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International audience; Powder discharging from a silo provokes an emission of dust. To understand and prevent this source of danger,3D simulations of a silo dischargewere performed using an Euler-Euler approach. The impact of the coupling between the flow inside the silo and the granular jet in free-fall is analyzed. Results show that the solid mass flow rate is correctly predicted and that gas back-flowing at the hopper exhaust appears responsible for the formation of a fluctuating and radially expanding jet. However, the radial expansion of the free-falling jet is underestimated. Stochastic fluctuations of the particles velocity at the hopper exhaust are introduced to evaluate their effect onthe downstream development of the free-falling jet. These fluctuations are found capable of generating a development of the jet similar to that observed experimentally. This suggests that the granular flow conditions at the hopper exhaust are responsible for the further dispersion of powder.

International audience; Multiphase pipelineflows are widely used in the oil and gas industry to transport solid-liquid or gas-solid mix-tures. Erosion caused by the impact of solid particles is a major challenge for equipment maintenance and safety,especially in complex geometries such as elbows. In this work, a CFD study on wall erosion in a 90° standardelbow has been performed using a multi-fluid approach, also called Euler/Euler, for poly-dispersedfluid-particleflows. A model is used for the erosion prediction taking into account particle turbulent kinetic energy obtainedfrom the Euler-Euler approach. A good agreement with experiments is observed. The effects of wall roughnessand solid massflow rate on the erosion rate are also investigated. For a certain amount of sand passing throughthe pipe elbow, there exists a solid massflow rate for which the particle impact damage is most dramatic.

International audience; In this paper, reactive unsteady three-dimensional numerical simulations of a chemical looping combustion (CLC) unit are presented. The configuration is a 120-kW pilot plant working with perovskite, CaMn0.9Mg0.1O3d, as the selected oxygen carrier. Numerical simulations were performed using NEPTUNE_CFD code in the frame of an Euler-Euler approach by computing both gas and solid phases in an Eulerian technique, accounting for specific closures in order to model the interphase mass, and momentum and energy transfers. Heterogeneous reduction and oxidation (gas–solid) reactions were modeled by means of a grain model to account for the competing mechanisms of the chemical reaction onto the grain surface, the gaseous diffusion through the product layer around the grain, and the external transfer through the gas mixture surrounding the particle. Results from numerical simulations were assessed against experimental measurements and analyzed in order to acquire insight on the local behavior of reactive gas-particle flow in the CLC system. The theoretical/numerical tool developed in this work can be used for the design upgrade of crucial parts of the system in the stage of scaling-up from pilot to industrial plants.

Conference: 9th International Symposium on Mixing in Industrial Processes Location: Birmingham, Angleterre Date: 2017/06/25-28; International audience; Cell growth in a chemostat is a well-documented research topic. How cells uptake the avail-able substrate to gain weight and engage cell division is not generally taken into account inthe modelling bioreactors. In fact, the growth rate is related to a population doubling timewhereas the microorganisms’ growth in mass is due to the mass transfer of substrates fromthe liquid phase to the biotic phase. Clearly, growth in mass precedes growth in number.Similarly, the transport of substrates down to the cell scale precedes the mass transfer. Thisarticle’s main feature is a two-dimensional population balance model that allows to uncou-ple growth in mass and growth in number when the equilibrium between a cell populationand its environment is disrupted. The cell length and the rate of anabolism are chosen asinternal variables. It is proved that the hypothesis “growth in number = growth in mass” isvalid at steady-state or in exponential growth only. The glucose uptake is assumed drivenby two transport systems with a different affinity constant for the substrate. This combina-tion of two regulated uptake systems operating in parallel explains a 3-fold increase in theuptake following a glucose pulse, but can also predict substrate uptake rates higher thanthe maximal batch value as observed in some experiments. These features are obtainedby considering carbon fluxes in the formulation of regulation principles for uptake dynam-ics. The population balance’s implementation in a multi-compartment reactor is a naturalprospective work and allows extensions to industrial processes.

This study deals with mathematical modeling and numerical simulations of reactive multiphase flows in dense fluidized beds. These flows involve complex physical mechanisms related to the coupling between the bed hydrodynamic and the reactions,which are still poorly understood. In this context, numerical simulations can provide explanatory access to the underlying physics taking place in the reactor, thus supplementing the experimental results. The present contribution focuses on the natural gas combustion in a dense fluidized bed reactor, for which experimental results are available from the literature (Dounit, 2001; Dounit et al., 2001, 2008). In their experiments, the authors pointed out the essential role played by the particle projection zone, above the bed surface, in the global thermal efficiency of the reactor operating at relatively low temperatures (600°C–800°C). In the present study, this point is further investigated by analyzing the results obtained by the numerical simulations. The unsteady 3D numerical simulations were performed using NEPTUNE_CFD code which is based on an Euler–Euler approach; the latter computes both the gas and the particulate phases in an Eulerian framework, accounting for specific closures modeling the interphase momentum and energy transfers. Time-averaged quantities were then computed and compared with the available experimental measurements. Numerical results (especially the gas temperature) were found to be very sensitive to the mesh refinement for the selected operating point. A further analysis at mesoscopic and macroscopic scales was carried out. This analysis pointed out the crucial role of the toroidal loop, which extends from the bed, close to the ejection zone, to the freeboard, on the fuel conversion. In numerical simulations, this loop must be accurately reproduced in order to provide reliable combustion predictions.

International audience; The aim of the paper is to introduce and validate a Monte-Carlo algorithm for the prediction of an ensemble of colliding solid particles, or coalescing liquid droplets, suspended in a turbulent gas flow predicted by Reynolds Averaged Navier Stokes approach (RANS). The new algorithm is based on the direct discretization of the collision/coalescence kernel derived in the framework of a joint fluid–particle pdf approach proposed by Simonin et al. (2002). This approach allows to take into account correlations between colliding inertial particle velocities induced by their interaction with the fluid turbulence. Validation is performed by comparing the Monte-Carlo predictions with deterministic simulations of discrete solid particles coupled with Direct Numerical Simulation (DPS/DNS), or Large Eddy Simulation (DPS/LES), where the collision/coalescence effects are treated in a deterministic way. Five cases are investigated: elastic monodisperse particles, non-elastic monodisperse particles, binary mixture of elastic particles and binary mixture of elastic settling particles in turbulent flow and finally coalescing droplets. The predictions using the new Monte-Carlo algorithm are in much better agreement with DPS/DNS results than the ones using the standard algorithm.

Euler–Euler two-phase model simulations are usually performed with mesh sizes larger than the small-scale structure size of gas–solid flows in industrial fluidised beds because of computational resource limitation. Thus, these simulations do not fully account for the particle segregation effect at the small scale and this causes poor prediction of bed hydrodynamics. An appropriate modelling approach accounting for the influence of unresolved structures needs to be proposed for practical simulations. For this purpose, computational grids are refined to a cell size of a few particle diameters to obtain mesh-independent results requiring up to 17 million cells in a 3D periodic circulating fluidised bed. These mesh-independent results are filtered by volume averaging and used to perform a priori analyses on the filtered phase balance equations. Results show that filtered momentum equations can be used for practical simulations but must take account of a drift velocity due to the sub-grid correlation between the local fluid velocity and the local particle volume fraction, and particle sub-grid stresses due to the filtering of the non-linear convection term. This paper proposes models for sub-grid drift velocity and particle sub-grid stresses and assesses these models by a priori tests.

International audience; In most modelling works on bioreactors, the substrate assimilation is computed from the volume average concentration. The possible occurrence of a competition between the transport of substrate towards the cell and the assimilation at the cell level is generally overlooked. In order to examine the consequences of such a competition, a diffusion equation for the substrate is coupled with a specific boundary condition defining the uptake rate at the cell-liquid interface. Two assimilation laws are investigated, whereas the concentration far from the cell is varied in order to mimic concentration fluctuations. Both steady and unsteady conditions are investigated. The actual uptake rate computed from the interfacial concentration is compared to the time-averaged uptake rate based on the mean far-field concentration. Whatever the assimilation law, it is found that the uptake rate can be correlated to the mean far-field concentration, but the actual values of the parameters are affected in case of transport limitation. Moreover, the structure of the far-field signal influences the substrate assimilation by the microorganism, and the mean interfacial uptake rate depends on the ratio between the characteristic time of the signal and the diffusional time scale, as well as on the amplitude of the fluctuations around the mean far-field concentration in substrate. The present work enlightens some experimental results and helps in understanding the differences between the concentration measured and that present in the microenvironment of the cells. (C) 2012 Elsevier Ltd. All rights reserved.