Dissolution of a porous medium creates, under certain conditions, some highly conductive channels called wormholes. The mechanism of propagation is an unstable phenomenon depending on the microscopic properties at the pore scale and is controlled by the injection rate. The aim of this work is to test the ability of a Darcy-scale model to describe the different dissolution regimes and to characterize the influence of the flow parameters on the wormhole development. The numerical approach is validated by model experiments reflecting dissolution processes occurring during acid injection in limestone. Flow and transport macroscopic equations are written under the assumption of local mass non-equilibrium. The coupled system of equations is solved numerically in two dimensions using a finite volume method. Results are discussed in terms of wormhole propagation rate and pore volume injected.

This work was carried out in order to quantify the impact of the pyrolysis heating rate both on the properties of the residual charcoal and on the behaviour during gasification by H 2 O of the charcoal. The experiments were conducted on 10 mm diameter beech wood spheres, pyrolysed at atmospheric pressure under heating rates covering the range from very slow, 2.6 K min K1 , to very rapid, over 900 K min K1 , i.e. the highest value that can be reached. When charcoal is submitted to gasification at 20% H 2 O i nN 2 at 1200 K, the ratio of the times for complete conversion reach 2.6. Such a difference is considerable as far as an industrial application is concerned. The initial properties of the charcoal such as apparent density, porosity, and pore surface area obtained by N 2 or Ar adsorption were measured in order to explain the differences in gasification kinetics within the charcoal. The charcoal particles exhibit densities as different as 219-511 kg m K3 and porosities between 87 and 70% for charcoal prepared at 900 and 2.6 K min K1 respectively. The specific surface area is higher than 600 m 2 g K1 for three charcoals. Influence of ash content of the initial charcoals, at 1.6-2.7%, is also regarded with particular attention to explain the observed differences in gasification kinetics.

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The present work deals with a study coupling experiments and modeling of charcoal gasification by steam at large particle scale. A reliable set of experiments was first established using a specially developed "macro-TG" apparatus where a particle was suspended and continuously weighed during its gasification. The main control parameters of a fixed-bed process were modified separately: steam gasification of beech charcoal spheres of different diameters (10 to 30 mm) was studied at different temperatures (830 to 1030 • C), different steam partial pressures (0.1 to 0.4 atm H 2 O), and different gas velocities around the particle (0.09 to 0.30 m/s). Simulations with the particle model were performed for each case. Confrontations with experimental data indicate that the model predictions are both qualitatively and quantitatively satisfactory, with an accuracy of 7%, until 60% of conversion, despite the fact that the phenomena of reactive surface evolution and particle fracturing are not well understood. Anisotropy and peripheral fragmentation make the end of the process difficult to simulate. Finally, an analysis of the thermochemical situation is proposed: it is demonstrated that the usual homogeneous or shrinking core particle models are not satisfying and that only the assumption of thermal equilibrium between the particle and the surrounding gas is valid for a model at bed scale.

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[1] Alteration and dissolution resulting from reactive fluid flows in vertical fracture are investigated from numerical and laboratory experiments. Due to fluid density contrast, buoyancy effects are observed leading to significant changes in fracture geometry. Buoyant and forced convection forces act here in the same direction. The experiments were carried out at two different flow rates. When buoyancy forces are preponderant (low injection flow rate), the dissolution rate increases with the vertical distance. By contrast, for convectiondominated transport (high injection flow rate), a uniform dissolution is observed. Using numerical simulations, four dissolution regimes were identified. The fracture patterns observed strongly depend on the characteristic dimensionless numbers of the process, respectively, the Richardson, Damköhler, and Péclet numbers. The good agreement between numerical simulations and experimental results in terms of fracture patterns highlights the capability of the numerical model to describe the complex coupling between flow dynamics, buoyancy, and chemical reaction. Finally, a 3-D behavior diagram is constructed to illustrate these interactions and as a means of relating the appropriate dimensionless parameters to the morphological changes observed.Citation: Olte´an, C., F. Golfier, and M. A. Bue`s (2013), Numerical and experimental investigation of buoyancy-driven dissolution in vertical fracture,