High-Viscosity Systems

As noted previously, mixing in highly viscous liquids is slow both at the molecular scale, on account of the low values of diffusivity, as well as at the macroscopic scale, due to poor bulk flow. Whereas in low viscosity liquids momentum can be transferred from a rotating impeller through a relatively large body of fluid, in highly viscous liquids only 

A simple relationship has been shown to exist, however, between much of the data on power consumption with time-independent non-Newtonian liquids and Newtonian liquids in the laminar region. This link, which was first established by Metzner and Otto 1 2 for pseudoplastic liquids, depends on the fact that there appears to be an average angular shear rate y mt, for a mixer which characterises power consumption, and which is directly proportional to the rotational speed of impeller  

The validity of the linear relationship given in equation 7.18 was subsequently confirmed by METZNER and Taylor1 i3i. The experimental evaluation of ks for a given geometry is 

The prediction of power consumption for agitation of a given non-Newtonian fluid in a particular mixer, at a desired impeller speed, may be evaluated by the following procedure. 

D-D Square-pitch marine propellers with 3 None, (i) shaft vertical at vessel axis, (ii) shaft 10 0.13 2.2-4.8 0.16-0.40 10 0.9 

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High-Viscosity Systems A axial-flow impellers become radial flow as Reynolds numbers approach the viscous region. Blending in... 

From a practical point of view, power consumption is perhaps the most important parameter in the design of stirred vessels. Because of the very different flow patterns and mixing mechanisms involved, it is convenient to consider power consumption in low and high viscosity systems separately. 

Research is currently being earned out for DIERS and in Europe to measure safety valve discharge coefficients for, two-phase flow, including high viscosity systems. Note that any two-phase discharge coefficient is the ratio of measured flow to flow calculated using a particular two-phase flow model. Discharge coefficients should therefore only be used with the flow model for which-they were derived. 

Since bubbly flow at high superficial velocity approximates to homogeneous flow, this could lead to the conclusion that the homogeneous vessel assumption is likely to be good for high viscosity systems. 

It is necessary to discuss another chemical feature related to water-soluble polymers cross-cross-linking — the component that separates viscous systems from gel systems. Viscous systems flow, and it follows, therefore, that they do not possess the tensile properties of muscles. High-viscosity systems have structural integrity, gels provide the necessary combination of tensile strength and elongation or stretch. 

High-Viscosity Systems All axial-flow impellers become radial flow as Reynolds numbers approach the viscous region. Blending in the transition and low-viscosity system is largely a measure of fluid motion throughout the tank. For close-clearance impellers, the anchor and helical impellers provide blending by having an effective action at the tank wall, which is particularly suitable for pseudoplastic fluids. 

High-viscosity systems small devices improve efficiency... 

DDRM is particularly useful for the binary polymer blends. The dynamic interfacial tension coefficient, Vj2, is determined from the time evolution of a distorted fluid drop toward its equilibrium form. Measurements of either low viscosity model systems or high viscosity industrial polymer mixtures led to a good agreement with values obtained from the widely used breaking thread method. DDRM enables to measure in polymeric blends of commercial interest — the high viscosity systems that frequently are impossible to characterize by other techniques. Furthermore, for the first time it is possible to follow the time dependence of Vj, thus unambiguously determine its dynamic and equilibrium values. 

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