Visualization of Micro- and Macromixing in Liquid Mixtures of Reacting Components (Keynote Paper)

Abstract

Micromixing is visualized inside a stirred vessel by using two different optical measurement techniques, the optical tomography and the Planar Laser Induced Fluorescence technique (PLIF). In order to distinguish between macro- and micromixing, a mixture of two dyes is injected into the mixing vessel. One of the dyes is an inert dye whereas the second dye is undergoing a fast chemical reaction with the vessel content. The distribution of the inert dye serves as a tracer for the macromixing but does not predicate the mixing quality on the nano scale. The chemical reaction requires mixing on the molecular scale. Therefore the reacting dye, which is changing its emission characteristics during the reaction, indirectly visualizes the micromixing. The tomographical dual wavelength photometry is used to measure the three-dimensional, transient concentration fields in the whole vessel at the same time. Measurements with the Planar Laser Induced Fluorescence technique are performed in an arbitrary plane of the vessel. This restriction on a two-dimensional concentration field is recompensed with a much higher spatial resolution which allows to visualize small scale structures in the order of mm. For both techniques low Reynolds number measurements are performed in a mixing vessel equipped with a Rushton turbine. Results are presented as two- or three-dimensional concentration fields. Areas of micromixing are detected by calculating the local degree of deviation from the concentration fields. They are depending on the injection position of the dye and are mainly found in the boundary layer of the lamellas.