Stability analysis for combustion fronts traveling in hydraulically resistant porous media

We study front solutions of a system that models combustion in highly hydraulically resistant porous media. The spectral stability of the fronts is tackled by a combination of energy estimates and numerical Evans function computations. Our results suggest that there is a parameter regime for which there are no unstable eigenvalues. We use recent works about partially parabolic systems to prove that in the absence of unstable eigenvalues the fronts are convectively stable.Comment: 21 pages, 4 figure

Lattice Boltzmann Simulation of Multicomponent Porous Media Flows With Chemical Reaction

Flows with chemical reactions in porous media are fundamental phenomena encountered in many natural, industrial, and scientific areas. For such flows, most existing studies use continuum assumptions and focus on volume-averaged properties on macroscopic scales. Considering the complex porous structures and fluid–solid interactions in realistic situations, this study develops a sophisticated lattice Boltzmann (LB) model for simulating reactive flows in porous media on the pore scale. In the present model, separate LB equations are built for multicomponent flows and chemical species evolutions, source terms are derived for heat and mass transfer, boundary schemes are formulated for surface reaction, and correction terms are introduced for temperature-dependent density. Thus, the present LB model offers a capability to capture pore-scale information of compressible/incompressible fluid motions, homogeneous reaction between miscible fluids, and heterogeneous reaction at the fluid–solid interface in porous media. Different scenarios of density fingering with homogeneous reaction are investigated, with effects of viscosity contrast being clarified. Furthermore, by introducing thermal flows, the solid coke combustion is modeled in porous media. During coke combustion, fluid viscosity is affected by heat and mass transfer, which results in unstable combustion fronts

Small-scale forward smouldering experiments for remediation of coal tar in inert media

This paper presents a series of experiments conducted to assess the potential of smouldering combustion as a novel technology for remediation of contaminated land by water-immiscible organic compounds. The results from a detailed study of the conditions under which a smouldering reaction propagates in sand embedded with coal tar are presented. The objective of the study is to provide further understanding of the governing mechanisms of smouldering combustion of liquids in porous media. A small-scale apparatus consisting of a 100 mm in diameter quartz cylinder arranged in an upward configuration was used for the experiments. Thermocouple measurements and visible digital imaging served to track and characterize the ignition and propagation of the smouldering reaction. These two diagnostics are combined here to provide valuable information on the development of the reaction front. Post-treatment analyses of the sand were used to assess the amount of coal tar remaining in the soil. Experiments explored a range of inlet airflows and fuel concentrations. The smouldering ignition of coal tar was achieved for all the conditions presented here and self-sustained propagation was established after the igniter was turned off. It was found that the combustion is oxygen limited and peak temperatures in the range 800-1080 °C were observed. The peak temperature increased with the airflow at the lower range of flows but decreased with airflow at the higher range of flows. Higher airflows were found to produce faster propagation. Higher fuel concentrations were found to produce higher peak temperatures and slower propagation. The measured mass removal of coal tar was above 99% for sand obtained from the core and 98% for sand in the periphery of the apparatus

Pore-scale study on porous media flows with chemical reaction using lattice Boltzmann method

Porous media flows with chemical reaction are common in nature and widely exist in many scientific and industrial applications. However, due to the complexity of coupled mechanisms, numerical modelling and comprehensive understanding of such flows face significant challenges. Therefore, this thesis develops novel lattice Boltzmann (LB) models to undertake pore-scale simulations of porous media flows with chemical reaction. These models, with new reaction source terms and boundary schemes, can describe both homogeneous reaction between two fluids and heterogeneous reaction (dissolution or combustion) at the fluid-solid interface. Unlike previous studies, current models recast heat and mass transfer equations to correctly consider the thermal expansion effects and the conjugate heat transfer and species conservation conditions. Separate LB equations are also developed to include different species properties. Density fingering with homogeneous reaction is studied at the pore scale. By changing species contributions to density, diffusion coefficients, initial concentrations, and medium heterogeneities, results obtained demonstrate that reaction can enhance, suppress, or trigger fingering. Then, pore-scale simulations of viscous fingering with dissolution reaction are performed. Effects of fluid diffusion, chemical dissolution, and viscosity contrast are extensively assessed. Results illustrate four fingering regimes as stable, unstable, reactive stable, and reactive unstable. Finally, pore-scale coke combustion in porous media is studied. General combustion dynamics are correctly produced, verifying the superior performance of the present LB model over previous ones. A parametric study demonstrates that the inlet air temperature and the driving force are influential factors and should be constrained within certain ranges for stable combustion fronts. These pore-scale findings provide valuable insights, like temperature fluctuations at the fluid-solid interface, porous structure evolutions, exact reaction and diffusion rates, and medium heterogeneity effects, which are more precise and explicit than macroscopic results. Furthermore, detailed fingering and combustion dynamics under diverse conditions are helpful in scientific and industrial fields