A mixing length model describes the recently observed (Bilger et al., 1991) effect of chemical reactions on turbulent diffusivities. From this viewpoint, the coupling between microscale reactions and macroscale mixing occurs because the reaction rate is a quasitransferable property which causes more reaction in fluctuating fluid lumps leaving regions of high rate than low rate. The net effect, which raises or lowers reactant diffusivities, depends on a characteristic macromixing time and the rate at which the mean reaction rate changes with mean concentration:
The model shows correct qualitative behavior and appears to agree quantitatively with much of the existing data. It adds an apparent flux, ετM(drA/dy), to the convective diffusion equations which can enhance or hinder the normal turbulent fluxes, but is likely to have little effect on the mean concentration field.