Pseudoplastic fluid properties

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. 

Radiation has been shown to be a feasible way to convert higher DP alginates to lower DP alginates with improved properties (King, 1994). Loss of t accompanying such depolymerizations can transform pseudoplastic fluids to Newtonian fluids. In carrageenans, radiolysis was shown to be initially rapid and then to decrease to a constant, low, radiation-insensitive DP the rate was faster in solution than in the solid state (Marrs, 1988). 

Gogarty, W.B., 1967. Rheological properties of pseudoplastic fluids in porous media. SPEJ (June), 149-160. 

Polymer solutions and melts, residual oils, rubber solutions, many petroleum products, paper pulps, biological fluids (blood, plasma), pharmaceutical compounds (emulsions, creams, and pastes), various food products (fats and sour cream) can serve as examples of pseudoplastic fluids. Dilatant properties are mainly exhibited by high-concentration or coarse-disperse systems (such as... 

Steady shear measurements were used to determine flow properties and to estimate the degree of structure breakdown with shear (Elliott and Ganz, 1977). The power law equation (Eq. 3) has been used to describe the shear stress-shear rate behavior of salad dressings (Figoni and Shoemaker, 1983 Paredes et al, 1988, 1989). The flow behavior index of five commercial salad dressings at different temperatures and storage times of up to 29 days were all less than one, indicating that they were pseudoplastic fluids. The consistency index (/f) decreased with the increase in product temperature. 

As there seems to be no single influence that can be associated with non-Newtonian properties it is not realistic to look for explanations which cover all the effects observed. However, it may be useful to consider behaviour that is related to the individual characteristics of pseudoplasticity and viscoelasticity. Pseudoplastic fluids have a lower viscosity in regions of high shear, as near an impeller, and a higher viscosity in regions of low shear, as distant from an impeller. It is possible that this characteristic leads to mixing behaviour different from that observed for Newtonian fluids, in particular for fluid motion to be more than usually confined to the impeller region. 

As it was aforementioned, the rheological properties of cement pastes are dependent on many factors. In Fig. 5.7 the evolution of the properties of pastes from the pseudoplastic fluids through the Bingham fluid to the showing dilatancy, at higher... 

In the case of non-Newtonian liquids according to 77 = /(y) the shear stress is usually expressed by the relationship r = k- y) with k = rj and w = 1 valid for the special case of a Newtonian liquid. The symbol k is the eonsistency and 77 the fluidity (77 < 1 pseudoplastic and n> dilatant). Such fluids are not discussed here. The fluid property viscosity quantifies the inner friction within a fluid or the friction between molecules and is zero for ideal fluids. Increasing temperatures lead to an increase of viscosity in gases but to a reduction of this property in liquids. 

EXAMPLE 3J-2. Turbulent Flow of Power-Law Fluid A pseudoplastic fluid that follows the power law, having a density of 961 kg/m, is flowing through a smooth circular tube having an inside diameter of 0.0508 m at an average velocity of 6.10 m/s. The flow properties of the fluid are n = 0.30 and K = 2.744 N s"7m. Calculate the frictional pressure drop for a tubing 30.5 m long. 

Most of the studies on heat transfer, with fluids have been done with Newtonian fluids. However, a wide variety of non-Newtonian fluids are encountered in the industrial chemical, biological, and food processing industries. To design equipment to handle these fluids, the flow property constants (rheological constants) must be available or must be measured experimentally. Section 3.5 gave a detailed discussion of rheological constants for non-Newtonian fluids. Since many non-Newtonian fluids have high effective viscosities, they are often in laminar flow. Since the majority of non-Newtonian fluids are pseudoplastic fluids, which can usually be represented by the power law, Eq. (3.5-2), the discussion will be concerned with such fluids. For other fluids, the reader is referred to Skelland (S3). 

Therefore, we have to use non-classical lubricants, which are pseudoplastic fluids, the power losses reducing properties of which have been experimentally verified by Nicolas [6] in 1979 using Guar and Polyox in plain bearings. 

Xanthan gum properties might also be utilized in instant puddings a mixture of locust bean flour, Na-pyrophosphate and milk powder with xanthan gum as an additive provides instant jelly after reconstitution in water. The pseudoplastic thixotropic properties, due to intermolecular association of single-stranded xanthan gum molecules, are of interest in the production of salad dressings, i. e. a high viscosity in the absence of a shear force and a drop in viscosity to a fluid state under a shear force. 

Gogarty, W.B. Rheological Properties of Pseudoplastic Fluids in Porous Media, SPEJ (June 1967) 149-60. 

A wide variety of nonnewtonian fluids are encountered industrially. They may exhibit Bingham-plastic, pseudoplastic, or dilatant behavior and may or may not be thixotropic. For design of equipment to handle or process nonnewtonian fluids, the properties must usually be measured experimentally, since no generahzed relationships exist to pi e-dicl the properties or behavior of the fluids. Details of handling nonnewtonian fluids are described completely by Skelland (Non-Newtonian Flow and Heat Transfer, Wiley, New York, 1967). The generalized shear-stress rate-of-strain relationship for nonnewtonian fluids is given as... 

Xanthan, used in EOR trials in the USA, and still being considered elsewhere, has found a niche in drilling fluids, which, together with other oilfield uses, accounts for some 2000 tons per year. Xanthan solutions have several useful properties they display a highly pseudoplastic rheology, are tolerant to salt, and have good thermal stability. There was we felt, however, some scope for improvement. 

Although most physical properties (e.g., viscosity, density, heat conductivity and capacity, and surface tension) must be regarded as variable, it is of particular value that viscosity can be varied by many orders of magnitude under certain process conditions (5,11). In the following, dimensional analysis will be applied exemplarily to describe the temperature dependency of the viscosity and the viscosity of non-Newtonian fluids (pseudoplastic and viscoelastic, respectively) as influenced by the shear stress. 

Until much more progress is made on these very fundamental approaches, greater accuracy is believed possible in engineering work that is based on the better developed discussion reviewed here. To this end a method has recently been proposed whereby the properties of all four time-independent fluids (Newtonian, pseudoplastic, dilatant, and Bingham plastic) may be quantitatively compared. 

The results of the latest research into helical flow of viscoplastic fluids (media characterized by ultimate stress or yield point ) have been systematized and reported most comprehensively in a recent preprint by Z. P. Schulman, V. N. Zad-vornyh, A. I. Litvinov 15). The authors have obtained a closed system of equations independent of a specific type of rheological model of the viscoplastic medium. The equations are represented in a criterion form and permit the calculation of the required characteristics of the helical flow of a specific fluid. For example, calculations have been performed with respect to generalized Schulman s model16) which represents adequately the behavior of various paint compoditions, drilling fluids, pulps, food masses, cement and clay suspensions and a number of other non-Newtonian media characterized by both pseudoplastic and dilatant properties. 

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... 

These examples serve to illustrate the ability of soluble polymer, interacting in a controlled fashion with colloidal particles, to transform both the equilibrium state and the mechanical properties of dispersions. All states are possible, from low viscosity fluids to pseudoplastic pastes with high yield stresses. 

Fluids with a Yield Stress. Both pseudoplastic and dilatant fluids are characterized by the fact that no finite shear stress is required to make the fluids flow. A fluid with a yield stress is characterized by the property that a finite shear stress, To, is required to make the fluid flow. A fluid obeying... 

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