A. W. Roberts

The influence of granular vortex motion on the volumetric performance of enclosed screw conveyors

This paper is concerned with the volumetric performance of enclosed screw conveyors with particular reference to the influence of vortex motion. Vortex motion arises as a result of internal friction, friction between the granular material and surface of the helical blade, and the infinitely variable helix angle of the helical flight from the outer periphery of the blade to the shaft. The vortex motion, together with the degree of fill, govern the volumetric efficiency and, hence, the volumetric throughput. An analysis of the vortex motion in vertical or steeply inclined screw or auger conveyors is presented. It is shown the vortex motion is characterised by the tangential component of the absolute grain velocity being substantially constant with the radial position of a point on the blade. On this basis, an expression for the volumetric efficiency is derived. The volumetric throughput can then be predicted. The paper also presents results from an experimental screw conveyor where it is shown that the analytical predictions correlate very closely to the measured results.

Prediction of lining wear life of bins and chutes in bulk solids handling operations

Abstract This paper is concerned with the prediction of lining wear life of bins and chutes in bulk solids handling plant. It focuses on abrasive wear and outlines the basic principles to be embodied in the development of a laboratory wear tester. Emphasis is given to a linear action wear tester developed jointly by the University of Twente, The Netherlands, and the University of Newcastle, Australia. The characteristics of abrasive wear in bins, hoppers and chutes are described and the application of test results to the prediction of wear life of lining materials is illustrated.

Particle Technology—Reflections and Horizons

This paper presents a review of particle technology as it relates to the storage, flow and handling of bulk solids. The paper traces salient aspects of the development of this subject from the 1880s to the present. The early research that commenced with studies of silo pressures and spanned some 25 years is briefly reviewed. The work in powder mechanics and soil mechanics that characterized the contributions over the first half of his century is indicated with themajor thrust coming in the 1950s through the fundamental work of Jenike. The work of Jenike laid the foundations for a wide range of significant research developments that have occurred over the past 40 years. The work has covered such topics as flow property testing, theories of flow, modelling of particle systems by continuumtheory and discrete elements, vibrations of powders, blending and mixing and wall loads under initial filling, flow and pulsing conditions. The importance of industrially orientated research is strongly emphasized. It is shown that in many areas of industrial operations, the range and complexity of the problems multiply at a faster rate than the research outcomes. Industrial interaction is essential in identifying research areas and associated professional development activities.

Flow dynamics or ‘quaking’ in gravity discharge from silos

Abstract This paper reviews the characteristics of pulsating or cyclic flow of bulk solids during gravity discharge in bins and silos. The dynamic load phenomenon is often referred to as “silo quaking” and is influenced by various factors related to the type of flow pattern developed in the bin and the flow properties of the bulk material. Of particular relevance is the influence of ‘slip-stick’ during shear flow, and the velocity at critical sections in the silo during discharge. An overview of recent and current research on this subject is presented.

An investigation into the effects of sinusoidal and random vibrations on the strength and flow properties of bulk solids

Abstract This paper is concerned with the determination of the effects of vibration frequency, amplitude and energy input on the shear strength, yield loci and flow function of bulk solids. A modified Jenike shear cell fitted with a vibration exciter is used, the equipment being designed to permit the shear strength to be measured during the application of both discrete frequency and broad band random vibrations. Studies using pyrophyllite and iron ore show that the shear strength and flow function can be reduced significantly by transverse vibrations, there being particular frequency bands over which the shear strength is minimised. Measurements of compressibility during lateral vibrations are obtained and the results are compared with the frequency versus shear data. Dynamic system identification analysis is performed to provide information on the dynamic properties of bulk materials, particularly with regard to the distribution of vibration energy as a function of frequency.

The interaction of bulk solid characteristics and surface parameters in surface or boundary friction measurements

Abstract This paper is concerned with wall or surface friction and related parameters that govern the behaviour of powders and bulk solids in contact with surfaces of handling plant such as hoppers and chutes. Various methods for measuring wall friction are reviewed and the influence of such factors as wall surface roughness, particle size distribution, moisture content and shear velocity or strain rate are examined. Particular attention is given to the characterization of surface roughness and its interaction with particle size distribution. During operation, these properties may change, the particle size becoming finer and the surface roughness changing due to wear. As a consequence, surface friction can change during plant operation.

Stress distributions in loaded wheat grains

For the purposes of understanding the way in which wheat grains are able to withstand the loads to which they are subjected during mechanical handling by conveying and processing equipment, it is necessary to have some knowledge of the stress distributions throughout the grain. For light load conditions it is shown that the grain stresses can be analyzed on the basis of elasticity theory employing the Hertz stress analysis. As the loads are increased up to the point of failure the stress distribution is rather complex and difficult to analyze theoretically. It is shown that with the aid of photo-elastic model studies in conjunction with microscopic deformation investigations on actual grains a qualitative analysis is possible. By means of this technique a better understanding of the mechanism of failure is obtained.

Gravity flow of granular materials in chutes: Optimizing flow properties

Abstract The problem of determining the profile of a gravity flow discharge chute so that flow characteristics are to be optimum in some given sense is one of modelling the physical process and solving the resulting describing equations. The paper discusses various possible types of forces retarding the flow of granular materials and describes analytical formulations and solution methods appropriate for finding chute profiles to maximize exit velocity or minimize transit time.

The effect of large particles on the flow properties of powders

Abstract This paper is presented to fill a gap in the knowledge of the effect of larger particles on the flow behaviour of finer powders. Very little has appeared in the literature on the matter and the study is justified in that it is common practice to remove coarse particles before testing on a shear tester. The assumption is made in removing larger particles that they do not affect behaviour and that it is the fines which cause the binding and hence the flowability problems. Four different dry fine powders, a fine white sand, an electrostatic precipitator dust, a mixture of zircon and pyrophyllite (ZAP75) and a local steaming coal were investigated. No generalisation can be made about the addition of various volumetric percentages of spheres of varying sizes or of irregular particles. Thus, with fine sand, balls had little effect on the shear strength whereas irregular coal particles produced marked effect increasing consistently with the relative amount added. On the other hand, with the ZAP75, an increase in strength at low normal loads was exhibited but a decrease (over strength of powder alone) was found at higher normal loads. Experiments were conducted on a whole coal with varying limits of upper size (and also with varying limits of lower size) and the shear strength was found to be strongly dependent on those size limits. It is concluded that whereas much more experimental work is necessary before generalisation can be made of the effect of the presence (or absence, depending on the viewpoint) of large particles on the flowability of a powder, such an effect cannot be ignored.

The interpretation of ring shear cell results

Abstract In testing some powders on a Portishead shear cell it was found that the sample sometimes becomes grossly inhomogeneous, some portions being much more compacted than others. A suggestion is made that this phenomenon may occur in other cell types without being detected, which would render interpretation of results more difficult, especially as similar inhomogeneities may occur in bulk handling plant. The Jenike shear cell was found to give values of strength of powders considerably greater than those of the Portishead cell.