Analysis of Macro- and Micromixing in Laminar Stirred Mixing Vessels Using Laser Optical and Numerical Methods

Abstract

During the mixing process the transport of species takes place on different length scales. The convection of several species is identified on large scales, whereas diffusion progresses on molecular scales. For a laminar mixing process these two distinguishable mechanisms of transport are analyzed experimentally and theoretically for two- and three-dimensional mixing systems. The local concentration fields of an injected mixture of two fluorescent dyes are measured by means of the 4D Two-Colour Laser Induced Fluorescence (LIF) in the volume of a mixing vessel. An inert dye is used for the identification of the convective transport. A second dye is reacting with an individual component in the mixing vessel in a fast chemical reaction. The formation of the reaction product indirectly indicates the molecular transport. Both measured concentration fields are used for the calculation of a local mixing quality. Particle Image Velocimetry (PIV) is used for measuring local velocity vectors. The source term of the local energy dissipation is calculated and visualized by means of the velocity gradients. The length scale of the forming lamellas depends on varying local energy dissipation. As a result of increasing energy dissipation local mixing quality enhances. The macro- and micromixing is analyzed depending on Reynolds number and position of injection. The flow and concentration fields are calculated by means of numerical methods. Therefore, a commercial CFD code is used. The geometries are equivalent to those of the experimental investigation. The experimental and numerical results are in good agreement.