Approche multi-échelle et phénomènes de transport en changement de phase

Les mécanismes de changement de phase solide-liquide (fusion, solidification, dissolution) ou liquide-vapeur (écoulements diphasiques) sont présents dans de nombreuses situations industrielles : générateurs de vapeur des centrales nucléaires, échangeurs thermiques (refroidissement par film, condenseurs), métallurgie (élaboration des matériaux), moteurs à combustion interne (aéronautique, automobile), transport pétrolier en conduite (production de vapeur par dépressurisation). L’une des particularités de ces mécanismes de changement de phase réside dans leur caractère multi-échelle. Par exemple, la solidification des mélanges multi-constituants fait intervenir au moins trois échelles qui sont celle de la dendrite (~ 100 μm), celle de la zone de croissance colonnaire (1-10 mm), et enfin celle du lingot (1 à 100 cm). De la même manière, dans les écoulements diphasiques, différentes échelles interviennent : taille des bulles, gouttelettes (100 μm à quelques mm), épaisseurs des films liquide ou vapeur en paroi (~ 100 μm), diamètre du tube (quelques mm à plusieurs cm)

A pore network modelling approach to investigate the interplay between local and Darcy viscosities during the flow of shear-thinning fluids in porous media

During the flow of non-Newtonian fluids in porous media, the relationships between macroscopic quantities are governed by extremely complex microscopic fluid dynamics resulting from solid-fluid interactions. Consequently, the Darcy-scale viscosity exhibited by a shear-thinning fluid depends on the injection velocity, contrarily to the case of Newtonian fluids. In the present work, pore network modelling is used to investigate the relationships between local and macroscopic viscosities during the flow of shear-thinning fluids in 3D porous media. Special efforts are devoted to 1) identifying the influence of the viscosity exhibited by the fluid within the constrictions of the preferential flow paths on the value of Darcy-scale viscosity and 2) proposing an analytical expression to upscale viscosity from the local viscosity values. To go further, the reduction in average hydraulic tortuosity stemming from the directional nature of shear-thinning behavior in 3D porous media will also be quantified. The results of the present study show that Darcy-scale viscosity can be accurately calculated as the flow-rate weighted average of local viscosities in the investigated media. Moreover, the velocity maps provided by the proposed pore network flow simulations are suitable to assess hydraulic tortuosity reduction as compared to the flow of a Newtonian fluid

First analysis of a numerical benchmark for 2D columnar solidification of binary alloys

International audienceDuring the solidification of metal alloys, chemical heterogeneities at the product scale (macrosegregation) develop. Numerical simulation tools are beginning to appear in the industry, however their predictive capabilities are still limited. We present a numerical benchmark exercise treating the performance of models in the prediction of macrosegregation. In a first stage we defined a "minimal" (i.e. maximally simplified) solidification model, describing the coupling of the solidification of a binary alloy and of the transport phenomena (heat, solute transport and fluid flow) that lead to macrosegregation in a fully columnar ingot with a fixed solid phase. This model is solved by four different numerical codes, employing different numerical methods (FVM and FEM) and various solution schemes. We compare the predictions of the evolution of macrosegregation in a small (10×6 cm) ingot of Sn-10wt%Pb alloys. Further, we present the sensitivities concerning the prediction of instabilities leading to banded channel mesosegregations

Modelisation et simulation numerique des instabilites hydrodynamiques lors d'un stockage en aquifere

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Natural convection in partially porous media: a brief overview

International audienc

Natural convection in partially porous media: a brief overview

International audienc

Averaged model for momentum and dispersion in hierarchical porous media

International audienceHierarchical porous media are multiscale systems, where different characteristic pore sizes and structures are encountered at each scale. Focusing the analysis to three pore scales, an upscaling procedure based on the volume-averaging method is applied twice, in order to obtain a macroscopic model for momentum and diffusion-dispersion. The effective transport properties at the macroscopic scale (permeability and dispersion tensors) are found to be explicitly dependent on the mesoscopic ones. Closure problems associated to these averaged properties are numerically solved at the different scales for two types of bidisperse porous media. Results show a strong influence of the lower-scale porous structures and flow intensity on the macroscopic effective transport properties

Large particule transport in porous media: effect of pore pluggins on the macroscopic transport properties.

Large particle transport in porous media plays an important role in chemical and mechanical engineering but also in the medical field, especially in cancer treatment. The major difficulty in large particle transport results from the fact that the particles themselves influence the effective transport properties by pore plugging due to particle entrapment. In this study, we used a random walk model describing the particle transport inside the pores. The effective macroscopic transport properties are determined using the results of the random walk model. It is shown that the permeability tensor strongly depends on the particle size and the injection point location, whereas the dispersion coefficients remain independent. We also determined the maximal particle radius for which particle transport can be described by the convection-diffusion equation. Another important point in the chemical and medical field is the final particle distribution, and particularly the distribution of the quantity of liquid transported by the particles. Our results show that large particles do not lead to a homogeneous liquid distribution. Hence when a large quantity of liquid should be homogeneously distributed in the porous medium, the use of smaller particles is recommended

Numerical analysis of the pore-scale mechanisms controlling the efficiency of immiscible displacement of a pollutant phase by a shear-thinning fluid

Knowledge of the pore-scale physics of underground multiphase flows is essential to devise efficient soil remediation methods. However, the immiscible displacement of pollutant through the injection of a shear-thinning fluid remains poorly understood. The current work presents a full set of direct numerical simulations in which a Newtonian contaminant is displaced by a Carreau fluid or, alternatively, by a Newtonian fluid, in three porous media with different degrees of microstructural complexity. Imbibition, drainage and neutral wettability cases are considered, and the sensitivity of residual pollutant saturation to Carreau's law parameters is also assessed. The present results allow for the quantification of the performance of immiscible displacement using shear-thinning invading fluids. This performance is shown to depend on the value of capillary number and the heterogeneity of the porous microstructure, which determine the relative importance of viscous fingering, capillary forces and pollutant trapping behind the invasion front

Coupled upscaling approaches for conduction, convection, and radiation in porous media: theoretical developments

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