Philippe A. Tanguy

Rheological modeling of concentrated colloidal suspensions

Abstract The use of concentrated colloidal suspensions is common in several industries such as paints, foodstuffs and pulp and paper. These suspensions are generally composed of strongly interactive particles. If the attractive forces dominate the repulsion and Brownian forces, the particles aggregate to form a three-dimensional network yielding a gel structure. Under flow, the micro-structure of suspensions can be drastically modified and the rheological properties are then governed by structure breakdown and build-up. In this work, we propose a structural network model based on a modified upper convected Jeffreys model with a single relaxation time and a kinetic equation to describe the flow-induced micro-structure evolution. Three distinct kinetic equations are tested for this purpose. The proposed model describes yield and thixotropic phenomena, nonlinear viscoelastic behavior and output signal distortions observed for relatively small strain amplitude during oscillatory measurements, and overshoots observed in stress growth experiments. A comparison of model predictions and experimental data for fumed silica and coating colors is also presented. However, different model parameters must be used to correctly predict the different flow properties indicating that a more versatile or generalized kinetic equation must be proposed.

A Comparative Assessment of the Performance of the Kenics and SMX Static Mixers

The performance of two commercial static mixers in the laminar creeping flow regime, the Kenics mixer (Chemineer) and the SMX mixer (Koch Engineering), are compared using 3D numerical simulations. Several criteria are chosen as the basis for the performance evaluation, namely, the pressure drop, the mixer length, the Lyapunov exponent, the mean shear rate and the intensity of segregation. It appears that the SMX mixer is more efficient than the Kenics mixer to achieve a difficult mixing task or when installation space is restricted.

Adaptive finite element simulations of fluid flow in twin-screw extruders

Abstract The objective of this work 1 is to present a new strategy for the transient simulation of fluid flow in geometries involving moving parts and small gaps. It is based upon a fictitious domain method and a mesh refinement technique that relies upon one single reference mesh. With this technique, at each time iteration, the reference mesh may be adapted locally according for instance to the position of the gaps in the computational domain. The method will be discussed in detail and applied to the two-dimensional (2D) simulation of fluid flow in twin-screw extruders where one of the key issues is the ability to predict accurately the shear rates in the gaps formed by the rotating screws.

Heat transfer, mass transfer and kinetics study of the vacuum pyrolysis of a large used tire particle

Abstract A two-dimensional transient pyrolysis model for a large tire particle has been developed. The model considers both transportation and kinetics phenomena. The high content of volatile matters (VM) and the large size of the tire particle sample make the proposed approach different from other pyrolysis models. One novel contribution is the modelling of the transportation of the pyrolysis products by a simplified “bubbling” mechanism and the quantitative description of the thermal and kinetic changes which occur during the pyrolysis process. The model was tested with independent experimental data and a good agreement was found.

Power-draw analysis of a coaxial mixer with Newtonian and non-Newtonian fluids in the laminar regime

Abstract The power consumption of a new coaxial mixer composed of a wall scraping arm and a series of rods and a pitched-blade turbine mounted on the same axis of revolution and operated in a contra-rotating mode has been characterized. The work is based on experimental measurements and 3D numerical simulations in the case of homogeneous Newtonian and non-Newtonian fluids in the laminar regime. Very good agreements between experimental and numerical results have been obtained. It has been shown that the Metzner–Otto concept can be extended to account for the speed ratio between the impellers, which allows to represent the power consumption results of the coaxial mixer on a single power master curve like with a single agitator mixer.

Mixing performance induced by coaxial flat blade-helical ribbon impellers rotating at different speeds

The mixing performance (pumping, dispersion capabilities, power consumption) of a new dual impeller mixer composed of a disc turbine and a helical ribbon impeller mounted on the same axis but rotating at different speeds is investigated. The methodology is based on a blend of experimental measurements and 3D numerical simulations in the case of Newtonian and non-Newtonian shear-thinning fluids. It is shown that the dual impeller mixer outperforms the standard helical ribbon in terms of top-to-bottom pumping when the fluid rheology evolves during the process. The power consumption of this new mixer is also studied which allows to derive a generalized power curve.

Mixing With Helical Ribbon Impellers

Batch mixing of rheological complex fluids using helical ribbon and helical ribbon screw impellers was investigated in the laminar mixing region. Eight models and one polysaccharide fermentation broth (gelkn) were used. These fluids exhibited different rheological properties: Newtonian viscosity, viscoelasticity, and pseudoplasti-city. The Ostwald-de Waele model (power law) described adequately the shear viscosity of pseudoplastic fluids, including the gellan broth ( n ranged from 0.14 to 0.65). Power consumption decreased as pseudoplasticity increased and a model was developed to predict the deviations from Newtonian power input due to pseudoplasticity. A dimensionless, unique representation of the power data using the predictions of the model, showed a shift of the upper limit for the laminar mixing region towards higher values of Re ( Re ≈ 100). The model predictions were found to be in good agreement with those obtained by using the classical approach of Metzner and Otto 1 . The constant of proportionality between the average shear rate and the impeller rotational speed, K s , was found to be a function of the power law index, especially for high shear thinning fluids. It was found that power consumption increased as the impeller blade pitch decreased or as the impeller blade width increased. Both impeller pitch and blade width do not have significant influence on K S .

Mixing Hydrodynamics in a Double Planetary Mixer

The mixing hydrodynamics in a double planetary mixer is investigated numerically and experimentally over the course of a cross-linking reaction. Using various visualization techniques, it is shown that this mixer provides good radial dispersion capabilities but poor axial (top-to-bottom) pumping, irrespective of the viscosity level. The power drawn by the mixer evolves dramatically from about 180 W m −3 at 40% conversion up to approximately 4kWm −3 at 85% conversion. Overall, the numerical predictions and the experimental results exhibit good agreement although at 85% conversion, the numerical model is not accurate enough to predict adequately the power consumption due to physical phenomena not considered in the computations.

On the imposition of friction boundary conditions for the numerical simulation of Bingham fluid flows

Abstract The augmented Lagrangian method is applied to impose a friction boundary condition on walls sustaining very large shear-stresses. If the flow field is such that the wall shear-stress is smaller than the threshold value, the fluid adherence to the boundary is imposed and we recover the classical no-slip condition. When the wall shear stress reaches the critical value, the fluid is allowed to slip along the boundary with a given friction coefficient. This can be viewed as a free boundary problem since the regions of the boundary where the fluid will adhere or slip are not known a priori. The numerical implementation of this boundary condition involves variational inequalities that are briefly described in the context of a Bingham fluid. Numerical examples will also be presented.

CFD analysis of several design parameters affecting the performance of the Maxblend impeller

A CFD characterization of the hydrodynamics of the Maxblend impeller with experimental validations has been carried out with viscous Newtonian and non-Newtonian inelastic fluids. The mixing cases investigated were the non-baffled configuration with Newtonian and shear-thinning fluids, and the baffled configuration with only Newtonian fluids. The study focused on the effect of the impeller bottom clearance and the Reynolds number on the power characteristics, the distribution of shear rates and the overall flow conditions in the vessel. It was found that the bottom clearance plays a significant role on the power consumption, and that the value of the Reynolds number and the power law index strongly affect the axial pumping efficiency and the shear rate profile. The best performance was obtained when the impeller Reynolds number is superior to 10.