Effect of particle shape and orientation

It has been claimed that particle shape, roughness and the nature of the material has little effect on the analysis [30] but there is considerable evidence that the size measured is the envelope of the particle. Comparison with other techniques gives good agreement for homogeneous spherical particles for non-spherical particles results may differ [31,32]. For porous particles the measured volume may be several times the skeletal volume, and the apparent volume for floes is greater than the volume of the particles that make up the floes [20]. 

Model experiments have been carried out by Marshall [37], Lloyd et. al. [38] and Eckhoff [39] but no firm conclusions can be drawn from them since the models used differ widely from the commercial Instruments. This work was extended by Harfield et. al. [40,41] using a large two- 

Pulses deviate from the ideal, single modal shape when they are generated from coincident particles. These pulses take on a variety of shapes always of longer duration and often having multiple peaks. 

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Effect of Particle Shape and Orientation to Flow. As indicated by Figure 10-2, the shape of the particle, and particularly its orientation to flow, affects the settling velocity. Particle shape is often quantifled by the sphericity, l , which is the ratio of the surface area of a spherical particle of the same volume to that of the nonspherical particle. Chapman et al. (1983) reported that for particles with sphericity between 0.7 and 1, it is sufficient to use eqs (10-3) and (10-4) and replace the particle diameter, dp, with the diameter of a sphere of equal volume. For particles with sphericity less than 0.7, the estimation of the settling velocity is complicated by the fact that the orientation to flow is a function of the Reynolds number. The effect of shape on the settling of such particles must be evaluated experimentally. Correlations presented by Pettyjohn (1948) and Becker (1959) are recommended only for preliminary estimates. 

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