Transferts de chaleur et de masse dans des structures poreuses multi-échelles : application à l'étude des filtres à particules Diesel

Les filtres céramiques sont des dispositifs prometteurs pour diminuer les émissions de particules carbonées par les moteurs Diesel et respecter les futures normes environnementales. L'analyse et la modélisation des phénomènes physiques intervenant dans leur fonctionnement est indispensable pour améliorer leurs performances et leur durabilité. Dans une première partie, l’écoulement du gaz, le transport et la capture des particules au sein d'un filtre présentant une structure poreuse multi-échelle sont étudiés. Une approche de type « milieu poreux effectif » est adoptée pour simuler l’étape de filtration à l’échelle du filtre. Le modèle s’appuie sur des équations ID, développées pour un élément unitaire de filtre, préservant le caractère inertiel de l’écoulement. Dans une deuxième partie, le comportement thermique du filtre est étudié. Les transferts de chaleur convectif et diffusif sont pris en compte dans un modèle macroscopique à une température obtenu par la méthode de prise de moyenne volumique. Un terme source de combustion permet de considérer la réaction d’oxydation des particules collectées, intervenant dans l’étape de régénération du filtre. ABSTRACT : Ceramic filters appear to be the most promising solution to deal with the future emission standard requirements for diesel-powered vehicles. Indeed, this kind of Diesel Particulate Filters is known to provide high collection efficiency for carbon particles. The analysis and modelling of the physical phenomenon involved in their functioning is required to improve their performances and durability. First, the gas flow, the particle transport and collection within the filter are studied. Given its particular multi-scale structure, an “effective porous media” analysis is used in order to model the mass transfers in the whole filter. The model is based on ID equations, obtained for a representative unit cell of the filter, preserving the flow initial behaviour. In a second part, the thermal behaviour of the apparatus is studied. Heat transfers within the apparatus are described using a macro-scale one-temperature model obtained from a volume averaging scale-change analysis. A combustion source term is used to describe the oxidation reaction involved in the filter cleaning

Effect of pine bark and compost on the biological denitrification process of non-hazardous landfill leachate: Focus on the microbiology

In an attempt to optimize the cost-efficiency of landfill leachate treatment by biological denitrification process, our study focused on finding low-cost alternatives to traditional expensive chemicals such as composted garden refuse and pine bark, which are both available in large amount in South African landfill sites. The overall objective was to assess the behaviour of the bacterial community in relation to each substrate while treating high strength landfill leachates. Denitrification processes in fixed bed reactors were simulated at laboratory scale using anaerobic batch tests with immature compost and pine bark. High strength leachate was simulated using a solution of water and nitrate at a concentration of 500 mg l−1. Results suggest that pine bark released large amounts of phenolic compounds and hydroxylated benzene rings, which both can delay the acclimatization time and inhibit the biological denitrification (only 30% efficiency). Furthermore, presence of potential pathogens like Enterobacter and Pantoea agglomerans prevents the applicability of the pine bark in full-scale operations. On the other hand, lightly composted garden refuse (CGR) offered an adequate substrate for the formation of a biofilm necessary to complete the denitrification process (total nitrate removal observed within 7 days). CGR further contributed to a rapid establishment of an active consortium of denitrifiers including Acinetobacter, Rhizobium, Thermomonas, Rheinheimera, Phaeospirillum and Flavobacterium. Clearly the original composition, nature, carbon to nitrogen ratio (C/N) and degree of maturity and stability of the substrates play a key role in the denitrification process, impacting directly on the development of the bacterial population and, therefore, on the long-term removal efficiency

Modélisation de type double milieu de l'infiltration dans un massif de déchets ménagers.

Les déchets ménagers mis en place dans les Installations de Stockage des Déchets (ISD) constituent un milieu poreux notoirement complexe et hétérogène. De plus, les nouvelles méthodes de gestion des ISD de type bioréacteur mettent en oeuvre une ré-injection des lixiviats visant à augmenter l'humidité du milieu afin de favoriser la biodégradation. Dans ce cadre, et afin de mieux caractériser les transferts de liquide dans ce matériau, une campagne expérimentale à méso-échelle a été réalisée durant l'année 2008. La méthode d'humidimétrie neutronique couplée au suivi détaillé du bilan hydrique a permis d'évaluer l'évolution des profils d'humidité dans les pilotes entre chaque phase d'injection. Cette étude met en évidence le comportement caractéristique d'un milieu à porosité étalée et mal connectée. Un modèle de Richards modifié prenant en compte un milieu capillaire à double porosité de type mobile/immobile avec deux modèles de rétention, unimodal et bimodal, pour le continuum mobile ont été évalués. Le modèle à double porosité associé avec des propriétés hydrauliques bimodales permet de restituer de façon satisfaisante les observations expérimentales. En utilisant une série d'essais expérimentaux (mesures de porosité, de perméabilité et de courbe de rétention), seuls trois paramètres associés au propriétés capillaires de la phase « immobile » et au transfert entre les deux phases doivent être calés pour renseigner le modèle qui apporte une avancée significative au loi d'écoulement de type Richards classiquement utilisées

A New Momentum Equation for Gas Flow in Porous Media: The Klinkenberg Effect Seen Through the Kinetic Theory

International audienceIn this paper, an original set of transport equations for gas in porous media is developed. As far as low pressure gases or very fine grained porous media are concerned, molecular effects are likely to promote a dependency of the permeability on the pressure. These phenomena are usually modelled using the Klinkenberg correction to the Darcy's law. The retained methodology brings a new interpretation of this particular problem. The approach is based on a volume averaging scale-change methodology applied to the Boltzman equation taking into account the presence of walls. It leads to a homogenized kinetic equation describing the problem at the macroscale. A proper closure is then applied following the strategy proposed by Levermore to obtain a hydrodynamic description. The hydrodynamic force applied by the porous structure on the gas exhibits a strong non-linearity with the gas velocity. However, a linearization is proposed, recovering formally the classical Darcy's law. The validity of the resulting permeability tensor is finally discussed. As its dependency with pressure is concerned, it opens an original interpretation of the nature of the Klinkenberg effect

Transferts de chaleur et de masse dans des structures poreuses multi-échelles (application à l'étude des filtres à particules Diesel)

Les filtres céramiques sont des dispositifs prometteurs pour diminuer les émissions de particules carbonées par les moteurs Diesel et respecter les futures normes environnementales. L'analyse et la modélisation des phénomènes physiques intervenant dans leur fonctionnement est indispensable pour améliorer leurs performances et leur durabilité. Dans une première partie, l'écoulement du gaz, le transport et la capture des particules au sein d'un filtre présentant une structure poreuse multi-échelle sont étudiés. Une approche de type "milieu poreux effectif" est adoptée pour simuler l'étape de filtration à l'échelle du filtre. Le modèle s'appuie sur des équations 1D, développées pour un élément unitaire de filtre, préservant le caractère inertiel de l'écoulement. Dans une deuxième partie, le comportement thermique du filtre est étudié. Les transferts de chaleur convectif et diffusif sont pris en compte dans un modèle macroscopique à une température obtenu par la méthode de prise de moyenne volumique. Un terme source de combustion permet de considérer la réaction d'oxydation des particules collectées, intervenant dans l'étape de régénération du filtre.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

Characterisation of the physico-mechanical parameters of MSW

International audienceFollowing the basics of soil mechanics, the physico-mechanical behaviour of municipal solid waste (MSW) can be defined through constitutive relationships which are expressed with respect to three physical parameters: the dry density, the porosity and the gravimetric liquid content. In order to take into account the complexity of MSW (grain size distribution and heterogeneity larger than for conventional soils), a special oedometer was designed to carry out laboratory experiments. This apparatus allowed a coupled measurement of physical parameters for MSW settlement under stress. The studied material was a typical sample of fresh MSW from a French landfill. The relevant physical parameters were measured using a gas pycnometer. Moreover, the compressibility of MSW was studied with respect to the initial gravimetric liquid content. Proposed methods to assess the set of three physical parameters allow a relevant understanding of the physico-mechanical behaviour of MSW under compression, specifically, the evolution of the limit liquid content. The present method can be extended to any type of MSW

Effect of X-raymicro-CT Resolution on the Computation of Permeability and Dispersion Coefficient for Granular Soils

International audienceX-ray micro-computed tomography (μCT) can produce realistic 3D-images of the pore structure of a material. Extracting its geometry enables the computation of effective properties of the material—such as the permeability (k) and the hydrodynamic dispersion coefficient ( Dh )—, through the solutions of the Stokes equation (SE) and Advection-Diffusion equation (ADE), respectively. In this study, the effect of the image resolution on theseproperties is discussed. For such purpose, four different resolutions are evaluated for both a real sample of Fontainebleau sand and a numerically generated sample created by degrading the Fontainebleau image with highest resolution. The SE was computed using the commercial software GeoDict. To solve the ADE, a Finite Volume software was developed which includes a high order total variation diminishing scheme for advection. The analy-sis of dispersion was based on numerical breakthrough curves. Our model was tested in a large range of Peclet numbers (Pe) and travel distances, accurately describing the transition between diffusion and advection dominated regimes of dispersion. The Dh exhibits a linear increase with travel distance for Pe > 10. This classical effect increases with increasing Pe.The percentage change on k and Dh increases with decreasing resolution in agreement with the corresponding behavior of porosity, specific surface and pore size distributions. The images directly scaled with the μ CT showed more discrepancy than the numerically scaled images. The criteria to estimate the quality of permeability from the pore size distribution proposed on our previous study remains valid. The Dh is less sensitive to resolution than k

Characterizing the Gas Permeability of Natural and Synthetic Materials

International audienceThe objective of this article is to propose an experimental method to compare the gas permeability of all the different materials used as gas barrier, such as compacted clay liners or geomembranes. This method is based on the falling pressure experiment, allowing the determination of a single coefficient whatever the material tested. This coefficient is the time constant τ, which is obtained by analytical solutions of the simplified equations describing the transport of gas through the material. The domain of validity is specified for porous media, thanks to a numerical solution of the complete equations system. Two applications are presented, showing the applicability of the method on compacted clay liners and on high density polyethylene geomembranes

Liquid and gas permeabilities of unsaturated municipal solid waste under compression

International audienceA novel set of experimental apparatus was designed and constructed to study the changes inthe fluid-flow properties of municipal solid waste (MSW) related to the physical evolution ofits structure under compression. The vertical liquid and gas permeabilities of MSW sampleswere measured in a laboratory-constructed cell termed an oedopermeameter. Another originaldevice, a gas pycnometer, was employed to assess the volumetric gas content of the porousmedium. Finally, the horizontal gas permeability of the compressed MSW sample wasmeasured using another laboratory-constructed cell called a transmissivimeter. The resultsmade it possible to characterise the intrinsic gas permeability as a function of porosity.Additionally, gas permeability measurements of samples with different liquid contents allowedthe derivation of gas permeability correlations as functions of the physical parameters of themedium. A unique relationship was found between the gas permeability and the volumetricgas content

Apprentissage par projet multidisciplinaire pour les élèves-ingénieurs en géotechnique et génie civil, afin d’acquérir des notions fondamentales

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