Pseudoplastic fluids —> Rheology

Dilatant Fluids. Dilatant fluids or shear-thickening fluids are less commonly encountered than pseudoplastic (shear-thinning) fluids. Rheological dilatancy refers to an increase in the apparent viscosity with increasing shear rate (3). In many cases, viscometric data for a shear-thickening fluid can be fit by using the power law model with n > 1. Examples of fluids that are shear-thickening are concentrated solids suspensions. 

K. S. Sorbie, P.J. Clifford and E.R.W. Jones, The rheology of pseudoplastic fluids in porous media using network modelling. /. Colloid Interface Sci., 130 (1989) 508-533. 

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

Figure 4-2. Flow curves for various ideal rheological bodies. A Newtonian liquid. B Pseudoplastic fluid. C Dilatant fluid. D Bingham plastic iii is the yield value). E Pseudoplastic material with a yield value. F Dilatant material with a yield value.

The viscous fermentation broth used in this project exhibited pseudoplastic rheology that is modeled quite well over a wide range of shear rates by the power law model. Consequently, the power law was used to model fluid rheology in this study for at 10, 15, 20% solids concentration. The upper and lower limits for n in this study were obtained from experiments with fermentation broth (Z mobilis cultures) and ranged from 0.46 to 0.97 during the portion batch fermentation. 

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... 

As previously observed, the xanthan C solution in sea water in the presence of proteins behaved like a thixotropic fluid. By contrast, an almost standard rheological behavior of a pseudoplastic fluid was found with the centrifuged solution in sea water with proteins added. A weak thixotropic effect was still observed probably because of the difficulty to completely eliminate cells from such a viscous fluid. 

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). 

Solution Rheology. Solutions of polyacrylamides tend to behave as pseudoplastic fluids in viscometric flows. Dilute solutions are Newtonian (viscosity is independent of shear rate) at low shear rates and transition to pseudoplastic, shear thinning behavior above a critical value of the shear rate. This critical shear rate decreases with the polymer molecular weight, polymer concentration, and the thermodynamic quality of the solvent. A second Newtonian plateau at high shear rates is not readily seen, probably because of mechanical degradation of the chains... 

The results of a number of studies on polymer rheology in porous media are then reviewed. Firstly, results for pseudoplastic fluids (mainly xanthan) are discussed and then results are reviewed for fluids showing some viscoelastic/extensional viscosity behaviour (e.g. HPAM, PEO). In all of these studies, flow is purely single-phase, and most experiments have been performed at 100% water saturation in the porous pack or core, although a few have been done at residual oil. 

The network calculations outlined here and expanded elsewhere (Sorbie et al, 1989c) have indicated why expressions of the type given in Equation 6.17 are useful for correlating in-situ rheological data for pseudoplastic fluids. The apparent viscosity, f/ pp, when plotted against the average in-situ effective... 

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

Slurry Viscosity. Viscosities of magnesium hydroxide slurries are determined by the Brookfield Viscometer in which viscosity is measured using various combinations of spindles and spindle speeds, or other common methods of viscometry. Viscosity decreases with increasing rate of shear. Fluids, such as magnesium hydroxide slurry, that exhibit this type of rheological behavior are termed pseudoplastic. The viscosities obtained can be correlated with product or process parameters. Details of viscosity deterrnination for slurries are well covered in the Hterature (85,86). 

Dilatant Basically a material with the ability to increase the volume when its shape is changed. A rheological flow characteristic evidenced by an increase in viscosity with increasing rate of shear. The dilatant fluid, or inverted pseudoplastic, is one whose apparent viscosity increases simultaneously with increasing rate of shear for example, the act of stirring creates instantly an increase in resistance to stirring. 

Cross, M. M. J. Colloid Sci. 20 (1965) 417. Rheology of non-Newtonian fluids a new flow equation for pseudoplastic systems. 

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. 

Shellflo-S, succinoglycan is an interesting new polymer for use in the oilfield, in many ways complementary to xanthan. It is particularly appropriate for use in well completion fluids at moderate temperatures, where a more pseudoplastic rheology than that provided by HEC, and a more rapidly breaking polymer than xanthan is required. 

Polymer rheology can respond nonllnearly to shear rates, as shown in Fig. 3.4. As discussed above, a Newtonian material has a linear relationship between shear stress and shear rate, and the slope of the response Is the shear viscosity. Many polymers at very low shear rates approach a Newtonian response. As the shear rate is increased most commercial polymers have a decrease in the rate of stress increase. That is, the extension of the shear stress function tends to have a lower local slope as the shear rate is increased. This Is an example of a pseudoplastic material, also known as a shear-thinning material. Pseudoplastic materials show a decrease in shear viscosity as the shear rate increases. Dilatant materials Increase in shear viscosity as the shear rate increases. Finally, a Bingham plastic requires an initial shear stress, to, before it will flow, and then it reacts to shear rate in the same manner as a Newtonian polymer. It thus appears as an elastic material until it begins to flow and then responds like a viscous fluid. All of these viscous responses may be observed when dealing with commercial and experimental polymers. 

Cross, M.M., Rheology of Non-Newtonian Fluids A New Flow Equation for Pseudoplastic Systems, /. Colloid ScL, 20, 417 (1965)... 

Dilatant Fluids. Dilatant fluids display a rheological behavior opposite to that of pseudoplastics (Figs. 2 and 3) in that the apparent... 

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. 

There are three types of non-Newtonian fluids plastic, pseudoplastic, and dila-tant. Figure 4.39 shows the rheological behaviors of Newtonian and non-Newtonian fluids. A plastic fluid does not move until the shear stress exceeds a certain minimum value, known as the yield value (f), and is expressed mathematically ... 

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