Using CFD to understand chaotic mixing in laminar stirred tanks



High-accuracy CFD results for laminar flow in a stirred tank agitated by three Rushton turbines are used as a starting point for an in-depth analysis of mixing. Asymptotic mixing performance is investigated as a function of the Reynolds number by means of Poincaré sections, which reveal large segregated regions with sizes and shapes that vary greatly over a relatively small range of Reynolds numbers. The spatial distribution of mixing intensities is also examined by computing the stretching field, which can be used to optimally choose injection locations for dispersing additives in the tank. When mixing dynamics is examined by particle tracking, the structures observed at short times expose the mechanism of laminar mixing by Rushton turbines. The computed mixing structures are compared with experimental images of dye concentration using planar laser-induced fluorescence. A remarkable agreement is observed for the short-term, as well as the asymptotic evolution, of mixing patterns. Simulations of dye concentration fields as a function of Re confirm large differences in mixing behavior for the four different flow conditions. Strong axial segregation revealed by the stretching field, as well as the location of poorly mixing regions, are accurately predicted.