FLUID MECHANICS AND TRANSPORT PHENOMENA 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 in®estigated as a function of the Reynolds number by means of Poincare sections, which re®eal large segregated regions with sizes and shapes that ®ary ́ greatly o®er a relati®ely 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 additi®es in the tank. When mixing dynamics is examined by particle tracking, the structures obser®ed 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 obser®ed for the short-term, as well as the asymptotic e®olution, of mixing patterns. Simulations of dye concentration fields as a function of Re confirm large differences in mixing beha®ior for the four different flow conditions. Strong axial segregation re®ealed by the stretching field, as well as the location of poorly mixing regions, are accurately predicted.