Fluids pseudoplastic

Numerous examples of polymer flow models based on generalized Newtonian behaviour are found in non-Newtonian fluid mechanics literature. Using experimental evidence the time-independent generalized Newtonian fluids are divided into three groups. These are Bingham plastics, pseudoplastic fluids and dilatant fluids. 

Pseudoplastic fluids have no yield stress threshold and in these fluids the ratio of shear stress to the rate of shear generally falls continuously and rapidly with increase in the shear rate. Very low and very high shear regions are the exceptions, where the flow curve is almost horizontal (Figure 1.1). 

A common choice of functional relationship between shear viscosity and shear rate, that u.sually gives a good prediction for the shear thinning region in pseudoplastic fluids, is the power law model proposed by de Waele (1923) and Ostwald (1925). This model is written as the following equation... 

Figure l.l Shear thinning behaviour of pseudoplastic fluids... 

Pseudoplastic fluids are the most commonly encountered non-Newtonian fluids. Examples are polymeric solutions, some polymer melts, and suspensions of paper pulps. In simple shear flow, the constitutive relation for such fluids is... 

Power consumption for impellers in pseudoplastic, Bingham plastic, and dilatant nonnewtonian fluids may be calculated by using the correlating lines of Fig. 18-17 if viscosity is obtained from viscosity-shear rate cuiwes as described here. For a pseudoplastic fluid, viscosity decreases as shear rate increases. A Bingham plastic is similar to a pseudoplastic fluid but requires that a minimum shear stress be exceeded for any flow to occur. For a dilatant fluid, viscosity increases as shear rate increases. 

Axial-flow turbines are often used in blendiug pseudoplastic materials, and they are often used at relatively large D/T ratios, from 0.5 to 0.7, to adequately provide shear rate in the majority of the batch particularly in pseudoplastic material. These impellers develop a flow pattern which may or may not encompass an entire tank, and these areas of motion are sometimes referred to as caverns. Several papers describe the size of these caverns relative to various types of mixing phenomena. An effec tive procedure for the blending of pseudoplastic fluids is given in Oldshue (op. cit.). 

Figure HA. Shear stress-shear rate relationships for dilatant and pseudoplastic fluids compared with...

Figure 8.5. Apparent viscosity-shear rate curves for dilatant fluid, a Newtonian fluid and pseudoplastic fluid which have the same apparent viscosity at zero shear rate...

Figure 4 shows the plots of apparent viscosity versus shear stress at 170°C for different mixes. It is evident that the materials behave as pseudoplastic fluids, and the viscosity decreases with increasing zinc stearate... 

As can be seen from this expression, there is no radial migration for Newtonian fluid (n = 1) for pseudoplastic fluid (n < 1), the migration occurs from the tube centre for dilatant fluid (n > 1), the migration is directed towards the tube centre. 

In general, for shear-thinning pseudoplastic fluids the apparent viscosity will gradually decrease with time if there is a step increase in its rate of shear. This phenomenon is known as thixotropy. Similarly, with a shear-thickening fluid the apparent viscosity increases under these circumstances and the fluid exhibits rheopexy or negative-thixotropy. 

Figure 7.8. Power curve for pseudoplastic fluids agitated by different types of impeller...

Pseudomonas fluorescens, 1 732 11 4 Pseudomonas putida, 11 4 Pseudomonas testosteroni alcohol dehydrogenase, 3 672 Pseudopelletierine, 2 81-82 Pseudoplastic flow, 7 280t Pseudoplastic fluids, 11 768 Pseudoplasticity, 10 679 Pseudoplastic with yield stress flow, 7 280t Pseudopolymorphism, 8 69... 

Wheeler, J. A. and Whissler, E.H., The friction factor-Reynolds Number Relation for the Steady Flow of Pseudoplastic Fluids through Rectangular Ducts. Part 1. Theory, Am. Inst. Chem. Eng. J., 11, 207 (1965)... 

Newtonian fluids containing a high concentration of rigid particles can show non-Newtonian flow behaviour with increasing shear rate, due to a break up of agglomerates in the shear field [4]. For many pseudoplastic fluid suspensions the... 

An extensive class of non-Newtonian fluids is formed by pseudoplastic fluids whose flow curves obey the so-called power law ... 

Figure 9 shows a dimensionless standardized material function of two pseudoplastic fluids often used in biotechnology. It proves that the examined polymers (CMC—a chemical polymer and Xanthane—a biopolymer) are not completely similar to each other if they were, the exponent m must not have been different by a factor of 2 (insert in Fig. 9). 

Figure 9 Dimensionless standardized material function of two pseudoplastic fluids [carboxy me thy 1-cellulose (CMC) and Xanthane] often used in biotechnology. Source From Ref. 12.

Since the shear-stress-shear-rate properties of pseudoplastic materials are defined as independent of time of shear (at constant temperature), the alignment or decrease in particle size occurring when the shear rate is increased must be instantaneous. However, perfect instantaneousness is not always likely if the foregoing causes of pseudoplastic behavior are correct, as they are believed to be. Pseudoplastic fluids are therefore sometimes considered to be those materials for which the time dependency of properties is very small and may be neglected in most applications. 

For pseudoplastic fluids the dependence of heat transfer coefficients upon flow rate should be the same as for Newtonians, but their numerical value should be greater at any flow rate by the factor /(3n + l)/4n. ... 

Fig. 10. Heat transfer correlation of Branch (B7) for pseudoplastic fluids.

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