The sheet flow viscometer

Abstract

The flow of non-Newtonian materials as a planar sheet is an industrially important topic. In order to characterise the rheological properties of these flows, rotational/tube viscometry can be used successfully. However in most cases, it is generally not possible to achieve low shear rate data with coarse particle slurries, especially in small tube diameters, or normal gap-size couette viscometers, whilst adhering to the empirical criterion that the measuring gap size be greater than ten times the largest particle diameter. A possible solution is to use laminar sheet flows to obtain flow curve data at lower shear rates thereby supplementing the tube rotational viscometer data. One of the primary objectives of viscometry is the establishment of the relationship between shear stress and shear rate. This is often referred to as a rheogram or a flow curve and can be cast in the general form relating the flow curve to the bulk shear rate. The objective of this paper is to develop the basic relationships connecting shear stress to bulk shear rate for sheet flow, and to show how these can be extrapolated for laminar flow open channel design. This process is validated by comparison with established experimental data in the literature. It is concluded that laminar sheet flows can be used as a viscometrically reliable geometry for the rheological characterisation of complex slurry flows. Furthermore, these flows can be conceptually extrapolated to a sheet flow paradigm for laminar flow open channel design. A basis for the scale-up of such laminar flows for engineering design purposes is established. Further research is required to establish the laminar-turbulent transition and the effect of Froude number surface deformation effects.