(248e) Laser Optical Measurements and Numerical Investigations of Macro- and Micromixing in Stirred Vessels

The research project contains the analysis of transport phenomena during the mixing process of two multicomponent liquids. This mixing process is of great technical interest but still the steps of macro- and micromixing are not fully understood. 3D-Two-Color laser induced fluorescence (LIF) is used for visualizing these transport processes on different scales simultaneously. The results are used for validating numerical calculations of a molecular mixing rate.

The experiments are done in a stirred vessel equipped with a multi-stage turbine. A system of two fluorescent dyes will be injected in the mixing vessel. One dye is inert, which visualizes the convective transport of large fluid elements. During their transport through the vessel these elements are stretched and folded, so that their length scale of segregation decreases. The second dye reacts with calcium ions. The fluorescence signal of the product of this chemical reaction is detected and informs indirectly about the molecular progress of the mixing process. The local concentrations of the two dyes are calculated from the measured fluorescence intensities. The process of the micromixing in the molecular scale occurs due to diffusion and decreases the intensity of segregation. A molecular mixing rate, the degree of deviation, is calculated with the local information of both concentration fields.

The mass transfer on microscopic scale varies because of the locally diverse dissipation of energy. Particle Image Velocimetry (PIV) is used to measure the local velocities of the flow field from which the local source terms of the energy dissipation are calculated. The correlation between micromixing and macroscopic flow field is examined for this mixing system with adjustable energy dissipations.

The concentration and velocity fields are measured sequentially by laser beams. These are expanded into a thin light sheet illuminating an arbitrary plane in the mixing vessel and exciting the fluorescent dye or the seeding particles in this area. CCD-cameras positioned vertical to the measurement plane detect the emitted light. The fluorescent light is passing optical filters which are suitable to separate the fluorescent light of the two dyes or of the seeding particles. Both experiments consist of optical measurement techniques with high temporal and spatial resolution.

The geometry of the experimental device is used for construction of the grid for the numerical calculations. All boundary conditions e.g. the Newtonian Fluid, Reynolds numbers are equivalent to the experimental investigations. The mixture of the two dyes is simulated by a multicomponent mixture. The numerical calculations are solved with the CFD program from ANSYS-CFX.

The results from the numerical calculations of the stationary flow and dissipation field are in good agreement with the experimental data. The transient calculations of the injected dyes correspond to the experimental measured 3D concentration fields. The local molecular mixing rate is conform to the measured degree of deviation. These investigations can be used for a theoretical prediction of mixing rates in several applications validated by experimental data in equivalent systems.