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Reference: Cooke, Michael H., (1976). Mixing cohesionless materials. DPhil. University of Oxford.Citable link to this page:


Mixing cohesionless materials

Abstract: When solid particles of different types are mixed together,a random distribution of the components is rarely produced anddeterioration of the mixture can occur on subsequent handling.Among the microscopic processes responsible, one importantmechanism for free-flowing materials is thought to be interparticlepercolation, the drainage of particles through theinterstices between larger ones. If the larger particles arestationary this is called spontaneous percolation, whereasif it is produced by shear strain the term strain-inducedpercolation is used. Here a quantitative evaluation of bothand some consequences are described.A practical application of spontaneous percolation hasbeen the design and construction of a new static mixer ordistributor, consisting of rows of angle bars mounted horizontallyin a vertical channel. Material fed to the top of a unit bouncesoff the bars and is distributed across the channel. Two mixerswere built; one dispersed material in one lateral directiononly and could be used for feeding material onto a belt ordistributing seed from a moving vehicle. The other produced atwo-dimensional dispersion and would be useful in distributingmaterial flowing into hoppers or whenever a good mixture wererequired. Optimisation of the design was investigated using acomputer program which simulated the motion of a sphericalparticle as it fell through such a mixer. Design data wasdeduced from the record of the position of the particle. Themixers were not suitable for use with fine materials.Interpretation of experimental results from this equipmentrequires suitable statistical indices and two were developed here.One related the variance of sample compositions to the number ofparticles fed to the mixer by assuming that the distributions ofmaterial were ordered. The second, using the correlationcoefficient between samples, related the variance to the samplesize in those situations where two orthogonal processes are inoperation. Both techniques are generally applicable to fieldsother than that of powder mixing.On the theoretical side, an existing model of spontaneouspercolation for inelastic materials has been extended andimproved. The original form did not account for the motion ofa particle between collisions with bulk particles but this hasnow been included. An entirely new semi-empirical model forpartly elastic materials has also been proposed. Both predictpercolation velocities which agree with experimental data.In order to extend earlier experimental studies on strain-inducedpercolation, a simple shear cell was modified byinstalling a hydraulic drive which enabled the cell to bedriven at a constant speed. Advantages of the use of such acell include the possibility of detecting a percolating particleon entry to and exit from the bed and the constant strain throughoutthe material. Reliable and accurate readings of residencetimes of percolating particles were recorded and percolationvelocities and both lateral and axial diffusion coefficientswere calculated. These were functions of the relative particlesize and density, the material properties of the percolatingparticle and bed conditions such as strain rate and normal stressDenser and softer particles percolated faster. Decreasing thediameter ratio between percolating and bulk particles from 0.67to 0.27 caused a twenty-five fold increase in the percolationrate. The dependence of this rate on particle diameter was interpreted using statistical mechanics. The percolation ratehas been shown to reach a constant value as the strain rateincreases, in contrast to the deductions drawn in earlierwork by Scott, whose procedure has been proved to be unsound.

Type of Award:DPhil Level of Award:Doctoral Awarding Institution: University of Oxford Notes:The digital copy of this thesis has been made available thanks to the generosity of Dr Leonard Polonsky


Bridgwater, J.More by this contributor



Dr. J. BridgwaterMore by this contributor


 Bibliographic Details

Issue Date: 1976Identifiers

Urn: uuid:22cb5134-6da5-46be-a9b2-0653c3b141df

Source identifier: 306416596 Item Description

Type: Thesis;

Language: eng Subjects: Cohesion Materials Mechanical properties Particles Tiny URL: td:306416596


Autor: Cooke, Michael H. - institutionUniversity of Oxford facultyMathematical and Physical Sciences Division - - - - Contributors Bridg

Fuente: https://ora.ox.ac.uk/objects/uuid:22cb5134-6da5-46be-a9b2-0653c3b141df


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