Qd Nguyen

Viscosity bifurcation in thixotropic, yielding fluids

Most concentrated colloidal suspensions such as cement, drilling fluids, paints, muds, etc., have been considered until now thixotropic fluids with a flow curve of an ideal yield stress fluid. We start by showing from inclined plane tests, intended to determine the yield stress, that these systems in fact exhibit peculiar properties. Unlike ideal yield stress fluids, they stop flowing abruptly below a critical stress, and start flowing at a high velocity beyond a critical stress, which in addition increases with the time of preliminary rest. In order to clarify these features we carried out a complete set of rheometrical tests with a model fluid, a bentonite suspension. Our results show that under controlled stress, in some cases after significant flow, there is bifurcation of the behavior towards either stoppage or rapid shear, depending on the relative values of the imposed and critical stresses. As an immediate consequence, we find that no (homogeneous) steady state flows at a shear rate below a critical value can be obtained. These results can be qualitatively predicted by a simple theoretical model that assumes that the viscosity of the material results from the competition between aging and shear rejuvenation, associated to, respectively, the organization or disorganization of the network of particle interactions. This shows that the flow curve in the steady state of concentrated colloidal suspensions and, more generally, of structured fluids, is strongly affected by their thixotropy.Most concentrated colloidal suspensions such as cement, drilling fluids, paints, muds, etc., have been considered until now thixotropic fluids with a flow curve of an ideal yield stress fluid. We start by showing from inclined plane tests, intended to determine the yield stress, that these systems in fact exhibit peculiar properties. Unlike ideal yield stress fluids, they stop flowing abruptly below a critical stress, and start flowing at a high velocity beyond a critical stress, which in addition increases with the time of preliminary rest. In order to clarify these features we carried out a complete set of rheometrical tests with a model fluid, a bentonite suspension. Our results show that under controlled stress, in some cases after significant flow, there is bifurcation of the behavior towards either stoppage or rapid shear, depending on the relative values of the imposed and critical stresses. As an immediate consequence, we find that no (homogeneous) steady state flows at a shear rate below a critic...

Application of rheology to solving tailings disposal problems

This paper is an overview of a research program conducted at the University of Melbourne on mineral tailings handling, dewatering and pipeline transport. Two case studies are described where measurement and use of rheological properties have assisted in solving large-scale tailings disposal problems in the Australian mining and mineral industry. The first example deals with the alumina industry where techniques have been developed for characterising the complex flow behaviour of concentrated bauxite tailings (red mud), and knowledge gained has been successfully applied to the development and optimization of a semi-dry disposal system by Alcoa of Australia in Western Australia. In the second example, taken from the coal industry, the techniques developed for red mud have been used to examine the rheology of clay-based coal tailings from the Hunter Valley in New South Wales in order to assess the feasibility of dewatering and pipeline transportation of the tailings to disposal.

Effect of particle size on the flow properties of a South Australian coal-water slurry

Abstract The rheological behaviour of a low-rank coal-water slurry from Lochiel, South Australia, has been studied as a function of solids concentration, particle size and size distribution. Coal slurries consisting of particles finer than 45 μm suspended in water were found to exhibit a wide spectrum of flow behaviour ranging from Newtonian as low solids concentrations to shear-thinning and viscoplastic with a yield stress at higher concentrations. By adding a narrow-sized coarse coal fraction (208–279 μm) to the fine coal slurry, the flow characteristic of the slurry, at a fixed total solids concentrations, could be changed significantly with a substantial reduction in the slurry viscosity. An optimum coarse-to-fine particle ratio of 40:60 exists at which the slurry is Newtonian with a minimum viscosity of a about 5 times lower than the viscosity of a slurry containing only fine particles at the same solids concentration. A further reduction in the slurry viscosity of 50% was achieved by adding a second coarse coal particle (279–325 μm) to the bimodal slurry. The significant improvement in the rheological behaviour with changing the particle size distribution may be explained in terms of spatial rearrangement of the particles, and an apparent dilution effect. The results obtained in this study indicate that, with a careful control of the particle size distribution, it is possible to prepare an optimum coal-water slurry which has a low viscosity but with high solids loadings.

Influence of broth dilution on the disruption of Escherichia coli

The effect of cell concentration (5 to 150 g/L wet wt after broth dilution) on homogenizer disruption efficiency and homogenate viscosity is reported for E. coli. Broth dilution increases homogenizer efficiency and decreases feed and homogenate viscosity. However, this increase in disruption efficiency is not sufficient to warrant dilution of the broth prior to homogenization. The optimal feed concentration is the maximum possible that does not lead to practical handling difficulties due to high viscosity.

Continuous rheometry for industrial slurries

Abstract Mineral solids are usually processed, produced or handled in the form of two-phase slurries. Accurate and reliable measurements of the rheological properties of settling slurries using conventional viscometric instruments can be difficult, if not impossible, since the solid particles settle rapidly under the influence of gravity. In this paper we report on the development of an on-line flow rheometer specially designed and constructed for characterising the rheological behaviour of settling slurries. The instrument design is based on the principles of helical flow, which is the combination of tangential Couette and axial Poiseuille flows. The data reduction procedure developed is firmly based on the theory of helical flow and has been extensively tested and validated with fluids of different rheological behaviour. Good results have been obtained using the rheometer for various Newtonian liquids, pseudoplastic polymeric solutions and mineral slurries.

A new rheometer for direct measurement of the flow properties of coal ash at high temperatures

Development of more effective technologies of utilising low rank coals for power generation has been driven by a demand for higher efficiency, low capital costs and minimal environmental impacts. Fluidised bed systems are regarded as one of the more promising alternative technologies for power generation to overcome the disadvantage of the existing pulverised coal burning power generation plants for low rank coals. However, ash deposition and bed agglomeration are potential problems in fluidised bed processing of coals with high alkali and sulphur contents. In order to gain a better understanding of the mechanism of agglomeration in fluidised beds, a good knowledge of the rheological behaviour of coal ash deposits at high temperatures and under the processing conditions is necessary. Rheological characterisation of materials at high temperatures is difficult due to lack of standard instruments and reliable measurement techniques. We have recently developed a rheometer that has the capability of measuring the rheological properties of coal ash slag over a wide range of temperatures from 600 to 1300°C and under different processing atmospheres. In this paper the features of this unique instrument are described and the experimental technique developed for flow property measurement is outlined. Some typical measured rheological properties of coal ashes from different Australian low-rank coals are presented and discussed to illustrate the potential applicability of the rheometer for high-temperature rheological characterisation. Furthermore, by means of the experimental results obtained it is demonstrated that the alkali content of the coal ash plays a significant role in controlling the rheological characteristics of the ash deposit, which in turn has an important implication on agglomeration in fluidised bed combustion processes.

Variational principles and variational inequalities for the unsteady flows of a yield stress fluid

A minimum principle, which has been derived for the steady, creeping flows of a yield stress fluid with shear-dependent viscosity, is extended to flows when the yield stress is also shear dependent, and the flow may be unsteady. As an application of the minimum principle, the unsteady squeezing flow between two co-axial and parallel disks is examined. Next, the variational principle is extended to a variational inequality, and situations where inertia may be incorporated into the latter are discussed. Using this, the specific forms of the variational inequalities are derived for five flows: unsteady pipe flows, flow past a solid at rest, the reservoir problem, the cavity driven flow, and, finally, for a class of problems with free surfaces. Further, the variational principle and the inequality are extended to deal with those problems where wall slip may be present. In a manner similar to the way the minimum principle has been extended, a maximum principle for the stress in the above class of yield stress fluids is established, and is easily reworded to include the case of wall slip as well. In addition, this principle is converted to a variational inequality for the stress. Finally, it is shown that the mimimum velocity functional and the maximum stress functional are identical when the velocity and stress fields satisfy the equations of motion and the relevant boundary conditions.

Continuous on-line rheological measurements for rapid settling slurries

A continuous flow rheometer based on the principles of helical flow has been developed for measuring the rheological properties of settling slurries. A special feature of the rheometer is its method for data analysis that allows the fundamental rheological parameters to be determined accurately and directly from experimental data without relying on any calibrations. Extensive testing with various fluids and mineral slurries has demonstrated the capability of the instrument for flow property characterisation of complex fluids. A comparison of the results obtained from the helical flow rheometer with those measured independently using a modified tube viscometer and a cone-plate rheometer shows excellent agreement. The numerical procedure developed for data analysis applies to fluids exhibiting any rheological behaviour, including viscoplastic fluids with a yield stress. Particle migration, which is known to affect rheological measurements of particulate systems, is found to have a minimal effect in the rheometer developed.

Rheological Properties of South Australian Coal-Water Slurries

The rheological behaviour of coal-water slurries from two low-rank coals in South Australia has been studied as a function of solids concentration, particle size and size distribution. Coal slurries consisting of particles finer than 45 micron dispersed in water were found to exhibit a wide spectrum of flow behaviour ranging from Newtonian at low solids concentrations to shear-thinning and viscoplastic with a yield stress at higher concentrations. By adding a coarser coal fraction (208–279 micron) to the fine coal slurries, the flow characteristics of the resulting mixtures at a fixed total solids concentration could be changed dramatically with a substantial reduction in the slurry viscosity. There exists an optimum coarse to fine particle ratio of 40:60 at which the slurry is Newtonian with a minimum viscosity of several orders of magnitude lower than the viscosity of a slurry containing only fine particles at the same solids concentration. A further reduction in the slurry viscosity was achieved by add...

Rheological Properties of Vegetable Oil‐Diesel Fuel Blends

Straight vegetable oils provide cleaner burning and renewable alternatives to diesel fuels, but their inherently high viscosities compared to diesel are undesirable for diesel engines. Lowering the viscosity can be achieved by either increasing the temperature of the oil or by blending it with diesel fuel, or both. In this work the viscosity of diesel fuel and vegetable oil mixtures at differing compositions is measured as a function of temperature to determine a viscosity‐temperature‐composition relationship for use in design and optimization of heating and fuel injection systems. The oils used are olive, soybean, canola and peanut oils which are commercially available. All samples tested between 20°C and 80°C exhibit time‐independent Newtonian behaviour. A modified Arrhenius relationship has been developed to predict the viscosity of the mixtures as functions of temperature and composition.