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

Random packings of spiky particles : Geometry and transport properties

Spiky particles are constructed by superposing spheres and prolate ellipsoids. The resulting nonconvex star particles are randomly packed by a sequential deposition algorithm. The geometry, the conductivity, and the permeability of the resulting packings are systematically studied, in relation with the individual grain characteristics. Overall correlations are proposed to approximate these properties as functions of the grain equivalent size and sphericity index

Barometric pumping of a fractured porous medium

International audienceBarometric pumping plays a crucial role in the release of trace gases from fractured porous media to the atmosphere, and it requires a rigorous and complete modeling in order to go beyond the approximate schemes available in the literature. Therefore, a coupled set of convection and convection-diffusion equations for a slightly compressible fluid in unsteady conditions should be solved. The numerical methodology is presented, and it is applied to conditions close to the ones of the Roselend Natural Laboratory (France). The precision of the code is assessed and the mechanism of barometric pumping is explained. The usual schematization by simple vertical fractures is shown to be only qualitative. Finally, barometric pumping is shown to be efficient in a narrow range of parameter values; its efficiency is a decreasing function of the matrix porosity and of the fracture density

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

Acoustic properties of saturated porous media

This work addresses the propagation of acoustic waves in porous media. The theoretical developments or earlier works are generalized to establish dynamic poro-elastic equations for a compressible fluid in a pore space which may consist in multiple independent pore components, closed or percolating. These equations involve several effective coefficients, to be determined by solving closure problems on the microscale. Systematic applications are presented, for various kinds of model reconstructed media, and for real media imaged by microtomography

Transport in rough self-affine fractures

Transport properties of three-dimensional self-affine rough fractures are studied by means of an effective-medium analysis and numerical simulations using the Lattice-Boltzmann method. The numerical results show that the effective-medium approximation predicts the right scaling behavior of the permeability and of the velocity fluctuations, in terms of the aperture of the fracture, the roughness exponent and the characteristic length of the fracture surfaces, in the limit of small separation between surfaces. The permeability of the fractures is also investigated as a function of the normal and lateral relative displacements between surfaces, and is shown that it can be bounded by the permeability of two-dimensional fractures. The development of channel-like structures in the velocity field is also numerically investigated for different relative displacements between surfaces. Finally, the dispersion of tracer particles in the velocity field of the fractures is investigated by analytic and numerical methods. The asymptotic dominant role of the geometric dispersion, due to velocity fluctuations and their spatial correlations, is shown in the limit of very small separation between fracture surfaces.Comment: submitted to PR

Critical point network for drainage between rough surfaces

In this paper, we present a network method for computing two-phase flows between two rough surfaces with significant contact areas. Low-capillary number drainage is investigated here since one-phase flows have been previously investigated in other contributions. An invasion percolation algorithm is presented for modeling slow displacement of a wetting fluid by a non wetting one between two rough surfaces. Short-correlated Gaussian process is used to model random rough surfaces.The algorithm is based on a network description of the fracture aperture field. The network is constructed from the identification of critical points (saddles and maxima) of the aperture field. The invasion potential is determined from examining drainage process in a flat mini-channel. A direct comparison between numerical prediction and experimental visualizations on an identical geometry has been performed for one realization of an artificial fracture with a moderate fractional contact area of about 0.3. A good agreement is found between predictions and observations