Mixing at high Schmidt number in a complex porous structure

• Mixing at a high Schmidt number (Sc = 2400) is analyzed in a turbulent flow through a foam structure.
• Vortex shedding changes the mean concentration plume profile from a single to a double peak.
• Strong mixing is found in the shear layers of the near wake regions of the ligaments.
• We show that the simple gradient diffusion hypothesis (SGDH) is not appropriate for this kind of flow.

Highly porous structures (porosity ≥75%≥75%), as metal foams or designed foam-like structures, are often used in industry to augment heat and mass transfer. In the present study, we focus on the mixing in highly porous structures in continuous millireactor characterized with a relatively low Reynolds number (ReD=240ReD=240 – based on the ligament diameter and bulk velocity) and a high Schmidt number fluid (Sc=2400). We apply a combined numerical and experimental approach. The numerical simulations employ a Large Eddy Simulation (LES) method with dynamic Lagrangian approach for subgrid-scale turbulent stress and turbulent mass flux. The results of the numerical simulation are compared with our own combined Particle Imaging Velocimetry (PIV) and Laser Induced Fluorescence (LIF) measurements. The application of combined PIV/LIF makes it possible to simultaneously measure velocity components, turbulent stresses, concentration and concentration fluxes. The mechanism of the mixing is analyzed in detail with specific focus on validity of a simple gradient diffusion hypothesis (SGDH) in modeling of the turbulent mass transfer in complex porous structure.