SIMPLIFIED FRAMEWORK FOR DESCRIPTION OF MIXING WITH CHEMICAL REACTIONS(Ⅰ)PHYSICAL PICTURE OF MICRO-AND MACROMIXING

1 INTRODUCTION When a fluid is injected into a turbulence field to mix with another fluid,twohomogenization processes take place simultaneously:micromixing(on the molecularscale)and macromixing(on the mixer scale).For micromixing,three successive stages arerecognized at high Schmidt number(1)turbulent dispersion,in which the original ma-terial lumps are dispersed by turbulence into discrete elements with smallest eddy sizeλ_k;(2)viscous deformation,in which the size of the elements is further

High speed microphotographic studies showed that fluid elements injected from a point source tended to exhibit slice-like and strip-like configurations rather than lamellar structure during the turbulent mixing of fluids with Sc〉〉1 . Based on this phenomenon a new micromixing model was developed. The model stated that micromixing was contributed by shrinkage deformation and molecular diffusion on a slab element surrounded by an infinite ambient fluid. Balance between the deformation and the diffusion resulted in an exponential increase in volume for the element. After a certain period of time tm , the volumetric increase of the feed material would be confined by the development and extension of the turbulent spreading zone. Therefore, three successive downstream regions, with different controlling steps and segregation states, could be recognized. A simplified picture for the point source mixing was then drawn, which might serve as a basis for describing chemical reactions during turbulent mixing.