The Back Extrusion Test as a Technique for Determining the Rheological and Tribological Behaviour of Yield Stress Fluids at Low Shear Rates

The original contribution is available at http://www.ar.ethz.ch/International audienceA new method is developed to determine the rheological and tribological behaviour of viscoplastic fluids using a back extrusion test. In back extrusion geometry, the material is forced to flow in the gap between the inner and the outer cylinder. Such a flow is modelled by a Bingham constitutive law under different wall boundary conditions (stick, slip with friction and perfect slip). When steady-state flow is reached, an apparent shear rate is computed. The analysis of the inner cylinder penetration force versus the penetration depth helps us to develop a method to identify the fluid rheological and tribological properties. This method is based on an inverse analysis to identify the fluid behaviour parameters from experiments performed at different ram velocities and with different apparatus geometries. In order to study more complex fluids (Herschell-bulkley rheological behaviour, for example), an equivalent flow curve is plotted from tests characterized by different average shear rates. The tribological behaviour is identified using different wall boundary conditions, varying the surface roughness of the cylinders. The method is applied to oil/sugar suspension and plasticine. Rheological and tribological behaviours are identified and results are compared with those obtained under steady state shear flow. The obtained rheological parameters are close to those provided by the common rheological methods (difference lower than 15 %)

Use of ram extruder as a combined rheo-tribometer to study the behaviour of high yield stress fluids at low strain rate

International audienceWe propose in this work to provide an efficient and simple extruder device able to evaluate the rheological and tribological behaviour of high yield stress fluids, such as extrudible materials. An extruder able to measure simultaneously both the friction force acting on the extruder wall and the total extrusion force is developed. Based on previous studies, an efficient and accurate method of data analysis is then proposed and applied in order to obtain both a flow curve and a tribological law. Experimental tests are performed on soft modelling clay, kaolin paste and cement-based materials. Results are compared to conventional rheometry measurements. This comparison helps to evaluate the accuracy of the proposed experimental device and procedure

Slipping zone location in squeeze flow

the original publication is available at http://www.springerlink.com/content/f1611lmr64836780/International audienceIn squeeze flow rheometry, the main problem is the boundary condition between the squeezed material and the plates. Therefore, the crucial assumption is to know the location and the shape of the sample part where wall slip may or may not occur. This question is investigated from experimental results. For this, squeeze flow experiments are carried out to visualize the flow pattern at the walls. Influence of boundary conditions is particularly studied using different plate surface condition. As a result, with wall slipping conditions, we propose a flow modelling divided into two zones: a circular central zone of the sample sticks on the plates and, beyond that zone, the sample slips at the plates with friction

Mircostructure and rheological properties of concentrated tomato suspensions during processing

Food processing comprises operations such as dilution (changing the concentration), homogenisation (changing the particle size), and subsequent pumping (shearing), among others. It is thus of great interest to gain a better understanding of the mechanisms governing the creation and disruption of structures during these engineering operations, and the way in which they are related to the textural and rheological properties of the material. The influence of processing on the microstructure and the rheological properties of tomato paste suspensions has been studied. The microstructure was characterised using light microscopy and particle size distribution analysis. The way in which particles of varying size are packed in a specified volume at different concentrations was estimated in terms of the compressive volume fraction. The rheological properties were studied using small-amplitude oscillatory tests, giving the elastic (G') and viscous (G'') moduli, as well as steady shear measurements, giving the viscosity (etha). In the latter case both a rotational and a tube viscometer were used. The results indicate that tomato suspensions consist of a collection of whole cells and cell wall material forming a network (G'>G''). During the process of homogenisation, the particles are broken down, resulting in a smoother and more evenly distributed network of finer particles. The effectiveness of homogenisation in decreasing particle size seemed to be governed by the inherent susceptibility of the particles to breakage (i.e. the type of paste), the viscosity of the suspending medium, and the concentration of particles. Higher viscosities and concentrations were found to prevent breakage to some extent. The presence of larger amounts of fine particles in the homogenised suspensions had a considerable effect on the rheological properties. The yield stress was found to increase, and time-dependent effects became more apparent. At low deformations (gamma G''. However, the dynamic and steady shear data obtained for tomato suspensions coincided when using a shifting factor of about 0.1 on the frequency , which was fairly constant for a large range of tomato paste concentrations (from 100 to 30%, all with G'>G'')

Non-newtonian fluid selection for achieving flow similarity in stirred vessels

Many key engineering applications in the chemical sector incorporate the “duty of agitation” through the use of mixing-based unit operations to achieve the desired transport phenomena governing the particular system. These industrial processes often occur in stirred vessels equipped with centrally-mounted impellers, with the material commonly associated with high levels of toxicity, expense, handling difficulty and complex rheological behaviour. The industrial fluids of interest to this project display viscoplastic behaviour, where the fluid deforms with shear-thinning characteristics, once a yield stress has been exceeded. The mixing of viscoplastic fluids in stirred vessels can lead to the formation of caverns, where regions of flow surround the central impeller, outside of which the material is stagnant. The optimisation of industrial processes often involves the implementation of appropriately scaled-down systems and the utilisation of suitable replacement model fluids. After an extensive literature review of typical viscoplastic materials and the agitation of these fluid types in stirred vessels, a vast number of viscoplastic model fluids were formulated, rheologically characterised and fitted with mathematical rheology models. A computational platform (the ‘Model Fluid Selection Tool’) was developed and acted as a rheological database, which was used as part of an operational function of the tool for suggesting suitable model fluids that most closely match the rheological properties of the ‘real’, industrial formulations. Highly transparent Carbopol model fluids were mixed in stirred vessels equipped with small, centrally-mounted impellers (Rushton turbines and pitched-blade turbines) over multiple scales, ranging in volume from 2-20 L, with key geometrical length scale ratios being maintained. A method for achieving cavern size similarity was determined, by scaling the dimensionless cavern diameter, Dc/D, against a combination of dimensionless parameters: Rem-0.3Rey0.6n-0.1ks-1, where Rem, Rey, n and ks are the modified power-law Reynolds number, yield stress Reynolds number, flow behaviour index and impeller geometry constant, respectively. The functional relationship between Dc/D and Rem-0.3Rey0.6n-0.1ks-1¬ was utilised in a second computational platform (the ‘Matching and Predictive Mixing Tool’) for predicting cavern sizes in systems of known scale and rheology, and matching dimensionless cavern sizes in a second system when the scale and/or rheology was changed. Finally, the mixing effectiveness in dual-impeller systems for agitation of viscoplastic fluids was assessed. This research project presents novel methods for matching fluids in terms of rheological properties, whilst presenting a protocol for scaling the mixing of these fluids in stirred vessel systems.Open Acces

The Stokes boundary layer for a thixotropic or antithixotropic fluid

We present a mathematical investigation of the oscillatory boundary layer (‘Stokes layer’) in a semi-infinite fluid bounded by an oscillating wall (the socalled ‘Stokes problem’), when the fluid has a thixotropic or antithixotropic rheology. We obtain asymptotic solutions in the limit of small-amplitude oscillations, and we use numerical integration to validate the asymptotic solutions and to explore the behaviour of the system for larger-amplitude oscillations. The solutions that we obtain differ significantly from the classical solution for a Newtonian fluid. In particular, for antithixotropic fluids the velocity reaches zero at a finite distance from the wall, in contrast to the exponential decay for a thixotropic or a Newtonian fluid. For small amplitudes of oscillation, three regimes of behaviour are possible: the structure parameter may take values defined instantaneously by the shear rate, or by a long-term average; or it may behave hysteretically. The regime boundaries depend on the precise specification of structure build-up and breakdown rates in the rheological model, illustrating the subtleties of complex fluid models in non-rheometric settings. For larger amplitudes of oscillation the dominant behaviour is hysteretic. We discuss in particular the relationship between the shear stress and the shear rate at the oscillating wall

Processing the Couette viscometry data using a Bingham approximation in shear rate calculation

The original contribution is available at http://www.sciencedirect.com/science/article/pii/S037702570800013XInternational audienceThis paper presents an approach to computing the shear flowcurve from torque-rotational velocity data in a Couette rheometer. The approximation techniques in shear rate calculation are generally dictated by the radius ratio between coaxial cylinders and the rheological behaviour of fluid tested. Here, the approach consists in analysing the sheared material as a Bingham fluid and computing an average shear rate when the fluid in the cylindrical gap is partially and fully sheared. We focus in particular on the applicability of the Bingham approximation in shear rate calculation. First, the approach is assessed by examining synthetic data generated with Newtonian, non-Newtonian and yield stress materials with known properties, varying the gap radius ratio. The results, which are compared with commonly used techniques in shear rate calculation, prove the relevance of the proposed approach. Finally, its efficiency is examined by applying it to process Couette data of yield stress fluids taken from published works

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