14 research outputs found
Impact of dynamic processes on the cloupling between fluid transport and precipitate deposition
No full textInternational audienceWe present a Lagrangian approach to study the clogging evolution of a one-dimensional homogeneous micro-fractured medium by deposition of one chemical species. Semi-analytical expressions are determined by means of the mass balance equations of #uid and solute in which the chemical and hydrodynamic disequilibrium makes an important dynamic contribution to the clogging mechanism. We describe some aspects of the coupling between the #uid transport and the chemical reaction with a qualitative analysis of these expressions by comparing the clogging evolution as a function of the imposed boundary conditions. In particular, a feedback e!ect underlies the solute transport if the pressure is imposed. Its analysis reveals a competition between local scales and large scales ampli"ed by the variation rate of the porosity, so that we have to distinguish the clogging process at short times and at long times. By means of the Lagrangian formulation, we are able to "nd an accurate analytical expression of the hydraulic conductivity of the overall medium. But it is necessary to use the successive approximation method to obtain quantitative results for the other quantities. Applications to heterogeneous media modeled by capillary tubes networks have displayed similar hydrodynamic behavior compared to homogeneous media if the deposition kinetics is not too sensitive to heterogeneities
CFD simulations of two stirred tank reactors with stationary catalytic basket
No full textInternational audienceAmong the different systems used for laboratory kinetic investigation, stationary catalytic basket stirred tank reactors (SCBSTRs) allow one to study triphasic reactions involving shaped catalyst with large size. The hydrodynamics of these complex reactors is not well known and has been studied experimentally in only a few cases. Despite the difference in the design of two commercial SCBSTRs reported in these works, the local measurements of the liquid–solid mass transfer coefficient inside the catalytic basket revealed the same velocity profile. The aim of the present work is therefore to investigate more accurately the hydrodynamics of the two reactors by means of CFD in order to compare the effect of the blade/baffle hydrodynamic interaction on the flow pattern. Owing to the geometrical complexity of the reactors, the hydrodynamic investigation is based on the kk–εε model and the Brinkman–Forsheimer equations. The agreement at the local level with the experimental data (PIV and mass transfer measurements) validates this preliminary work performed with the standard values of the parameters present in the turbulent model and the Brinkman–Forsheimer equations. The simulations reveal in both reactors a ring-shaped vortex around the impeller in the agitation region. The high axial location of its centre induces a reverse flow at the tips of the basket. Owing to the fluid friction in the porous medium, the azimuthal flow in the core region is transformed into a radial flow in the basket where the flow decreases abruptly. Vertical vortices are located at the blade tips and at the downstream face of the baffles or they are located in the basket on both sides of the baffles, depending on the design and the location of the baffles. At the inner radius interface of the basket, the vertical blade impeller induces a rather homogeneous velocity profile, but the pitched blade impeller imposes a high velocity at the plane of symmetry. Therefore the simulations demonstrate that two different local velocity patterns and two different porous media may induce the same mass transfer properties
Hydrodynamic and transport properties of packed beds in small tube-to-sphere diameter ratio: pore scale simulation using an Eulerian and a Lagrangian approach
No full textIn fixed bed catalytic reactors radial heterogeneities of the granular structure are present owing to topologic constraints imposed by the reactor wall. In order to analyse the influence of this structure on the fluid flow and the radial mass transfer properties, the study of sphere packings in cylindrical container and flow simulations at the pore scale are carried out. A collocated finite volume is used to solve the 3D Navier–Stokes equations. The Reynolds number ranges from 7 to 200 allowing to use the direct numerical simulation method in stationary flow regime combined to the no-slip condition at the interface solid/fluid. Therefore no correlation are used in this study. Furthermore, representative fixed beds, composed of several hundreds of spheres, are used for a diameter ratio of 5.96 and 7.8. Radial profile of the longitudinal velocity and the probability density function of the velocity components agree with the experimental data found in the literature. At low Reynolds number, the computation of current lines reveals the presence, at the reactor wall, of a layer whose width is around one-fourth of the sphere diameter. In this layer, the fluid flow is longitudinal and tangential. Particle tracking reveals also the existence of a second layer all along the spheres in contact with the reactor wall. Mass transfer in these two regions is controlled by the diffusive mechanism at low Reynolds number. The flow structure at high Reynolds number contains lots of eddies distributed homogeneously in the fixed beds. These structures are not recirculating zones (particle traps). On contrary, they accelerate the radial mass transfer so that the layers found at low Reynolds number tend to disappear at high Reynolds
1994). Réflexions sur la modélisation de la propagation de polluants dans les hydrosystèmes souterrains
No full textLes modèles de simulation de propagation de polluants dans les eaux souterraines sont de plus en plus utilisés comme outils de gestion de cette ressource. La qualité des simulations dépend étroitement des connaissances que l'on a des processus et des paramètres de transport nécessaires à la mise en application des modèles. La fiabilité des résultats repose sur: - le choix du bon modèle en fonction de l'échelle d'observation - la mesure des paramètres représentatifs du transport liée à l'échelle de discrétisation du site - la spatialisation de mesures locales. Compte tenu des spécificités des hydrosystèmes souterrains (invisibilité, accès coûteux), la connaissance du milieu restera trop fragmentaire pour réaliser des simulations fines. Seules les approches stochastiques permettent alors d'intégrer ces incertitudes dans les simulations.Groundwater quality modelling has become a tool for water management. The accuracy of the simulations closely depends on the available knowledge concerning the transport processes and the parameters used in the model. The accuracy of the results depends on the choice ofa suitable model adapted to the observation scale, the measurement of the effective parameters linked to the discretization of the field and the spatialization of the local measurements.The mathematical model used to describe mass transport in porous media is the dispersion-convection equation. The velocity is calculated by solving the flow equation. For heterogeneous media, numerical schemes which simultaneously solve heads and velocities have to be preferred to classical finite element or finite differences techniques. The dispersion coefficient represents the velocity fluctuations around the average velocity. Therefore, it strongly depends on the dimension and the scale of the discretization.A predictive simulation of the Twin Lake Tracer Test experiment has been done. After a very fine calibration of the flow (differences between measured and calculated heads less than 1 cm), the transport simulation did not succeed. The headgradients were not calculated with enough accuracy and the simulated plume travelled in a wrong direction.Due to the nature of groundwater (invisible, expensive rneasurements), knowledge of the structure of the aquifer will always be too incomplete to perform very detailed simulations. Stochastic computations may be the way to take into account uncertainities in groundwater modening
1994). Réflexions sur la modélisation de la propagation de polluants dans les hydrosystèmes souterrains
No full textCFD and kinetic methods for mass transfer determination in a mesh microreactor
No full textThe global gas-liquid-solid volumetric mass-transfer coefficient KLa of a catalytic multiphase microstructured film contactor, featuring 5 m dimensions, a 155 m liquid film thickness, and a 15 m thin catalytic layer is determined, using the very fast hydrogenation of -methylstyrene with a Pd/-alumina catalyst. The volumetric mass-transfer coefficients measured experimentally fall in the range 0.8 - 1.6 s-1 above that predicted by the film model and those obtained from a CFD (3-D model) simulation, and from an analytical solution
Theoretical modeling of front propagation of CdS nanoparticles in a gel
No full textWe report a theoretical model to describe the spatiotemporal dynamics of a new system consisting of sulfide ions diffusing into an organic gel containing mercaptoethanol-capped cadmium ions. The product, cadmium sulfide, exhibits a faint yellow transparent propagating front starting at the gel-outer electrolyte interface. When subjected to UV light, this system reveals fluorescing CdS nuclei localized spatially in a narrow region (constant width), called pulse, that leads the front and propagates down the tube. The reported model is based on reaction-diffusion equations coupled to dynamical competitive particle growth. The resulting evolution equations were solved numerically and the results are shown. © (2010) Trans Tech Publications, Switzerland.Al-Ghoul M, 2009, J PHYS CHEM B, V113, P11594, DOI 10.1021-jp9022647; Bao YH, 2007, J AM CERAM SOC, V90, P1063, DOI 10.1111-j.1551-2916.2007.01504.x; Barkema GT, 1996, PHYS REV E, V53, pR2017; Baroud CN, 2003, PHYS REV E, V67, DOI 10.1103-PhysRevE.67.060104; Cabarcos EL, 1996, PHYS REV LETT, V77, P2834; Chong TH, 2001, CHEM ENG SCI, V56, P5391, DOI 10.1016-S0009-2509(01)00237-8; GALFI L, 1988, PHYS REV A, V38, P3151, DOI 10.1103-PhysRevA.38.3151; HAMPTON JHD, 1993, CHEM ENG SCI, V48, P1601, DOI 10.1016-0009-2509(93)80120-F; KOO YEL, 1991, J STAT PHYS, V65, P893, DOI 10.1007-BF01049588; Kravchenko VV, 1999, DOKL AKAD NAUK+, V364, P114; Kravchenko VV, 1999, DOKL AKAD NAUK+, V364, P687; LARRALDE H, 1992, PHYS REV A, V46, P855, DOI 10.1103-PhysRevA.46.855; Magnico P, 2000, CHEM ENG SCI, V55, P4323, DOI 10.1016-S0009-2509(00)00047-6; Ortoleva PJ, 1994, GEOCHEMICAL SELF ORG; Park SH, 2007, PHYS REV E, V75, DOI 10.1103-PhysRevE.75.026107; Park SH, 2001, PHYS REV E, V64, DOI 10.1103-PhysRevE.64.055102; Sinder M, 2000, PHYS REV E, V62, P3340, DOI 10.1103-PhysRevE.62.3340; TAITELBAUM H, 1992, PHYS REV A, V46, P2151, DOI 10.1103-PhysRevA.46.2151; Taitelbaum H, 1996, PHYS REV LETT, V77, P1640, DOI 10.1103-PhysRevLett.77.1640; TAITELBAUM H, 1991, J STAT PHYS, V65, P873, DOI 10.1007-BF01049587; Yen A, 1996, PHYS REV E, V54, P2447, DOI 10.1103-PhysRevE.54.2447; 2002, PHYS CHEM CHEM PHYS, V4, P125312
Probabilistic learning of gas wall interaction from Molecular Dynamics simulations and application to gas transport problems in nano micropores
No full textInternational audienceModeling and characterizing gas-wall interactions at the atomic scale are crucial for understanding transport behavior in micro- and nanopores and for accurately simulating gas flows in porous materials. It is well known that gas displacement in extremely tight channels is complex and significantly influenced by adsorp- tion/desorption physics and surface diffusion mechanisms at the boundary walls. In this work, the collisions of helium atoms with graphite plates in thermal equilibrium are simulated using Molecular Dynamics meth- ods at various temperatures [1]. It is observed that at temperatures as high as 200 K, gas atoms reflect almost instantaneously, and pre- and post-collision velocities are strongly correlated. However, at lower tempera- tures, a significant proportion of gas atoms are adsorbed and move randomly on the surface before being desorbed. The velocity correlations are also weaker and reduced with temperature. A detailed analysis of the Potential Energy Surface (PES) and Mean Square Displacement (MSD) reveals a two-stage ballistic-diffusive behavior under weak energy barriers and low friction conditions. The velocity correlation coefficient, which is directly related to the tangential momentum accommodation coefficient (TMAC), is also determined, and an empirical relation between TMAC and temperature T is proposed.From the collision data, including particles’ velocity, residence time, and surface displacement, a surrogate stochastic wall model is constructed using probabilistic learning approaches [2]. The model is designed to replace atomic walls by predicting the probability distribution of residence time τ , surface displacements ∆x, ∆y, and post-collision velocities vx, vy, vz for a given pre-collision velocity vx′ , vy′ , vz′ in the form: (vx, vy, vz, ∆x, ∆y, τ ) = f (vx′ , vy′ , vz′ , U ). The latent gaussian variables U are reduced unknowns representing the microstate of the solid walls at tem- perature T . The function f is composed of orthogonal polynomials of random variables, whose parameters are determined by minimizing probalistic distance. Since the data is generated under equilibrium conditions, special attention is given to ensuring the equilibrium distribution of classical particle velocities and respect- ing the principle of time reciprocity. An example of a Monte Carlo simulation of Knudsen diffusion for gas particles traveling between two parallel walls using the stochastic wall model is presented.[1]Magnico P., To Q.D. (2023) Collisions and diffusion of Helium gas in nanometric graphitic channel. Inter- national Journal of Heat and Mass Transfer, 214, pp.124371.[2]Soize C., To Q.D. (2024) Polynomial-chaos-based conditional statistics for probabilistic learning with het- erogeneous data applied to atomic collisions of Helium on graphite substrate. Journal of Computational Physics, 496, pp.112582
Characteristics of liquids lugs in gas–liquid Taylor flow in microchannels
No full textThe hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (Ls/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity UTP. The product of the liquid slug residence time and the recirculation rate is independent of UTP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug
