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Use these techniques to ensure a slurry's rheological properties have been evaluated under relevant test conditions, and that measurements are accurate.
EXECUTING THE PROPER TECHNIQUE FOR measuring the Theological properties of slurry for engineering applications is not an easy task. Literature offers many overviews on the measurement of the rheological properties of single and multiphase fluids. Some sources consider how to make accurate rheological measurements, but few deal with how to obtain relevant Theological data for a specific application. This article discusses how to make both accurate and relevant flow curves (plots of the shear stress, [tau], vs. shear rate, [gamma](dot above), the ratio of which is the fluid's viscosity, [eta]),* and how parameters describing the flow curve may be estimated for use with correlations suited to specific engineering applications.
Slurries can contain "non-settling" particles, particles that settle "rapidly" under gravity, or particles with a wide range of sizes. If particles settle rapidly, the slurry usually cannot be considered pseudo-homogeneous; thus, measurement of its laminar flow properties is not only difficult, but generally fruitless. Nevertheless, attempts are still made to measure the viscosity of settling slurries and to apply the questionable data during engineering design.
For example, pipeline design for rapidly settling slurries cannot be based on laminar flow properties, as depicted by the flow curve. In these cases, pressure drop (dP) vs. volumetric flowrate (Q) correlations involve parameters such as: the differences in density between particles and the suspending liquid; particle size; and particle drag coefficient. On the other hand, dP vs. Q correlations for pseudo-homogeneous, non-settling slurries do require knowledge of the flow curve.
Shear rate and shear stress ranges
For many structured slurries, the variation of [eta] with [gamma](dot above) as a function of solids content is complex. Figure 1 was developed originally for fully deflocculated slurries. However, partially or highly flocculated slurries show similar behavior, although the incidence of shear-thickening is reduced (even non-existent), due to the inability to increase the solids concentration to a level high enough for this behavior to occur.
At low solids concentrations, the viscosity curve ([eta] vs. [gamma](dot above)) indicates shear-thinning behavior ([eta] decreases with [gamma](dot above)) with well-defined zero-shear and infinite-shear viscosities. At higher solids concentration, shear-thickening behavior ([eta] increases with [gamma](dot above)) is...