Smoldering Combustion in Oil Shales: Influence of Calcination and Pyrolytic Reactions

A three-dimensional numerical tool for the microscale simulation of smoldering in fixed beds of solid fuels is presented. The description is based on the local equations and accounts the local couplings of the transport and reaction mechanisms. The chemical model includes devolatilization and cracking of the kerogen, calcination of the carbonates contained in a mineral matrix and oxidation of the carbon char left by the pyrolysis. An extensive survey of the functioning regimes exhibits features that have to be taken into account in the operation of a reactor and in its macroscopic modeling. Three dimensionless numbers are shown to control the phenomenology, which embody the effects of the constituent properties and of the operating conditions. One of them, PeF,s, provides an a priori criterion for the validity of a local equilibrium hypothesis and for the applicability of standard homogenized formulations. The numerical observations comply when PeF,s is small with the expectations from a simple homogenized description, including quantitative predictions of the mean temperature profile, of the consumption of the various reactants and of the relative positions of the reaction fronts. Conversely, local equilibrium is not satisfied when PeF,s is large and these approaches fail in several respects. The simple upscaled transport equations are unable to predict the evolution of some of the locally average state variable. Furthermore, strong local deviations of the state variables from their local averages, combined with the nonlinearity of the kinetic laws, cause the overall reaction rates to differ from those deduced from the mean values. Nevertheless, a successful heuristic model for the spread of the hot and potentially reactive region can be stated, which provides an avenue for further studies

Dynamic simulation of the formation of granular media and study of their properties

A numerical model is presented which describes the evolution of a system containing a large number of deformable spherical grains based on Newton's second law. Starting from an initial state with fixed positions, velocities and grain characteristics, the system evolution is simulated by successive steps. The acceleration of each grain results from the application of an external force and from interactions with other particles. These contact forces are evaluated as functions of the grain deformations during the collisions considered as elastic. The grain bed can be deposited between vertical walls as well as with periodical conditions in the lateral directions. The properties of these packings submitted to mechanical stresses are characterized by using numerical codes which operate on unstructured tetrahedral grids on the scale of the individual grains

Propriétés acoustiques des milieux poreux secs et saturés

Les propriétés acoustiques de milieux poreux sont étudiées dans le cadre général de la théorie de l'homogénéisation, en supposant que l'échelle caractéristique des pores est petite devant la longueur d'onde, en appliquant les développements successifs du formalisme de Botin et Auriault (1993). Pour les milieux secs, on détermine la célérité d'une onde plane (de compression ou de cisaillement), et éventuellement la correction de polarisation, la dispersion de célérité ainsi que l'atténuation due à la diffraction de Rayleigh. Différents types de milieux modèles ainsi que des matériaux réels imagés par microtomographie X sont étudiés. Dans les milieux poreux saturés, le comportement acoustique est décrit par une équation de type élastique dans la phase solide et par les équations de Navier-Stokes dans le fluide interstitiel. La perméabilité dynamique et les célérités des ondes de compression et de cisaillement sont déterminées pour les mêmes milieux que précédemment

Thermal degradation of solid porous materials exposed to fire

Thermal degradation processes in solid materials are crucial in provoked or hazardous fire events, since pyrolysis is the main source of gaseous combustible matter which feeds the fire. The simulation of fire events on the global scale requires a good description of these processes as a function of the ambient parameters such as temperature and oxygen concentration. But characterizations of thermal decomposition by small or large scale measurements often yield different responses, due to the coupling in the latter case between the chemical reactions and the various heat and mass transport mechanisms. The purpose of this work is to make the connection between these two points of view, and to predict the macroscopic behavior as a function of the constituent properties, of the micro-structural characteristics and of the ambient conditions. A chemical model, including a reaction scheme and associated thermo-kinetic parameters, can be obtained by thermogravimetric measurements on samples small enough to prevent any limiting influence of heat and mass transports. Numerical simulations can then be conducted on a larger scale, to determine the material response in a given geometrical configuration, undergoing any scenario of ambient conditions. These simulations are performed on the Darcy scale in prescribed scenarios which correspond to standardized physical tests. This will allow establishing a typology of behaviors, identifying the key processes and their governing parameters, and validating the numerical predictions. In a later stage, microscopic investigations could be conducted for a better characterization of some processes and for the determination of relevant effective coefficients. Ultimately, this description of the material degradation could be directly coupled with a fire simulation tool on the global scale, which would provide the time dependent ambient conditions, and would account for the influence of the emitted species on the fire development

Percolation, Faults and Fractures in Rock

International audienc

Percolation, and Faults and Fractures in Rock

International audienc

Random fracture networks: percolation, geometry and flow

International audienceThe basic properties of fracture networks are derived numerically and rationalized. Simple formulas are provided for an easy estimation of orders of magnitude, based on the excluded volume, on an associated dimensionless density and on shape factors. A general correlation is proposed for the percolation threshold in various types of networks, the blocks which are cut in the solid matrix by the network are characterized and an empirical formula is proposed for the permeability

Transmissivity and conductivity of single fractures

International audienceA fracture can be seen as a void space between two rough surfaces in partial contact. Transmissivity and conductivity can be determined numerically by solving the Stokes and Laplace equations between these two surfaces. These problems were first solved and published by the same authors between 1995 and 2001. Updated, more complete and precise results are presented here.Each surface of a fracture can be schematized as a random surface oscillating around an average plane, characterized by the probability density and autocorrelation function C(u) of the heights; their standard deviation is the roughness sigma. The two surfaces are separated by a mean distance b_m and their heights are correlated with an intercorrelation coefficient theta. The two major classes for C(u), namely the Gaussian and the self-affine autocorrelations with Hurst exponent H, are both characterized by a length scale l_c, which is a typical scale for the surface features in the Gaussian case and a cut-off length in the self-affine case. Gaussian surfaces are statistically homogeneous while the mean properties in the self-affine case are size-dependent.Systematic calculations were performed for these two classes, with recent emphasis put on the Gaussian fractures. The results are modeled as functions of b_m/sigma, l_c/sigma and theta. Cubic law applies for large b_m/sigma in terms of the aperture reduced by the hydraulic thickness of the surface rugosity. Another cubic law applies in the opposite limit of tight fractures with an offset depending on theta and a prefactor which depends on theta and l_c. A transition takes place between these two regimes. It is also shown that the Reynolds approximation may overestimate the true transmissivity by almost an order of magnitude. Similar calculations were performed for conductivity. The whole work is summarized by a series of master curves and models which can be used to estimate the properties of real fractures

The percolation threshold of fracture networks

International audienceThe percolation threshold of fracture network

Modélisation à l'échelle microscopique de transports avec réaction en milieu poreux (combustion en lit fixe)

La combustion en milieu poreux est traitée par le biais de simulations numériques directes et détaillées, à la microéchelle, dans une extension du travail de Debenest (2003, 2005) qui porte principalement sur un enrichissement du modèle chimique. On considère plus particulièrement la combustion en lit fixe de particules solides, avec comme première application le brûlage de schistes bitumineux. Les processus de transport (convection, diffusion, conduction) et les réactions chimiques sont explicitement décrits à l'échelle des pores, ce qui permet d'exhiber leurs couplages et de révéler les phénomènes locaux qui déterminent les comportements globaux. Les simulations sont conduites principalement dans deux configurations bidimensionnelles, milieu stratifié ou réseau de cylindres, en examinant les effets des réactions pyrolytiques (cracking du kérogène et calcination des carbonates), et avec un schéma d'oxydoréduction qui fait intervenir jusqu'à quatre réactions. Une typologie phénoménologique est établie, incluant notamment l'existence de deux régimes principaux, avec ou sans flamme dans les pores. Des plages de fonctionnement sont identifiées, suivant les paramètres opératoires. On peut en rationaliser les tendances à l'aide de considérations théoriques, et montrer qu'une description macroscopique peut nécessiter des formulations différentes, selon les situations.Combustion in porous media is addressed by means of direct, detailed numerical simulations, on the microscale, in an extension of the work of Debenest (2003) where the main improvements are related to the chemical model. More specifically, fixed bed combustion of solid particles is considered, with application to the burning of oil shales. The transport processes (convection, diffusion, conduction) and the chemical reactions are explicitely described on the pore scale, which allows to account for their local couplings and to identify the small-scale phenomena which control the global behaviors. The simulations are conducted mainly in two two-dimensional configurations, a stratified medium and a network of cylinders. The effect of pyrolytic reactions (kerogen cracking and calcination of the carbonates) are examined, as well as the oxydative processes, with a chemical scheme involving four species and up to four reactions. A phenomenological typology is established, which features for instance two main regimes, with or without a flamme in the pore space. Ranges of functioning modes are identified, according to the operating parameters. Their trends can be rationalized by theoretical considerations, and it is shown that different situations may require different formulations in a macroscopic descriptionPOITIERS-BU Sciences (861942102) / SudocSudocFranceF