CFD-DEM simulations of early turbulent solid–liquid mixing: Prediction of suspension curve and just-suspended speed

Solid–liquid mixing as a unit operation still faces considerable challenges, notably regarding the prediction of the impeller speed required to suspend the particles (Njs), the fraction of suspended solids and the homogeneity of the suspension at a given speed. In this work, we extend to the turbulent regime, by means of large eddy simulation (LES), a CFD-DEM model developed recently in our group for solid–liquid mixing. The resulting model is used to study the mixing of glass particles in a baffled stirred tank equipped with a down-pumping pitched blade turbine. Various characteristics of the liquid dynamics as well as the distribution and motion of the solids are investigated. The fraction of suspended solid particles predicted by the model is validated against experimental data obtained via the pressure gauge technique (PGT). Two new methods to calculate the fraction of suspended particles in a Euler–Lagrange simulation, the so-called Lagrangian suspended fraction analysis (LSFA) and the decorrelated fraction analysis (DFA) techniques are introduced. The results obtained with these two methods, as well as with many others taken from the literature, are compared to the Zwietering correlation and to the results obtained by the PGT. It is found that some techniques proposed in the literature, namely the local concentration, the power consumption and the transient solids concentration analysis techniques, cannot be applied adequately in this case. On the other hand, the LSFA, DFA and PGT techniques are observed to predict accurately the fraction of suspended solids when compared to experimental PGT data

Is the Maxwell–Garnett continuum model valid to predict the thermal conductivity of particle-stabilized (Pickering) emulsions?

An experimental heat transfer measurement apparatus is constructed to measure the thermal conductivity in two-phase systems using an internal standard. This apparatus is validated and used to obtain the thermal conductivity of glass-bead-stabilized oil-in-water (o/w) emulsions. The experimentally obtained values are found to be in good agreement with the predictions from the Maxwell–Garnett continuum model, thus confirming that no preferential heat-transfer route is formed through the glass beads in the emulsions

Simulation of granular flow in a rotating frame of reference using the discrete element method

Over the years, the Discrete Element Method (DEM) has attracted significant attention for its capacity to simulate granular flows because it captures physical phenomena that cannot be observed using continuum methods. However, the simulation of granular systems with DEM is computationally demanding, especially in the case of systems in rotation. One solution is to perform simulations in a non-inertial rotating frame of reference, which requires the addition of fictitious velocity-dependent forces such as the Coriolis force. We assess the numerical feasibility and accuracy of such DEM simulations. We show that the velocity Verlet scheme in its classical form no longer defines a symplectic map and is no longer of second order when there are velocity dependent forces. Nevertheless, our study of a dense particle flow within a rotating hourglass shows that the relevant properties of such flow are accurately reproduced in a non-inertial frame and that computational performance is improved

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 () 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

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

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

Natural regeneration potential and dynamics in boreal lichen woodlands of eastern Canada following soil scarification

Boreal lichen woodlands (LWs) are stable low tree-density zones of the Canadian boreal forest whose afforestation has been proposed as a way to create new C sinks and thus mitigate climate change. Planting operations in these remote areas are however costly and time-consuming, and may not be necessary when soil scarification is followed by dense natural regeneration. In the present study, we assessed the natural regeneration potential and dynamics in six boreal LWs of Québec, Canada, 11 years after soil scarification. The number, size (height and stem diameter) and age of seedlings were measured in 2-4 sampling plots per site (18 plots in total). Our data show that scarification operations produced on average 1,400 m2 ha–1 of exposed mineral soil (scarification intensity of 14%) with, however, a large within-site variability. The natural regeneration was mainly composed of black spruce seedlings (> 95%), averaged ∼12,000 seedlings ha–1 across the six sites and significantly varied among sites, mostly due to the variation in scarification intensity. Seedling density averaged ∼9 seedlings m–2 of exposed mineral soil and increased with seed tree mean diameter at breast height (DBH) (R2 = 0.51; P < 0.05) but not with the density of seed trees, revealing the importance of old and large seed trees in natural regeneration success. Together, scarification intensity and the DBH of remaining seed trees explained ∼60% of the variation in natural regeneration density across the 18 sampled plots. The rate of establishment of seedlings was generally high – with on average 60% of the carrying capacity of the substrate being reached within three years following scarification – and increased with seed tree mean DBH (R2 = 0.77; P < 0.05). However, the growth rate of seedlings was very low. Eleven years after scarification, 60% of the seedlings were < 15 cm and the height of 10-yr-old seedlings averaged 27.5 cm. Thus, even though seedling establishment was successful, the biomass accumulated by the natural regeneration was negligible in the span of a decade. Therefore, the implementation of afforestation following scarification appears to be necessary to create significant C sinks in the midterm

One-step processing of highly viscous multiple Pickering emulsions

ABSTRACT: Hypothesis Solid-stabilized Pickering emulsions have attracted a lot of attention recently due to their surfactant-free character, and exceptional stability. At the moment, how the viscosities of the liquid phases impact the processing of Pickering emulsions remain to be clearly understood – it is however an important parameter to consider when developing chemical engineering processes employing these multiphase liquids. Our first assumption was that the amount of emulsified dispersed phase would drastically decrease as viscosity increases. Experiments and findings In this work, we demonstrate that double water-in-oil-in-water (W/O/W) Pickering emulsions are obtained in a single processing step when using very high viscosity silicone oils (≥10,000 cSt) and a single type of sub-μm silica particles modified with two grafted silanes and sodium alginate. The formation of water sub-inclusions proceeds via a phase-inversion mechanism. These sub-inclusions are subsequently stabilized and retained in the oil phase due to its viscosity, limiting sub-inclusions mobility, and the presence of adsorbed particles forming dense layers at oil-water interfaces, acting as barriers. The process we present is simple, requires a minimum number of components, and allows the preparation of multiple emulsions which could then be used to efficiently protect and/or transport a variety of sensitive encapsulated compounds

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

Tuning particle–particle interactions to control Pickering emulsions constituents separation

We demonstrate that the separation and recovery of solid-stabilized (Pickering) emulsion constituents are significantly improved via a simple filtration approach – without any additional chemical agent – by initially grafting sodium alginate (SA), a natural polysaccharide, onto silane-modified sub-micrometer silica particles. The combination of surface-grafted trimethoxy(propyl)silane (TMPS) and (3-aminopropyl)trimethoxysilane (APTMS) controls particle wettability, verified via zeta potential and contact angle measurements. Rheometry and filtration experiments reveal that further grafting of SA via APTMS enhances particle–particle and droplet–droplet interactions. This work provides an approach towards the design of environmentally friendly Pickering emulsion based chemical engineering processes with easy-to-separate and reusable particles, allowing waste reduction and reduced toxicity advantages

L’EROP : 10 ans pour le rétablissement des oiseaux de proie au Québec

L’Équipe de rétablissement des oiseaux de proie du Québec (EROP) a été fondée en 2004, à la suite de la fusion des équipes de rétablissement du faucon pèlerin (Falco peregrinus), du pygargue à tête blanche (Haliaeetus leucocephalus) et de l’aigle royal (Aquila chrysaetos). À ces espèces d’intérêt pour l’EROP s’est ajouté récemment le hibou des marais (Asio flammeus). À l’aide des plans de rétablissement de chacune des espèces, l’EROP veille à la mise en oeuvre de mesures de conservation (p. ex. acquisition de connaissances, sensibilisation, protection) visant à redresser la situation des populations d’oiseaux de proie en situation précaire au Québec. Cet article présente le mandat, le mode de fonctionnement et les principales réalisations de l’EROP au cours de la dernière décennie. The Québec raptor recovery team (EROP) was established in 2004 following the merger of the peregrine falcon (Falcoperegrinus), bald eagle (Haliaeetus leucocephalus) and golden eagle (Aquilachrysaetos) recovery teams. Recently, the short-eared owl (Asioflammeus) has also been added to the list of species covered by EROP. The team aims to foster conservation measures (e.g., data collection, public awareness and protection) to improve the population status of endangered birds of prey, and achieves this through the publication of species recovery plans. This article outlines the mission, and the working and main achievements of EROP over the past decade