A single-domain approach for modeling flow in and around porous media applied to buoyant reacting plume formation and ignition

Abstract

Many processes involve mixed porous-solid fluid domains where fluid flow, heat transfer, and chemical reactions interact over disparate length scales, such as the combustion of multi-species solid fuels. Although many studies have concentrated on detailed physics within the fluid or porous phase, few consider both phases, in part due to the challenge in determining suitable boundary conditions between the regions, particularly in turbulent flows where eddies might penetrate the pores. Here, we apply a single-domain approach that eliminates the need for boundary conditions at the interface, and simulate scenarios involving porous solids and a surrounding fluid. Similar to large eddy simulation, the method averages properties over a small spatial volume -- but over the entire domain. We focus on ignition and related interfacial phenomena. After verifying and validating the model, we examine the emission of buoyant reacting plumes from the surface of a heated solid and the near-field flow dynamics. The results indicate flow instabilities similar to Rayleigh--Taylor and Kelvin--Helmholtz phenomena. A combination of viscous and baroclinic torques triggers vorticity generation near the interface and its growth in the surrounding fluid region. Furthermore, we explore the effect of interface morphology, finding that geometrical characteristics such as asymmetry or gap size can alter ignition time and location, or even suppress it. Asymmetry-induced oscillations initially cause negative heat fluxes, which prevent the temperature from reaching the critical level necessary to trigger ignition. These behaviors could significantly influence the mixing of oxidizer and fuel, ignition processes, and fire propagation.