Flow instabilities, mixing, and combustion in porous solid-fluid domains

Many natural and industrial processes involve mixed porous-fluid domains where multiple physics interact over disparate lengths and time scales, such as the combustion of multi-species biofuels. Although many modeling studies so far, have concentrated on detailed physics within the single fluid or porous phase, there rarely are reports of a comprehensive model considering both phases. In this work, we numerically study different scenarios involving porous-fluid regions using a single-domain approach. The model considers the compressible Navier-Stokes/Darcy-Forchheimer equation and two separate energy conservation equations for the heat transfer in the solid matrix and the interstitial fluid in the porous region, along with the multi-species transport and detailed chemical kinetics. We validate the method against a set of experimental and direct numerical simulation results including the thermal decomposition of biomass particles. We then investigate the thermally- and buoyancy-driven fluid instabilities (Rayleigh-Bénard like) through three-dimensional large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) modeling. We focus on the interfacial porosity variation and configurational effects in turbulence mixing and its implications for ignition and fire spread.