Louis Fradette

Development of an unresolved CFD-DEM model for the flow of viscous suspensions and its application to solid-liquid mixing

Although viscous solid-liquid mixing plays a key role in the industry, the vast majority of the literature on the mixing of suspensions is centered around the turbulent regime of operation. However, the laminar and transitional regimes face considerable challenges. In particular, it is important to know the minimum impeller speed ( N j s ) that guarantees the suspension of all particles. In addition, local information on the flow patterns is necessary to evaluate the quality of mixing and identify the presence of dead zones. Multiphase computational fluid dynamics (CFD) is a powerful tool that can be used to gain insight into local and macroscopic properties of mixing processes. Among the variety of numerical models available in the literature, which are reviewed in this work, unresolved CFD-DEM, which combines CFD for the fluid phase with the discrete element method (DEM) for the solid particles, is an interesting approach due to its accurate prediction of the granular dynamics and its capability to simulate large amounts of particles. In this work, the unresolved CFD-DEM method is extended to viscous solid-liquid flows. Different solid-liquid momentum coupling strategies, along with their stability criteria, are investigated and their accuracies are compared. Furthermore, it is shown that an additional sub-grid viscosity model is necessary to ensure the correct rheology of the suspensions. The proposed model is used to study solid-liquid mixing in a stirred tank equipped with a pitched blade turbine. It is validated qualitatively by comparing the particle distribution against experimental observations, and quantitatively by compairing the fraction of suspended solids with results obtained via the pressure gauge technique.

3D finite element simulation of fluid flow through a SMX static mixer

Abstract Static mixers do not need an introduction anymore since they now cover a wide range of industrial applications. However, despite these numerous uses, the principle of action or in simple words, the way they work, is still to demonstrate. This paper presents an example of useful information that can be extracted from simulations results. It is also discussed why local values of power and shear at various points in the mixer are the only true useful information in the understanding of a mixers performance. It is then concluded that computational fluid dynamics software will tend to become essential for this investigation since experiments can hardly provide precise and versatile means of local measurements with such complex geometries as the ones povided by static mixers.

CFD phenomenological model of solid-liquid mixing in stirred vessels

Abstract Particle migration in a concentrated viscous suspension subjected to a non-homogeneous shear field was computed using a 3D extension of the diffusion model of Phillips et al. [Phillips, R. J., Armstrong, R. C., Brown, R. A., Graham, A. L., & Abott, J. R. (1992). A constitutive equation for concentrated suspensions that accounts for shear-induced particle migration. Physics of Fluids A , 4 , 30–40]. The numerical results were compared to experimental data from the literature for simple flows and to our own data in the case of two helical ribbon based mixing systems. It is shown that this type of diffusion model, which was developed to predict the behavior of concentrated suspensions dominated by particle–particle interactions, can predict migration trends but definitely requires additional improvements.

On the determination of heat transfer coefficient between pvc and steel in vacuum extrusion calibrators

The design of vacuum calibrators for the cooling of complex PVC profiles is central to the production of high quality extrudates. One important parameter governing cooling efficiency is the heat transfer coefficient at the interface between the stainless steel calibrator and the PVC extrudate, whose value is often taken as constant regardless of the extrusion velocity and the applied pressure vacuum. In this paper, a method is proposed to evaluate the variation of the heat transfer coefficient over the entire calibrator length. The idea is to use temperature measurements together with heat transfer simulation to derive a heat transfer correlation that can be used in practical design cases.

A semi-implicit immersed boundary method and its application to viscous mixing

Abstract Computational fluid dynamics (CFD) simulations in the context of single-phase mixing remain challenging notably due the presence of a complex rotating geometry within the domain. In this work, we develop a parallel semi-implicit immersed boundary method based on Open∇FOAM, which is applicable to unstructured meshes. This method is first verified on academic test cases before it is applied to single phase mixing. It is then applied to baffled and unbaffled stirred tanks equipped with a pitched blade impeller. The results obtained are compared to experimental data and those predicted with the single rotating frame and sliding mesh techniques. The proposed method is found to be of comparable accuracy in predicting the flow patterns and the torque values while being straightforwardly applicable to complex systems with multiples impellers for which the swept volumes overlap.

Experimental Flow Visualization and Residence Time Distributions in a Co-Kneader

Abstract Flow patterns and residence time distributions have been investigated in a transparent laboratory-scale Co-Kneader with a viscous fluid and different combinations of operating conditions. An image analysis technique, already successfully applied in stirred tanks, has been applied here for the first time in a continuous system. The flow visualization results are presented as mixing curves, which show quantitatively the evolution of the macromixing along the Co-Kneader. It has been found that the image analysis technique gives new insight into the mixing mechanism of the Co-Kneader and may be used as a simple experimental method to quantify the degree of backmixing. Residence time distributions have been used to obtain axial dispersion coefficients. A comparison of the cumulative distribution functions between different systems is also presented.

Sodium alginate-grafted submicrometer particles display enhanced reversible aggregation/disaggregation properties

In this article, we demonstrate that submicrometer particles with surface-grafted sodium alginate (SA) display enhanced and reversible aggregation/disaggregation properties in aqueous solution. 300 nm silica particles were first functionalized with an aminosilane coupling agent, followed by the grafting of pH-sensitive SA, as confirmed by zeta potential, XPS and FTIR analyses. The SA-modified particles show enhanced aggregation properties at acidic pH compared to unmodified silica, with a 10 times increase in average aggregate diameter. The process is reversible, as the aggregates can be broken and dispersed again when the pH is increased back to 7.0. As a result, the sedimentation rate of SA-modified particles at pH 3.0 is both significantly faster and complete compared to the unmodified particles. This enhanced aggregation is most likely due to the formation of intermolecular hydrogen bonds between neighboring SA-modified particles. This work illustrates how surface-grafted macromolecules of natural origins can be used to tune interparticle interactions, in order to improve separation processes.