Homogeneous Pseudoplastics

Fig. 23 Homogenization characteristic of a screw stirrer in draught tube for non-Newtonian liquids with pseudoplastic viscoelastic properties. For the sign explanation see Fig. 22, from [41],... 

Using the approach of Ford and Ulbrecht, influence of rheological properties (Newtonian, pseudoplastic, viscoelastic) on the homogenization characteristics was also satisfyingly taken into account for turbine stirrers [44] and other stirrer types [45]. 

Homogenization characteristics of a mechanically agitated polymerization reactor in terms of NO = /(Reeff = Ndfp/pe( ) have been published for various stirrer types and pseudoplastic liquids with power-law behavior (Opara, 1975, Tebel et al, 1986). The homogenization time is compared to that for Newtonian fluids (Opara, 1975), and the homogenization properties of a... 

One can see that the index n of a power-law fluid substantially affects the velocity profile. With increasing pseudoplasticity the distribution of the velocity becomes more and more homogeneous, approaching a quasisolid distribution with profile V = (V) = const in the limit as n —> 0. On the contrary, dilatancy makes the flow field more and more nonuniform, and as n - oo the velocity profile approaches the triangular shape given by... 

It has been found that in the case of viscometers with co-axial cylinders, the flow is not homogenous if the distance between cylinders is too wide [1, 18]. The maximum distance should be not higher than 1 mm [ 1]. At higher distance the flow curves correspond to the pseudoplastic fluids with no clearly marked yield stress value. For low shear rate the flow of paste in the gap between the cylinders is not uniform as long as the stress does not exceed the yield stress value this can be derived from the Reiner-Rivhn equation ... 

Slow sedimentation of particles will occur, for example, in an activated sludge or in fine particle catalyst suspensions. For those lands of systems, a homt eneous distribution of solids is characteristic. Here, the liftoff from the vessel bottom as well as the state of a homogeneous suspension can be achieved with a comparably low power input or only slight movement of the liquid. On the other hand, at higher solids concentrations a pseudoplastic flow characteristic of the suspension can occur. As an example, concentrations of only 6% of fibrous material - typically known from paper industry - can lead to this non-Newtonian behavior Frequently observed in suspensions with high solids concentrations is a Bingham plastic behavior. In this case, if a certain amount of shear is not introduced by agitation, the system behaves like an elastic solid body or a gel. 

Pseudohomogeneous pseudoplastics behave similarly to their homogeneous counterparts. Clay suspensions and magnetite-based slurries demonstrate an exponential relationship between n and as shown in Figure 3-12. The power law factor A has a more complex relationship with C, as shown in Figure 3-12. 

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