micromixing

Micromixing is the process by which turbulent flows bring reacting species into intimate contact at the smallest scales, enabling diffusion to complete mixing within small volumes. In a turbulent field, large eddies accomplish macromixing, but further homogenization occurs inside thin diffusion layers and the smallest eddies, a stage known as micromixing. The rate and efficiency of micromixing can control the overall rate of chemical reactions in fast or highly exothermic systems, where reaction times are comparable to or shorter than mixing times. If micromixing is slower than reaction rates, products may form inhomogeneously, with hot spots or unreacted pockets.

Modeling micromixing in CFD and combustion simulations uses several approaches. The eddy dissipation model (EDM) assumes that reactions occur within finely stirred eddies and ties reaction rates to the turbulent dissipation rate and scalar differences. The interaction by exchange with the mean (IEM) model represents mixing as stochastic exchange between a fluid parcel and the mean composition over a prescribed micromixing timescale. Probability-density-function (PDF) methods implement subgrid micromixing closures to evolve the full scalar distribution, and are often coupled with Lagrangian or Eulerian formulations. More detailed particle-based or joint-pdf approaches provide a finer representation of micromixing, at higher computational cost.

Applications span combustion, gasification, spray drying, and polymerization, where accurate micromixing treatment affects ignition delay, flame structure, product selectivity, and emissions. The concept originated in turbulence-chemistry research in the 1960s and 1970s and remains a central element of subgrid modelling for reacting flows. Experimental measurements and high-fidelity simulations use laser-based diagnostics and DNS/LES with micromixing closures to quantify mixing efficiency and timescales.