Measurements of macro- and microscale mixing by Two-Color Laser Induced Fluorescence

Two-Color Laser Induced Fluorescence (LIF) Technique enables the measurement of the progress in mixing on macroand microscale simultaneously. This is done by injecting a mixture of an inert and a reacting fluorescent dye into the vessel. The inert dye serves as a tracer for the macromixing but does not predicate the mixing quality on the nano scale. Since the chemical reaction requires mixing on the molecular scale, the reacting dye visualizes the micromixing indirectly. Low Reynolds number measurements are performed in a mixing vessel equipped with a Rushton turbine. Repetitive layers form as the impeller blades create new folds each time they pass by the viewing plane. These lamellar structures can be clearly resolved. Areas of micromixing are detected by calculating the local degree of deviation from the measured concentration fields. They are mainly found in the boundary layer of the lamellas. By choosing a suitable border value for the degree of deviation the lamellas can be classified into a center region which is not micromixed yet, and the boundary layer with a high degree of micromixing. The center and border region is separated by means of mathematical filters and their distance is measured. This is the macroscopic length scale of segregation, which is not influenced by micromixing.