the feeder. Let us consider a multichamber feeder filled with FM from a common service bin. Without violating the commonality of arguments, we shall assume that there are three chambers and their capacity can be chosen from the equation

Automatic flow-through moisture meter for free-flowing materials with high active conductivity

Automatic flow-through moisture meter for free-flowing materials with high active conductivity

The effect of the shape of the feed opening on the performance of a horizontal screw conveyor

An experimental investigation was carried out in order to confirm the influence of the shape of the feed opening to an enclosed horizontal screw conveyor on its performance. While the shape of the feed opening is not critical when dealing with free-flowing materials it has a very strong effect on performance when fibrous materials are being fed into the screw. In order to avoid blockages, the inlet of the casing should form an acute angle with the flight, in order to reduce the sliding of the materials along the casing edge and to encourage the cutting of the materials between the moving flight and the stationary edge of the casing. The included angle is related to the helical angle of the flight and the angle of friction between the materials and the flight.

Tubular dispensers for free-flowing materials

Tubular dispensers for free-flowing materials

Vibrational mixing of highly dispersed powders

Highly dispersed powders having a well developed interphase surface belong to the class of difficult-to-mix free-flowing materials. Different types of mixers are being used for mixing them [i]. However, most of them fail to ensure a high degree of homogeneity (uniformity) during the mixing process because of the difficulties in attaining the maximum possible (limiting) destruction of the structure of the material. In view of this, effective realization of the mass transfer processes becomes impossible under the conditions of continuous movement (displacement/shear) since the structure of the material is not fragmented into individual particles and discontinuous localized layers of the dispersed systems (zones) or blocks (particle aggregates) form [2]. Complete and uniform distribution of various phases in the macro and micro-volumes of the system (i.e., attainment of the limiting homogeneity) is possible only when the maximum possible (limiting) destruction of the structure of the entire dispersed system and all the particle aggregates of the solid phase occurs and their complete separation is ensured.

Calculation of box-type constructions

The ribs of box-type constructions situated underground or underwater, and also those of box-type capacities for fluids and free-flowing materials, can be considered as fixed. We present an algorithm for the calculation of such constructions, oriented toward the use of computers.

Particle mixing and segregation in failure zones—theory and experiment

During the motion of powders, it is found that such of the material moves as blocks with regions of high strain, termed failure zones, between. For free-flowing materials, it is shown how a general theory of particle motion can be developed in order to understand mixing and segregation in failure zones. The theory shows how complete segregation can arise. It can also be applied to systems of low concentrations of small particles, employing previously obtained data on inter-particle percolation rates, to give a model free of adjustable parameters; this shows satisfactory agreement with experiment.

Variation of void fraction in moving dense bed of fine-grain material in vertical standpipes

In hydrodynamic calculations and also in determining the pressure on the sidewalls of vertical channels with gravitational movement of free-flowing materials in a packed bed, the void fraction epsilon is usually assumed to be constant with respect to the height h in the bed, since no reliable data are available on the variation of epsilon. In this connection, the authors have investigated the variation of the void fraction with the height in a dense bed of fine-grain material moving in a vertical pipe and report their findings in this paper.

The simulation of a particle disperser

Bar dispersers provide a convenient way of distributing free-flowing materials. The effect of internal structure of a bar disperser on performance has been assessed by a computer simulation, a procedure much simplified by the recognition of a means of separating motion in the two horizontal directions. Results are presented in the form of a dimensionless velocity and as dimensionless diffusion coefficients. Disperser performance is improved by the use of smaller bars and is, in general, favoured by increase in free space within the equipment.

Pressures in hoppers filled with fine powders

In a previous paper [18] the authors showed that air pressures occurred between the grains of fine powders when they were loaded into hoppers and presented data where the pore air pressure was 60% of the total stress measured at the bunker wall when the hopper was first loaded. The paper included a mathematical model to describe the dissipation with time of these pore air pressures. The model was used to scale up from a bench scale deaeration test to give the rate of air pressure dissipation in a large scale hopper, which was broadly in agreement with that observed. The present paper continues the work, but the emphasis is experimental. The total stresses measured in large scale hoppers of various configuration and dimensions when first loaded with a fine powder are demonstrated to be hydrostatic in nature. The total stresses and pore air pressures near the wall during discharge of a fine powder are then presented. Different types of discharge are considered, namely mass flow, flow from eccentric outlets, and vibration-assisted flow. The total stress is observed to fluctuate with time, but these fluctuations are generally less regular and of lower magnitude than those observed in mass flow with free-flowing materials, such as sand. The total stresses on discharge by mass flow may rise above the initial hydrostatic stresses but it is shown that for the fine particulate material considered, the hydrostatic stress represents a reasonable approximation for one of the loading cases that should be considered during design.

Vibration mixers for free-flowing materials

The VNIItsvetmet (All-Union Scientific-Research Institute of Nonferrous Metallurgy) and Ust'-Kamenogorsk Highway Engineering Institute developed batch and continous vibration mixers. The functioning of these mixers is based on the principle of use of circular or elliptic vibrations perpendicular to the longitudinal axis of a horizontal cylindric or V-shaped vessel provided inside with rigidly fixed reverse screw guides interset with some gap. Finds that mixers of the proposed design permit effective mixing of free-flowing materials with any initial degree of nonuniformity and any difference in densities. The economic benefit derived from the introduction of 2 pilot-scale mixers was 58,000 rubles.

Measurement of forces and speeds of the striking parts of a hammer in the impact pressing of free-flowing materials

Measurement of forces and speeds of the striking parts of a hammer in the impact pressing of free-flowing materials

Non-spill discharge characteristics of bucket elevators

One of the well-known disadvantages of a simple type bucket elevator is still the backflow or spill. The accordingly lower capacity and increased power consumption are not always the worst consequences, provided that the boot does not become too full. With the considerable heights of modern bucket elevators, up to 225 ft and over, serious damaging of the conveyed material, an intensified noise level and increased wear can be far more inconvenient. The discharge of the buckets has been recognised as an extremely complicated phenomenon which strictly speaking cannot be analysed theoretically. This holds even more for free-flowing materials. Nevertheless, an analytical approximation has been worked out for the relatively simple case of cylindrical buckets filled with cohesive bulk material, to start with. With the developed approximative theory a spill-free combination of the relevant parameters has been found.

Theoretical computer investigation of the high-speed shaping of free-flowing materials

1. An analysis of the process of high-speed shaping of free-flowing materials has been carried out on the basis of theoretical premises of continuum mechanics and plastic flow of metals. 2. Calculation formulas have been derived for the specific pressures, specific energy of shaping, and temperature rise in high-speed processes as functions of properties of free-flowing materials (σt.ψn, A, and Cv) and shaping conditions. 3. The formulas yield results which are in good agreement with experimental data, and can therefore be employed in design calculations of technical processes and installations.

Particle-size distribution of fine fractions of glass batch

The intense dustiness at various stages in the treatment of free-flowing raw materials during the preparation of the batch and the supply of batch to the furnace as well as the intense dust abrasion and destruction of the furnace refractories are mainly determined by the concentration of fractions measuring less than 0.i mm in free-flowing materials. In planning suction and dust-removal apparatus and compressed-air transportation equipment in the glass industry, it is essential to have information on the particle-size distribution of the fine fractions of the free-flowing raw materials.

Drier for free-flowing materials

Drier for free-flowing materials

A general flowability index for powders

In order to meet the need of industry for a universal index of flowability, we propose the slope value of the function linking material flow factor FF (Jenike's ratio of major principal stress Σ 1 to unconfined yield stress f) with inverse specific tension 1/T s (the reciprocal of tension T divided by its corresponding value of steady-state normal stress σ L). We find that for “regular” powders (those that show both a constant shear index, n, and angle of static internal friction, ψ), this value is nearly constant over a wide range of stress and may be used as a general index of flowability, ζ, which we express as a percentage, thus: ζ = [1−( n−1)sinψ][1−( n-1)sin 2ψ] 2 nsinψ × 100% The small minority of powders classed as “irregular” (those that show a variable n and ψ with increasing stress, temperature or dampness) may be given an instantaneous percentage flowability, ζ i, at the specified conditions ζ i = [1−( n−1)sinψ][1−sinψ][ G−( n−1) G2 nsinψ( G−1) [1−( n−1)sinψ][1−sinψ][ G−( n−1)G 2 nsinψ( G−1) × 100% where G 2 = 1 + n 2 cot 2 Δ and Δ is the major angle of internal friction. The resulting flowability scale ranges from over 90% for very free-flowing materials down to single figure percentages for severely cohesive ones. A classification table for 60 regular powders is presented.

Determining the dimensions of outlet openings of hoppers for loose, free-flowing materials

Determining the dimensions of outlet openings of hoppers for loose, free-flowing materials

Geometric representation of the deformation of powder materials

An investigation was conducted, using Pavlov's approach, into the deformation of powder materials. The relation between deformation coefficients is expressed, on the basis of the law of the conservation of mass, by the same mathematical description for free-flowing, porous, and dense bodies, while the geometric representation of the deformation of these bodies is a hyperbolic paraboloid. This is evidence that, in the relative density range l ≥ τ > 0, a single process characterizes the shaping of these materials by the application of pressure. In spite of the large difference in properties between free-flowing, porous, and dense materials, the theory of shaping of powder materials by the application of pressure may be regarded as the most comprehensive, in which the working of dense bodies is but one particular case.