Investigation of impeller type and baffle effects on isolated mixing regions in a laminar stirred tank

The isolated mixing regions (IMRs) or segregated regions that exist above and below the rotating impeller for several hours during the mixing of high viscous fluids in agitated vessels are substantial barriers to efficient mixing. The segregated regions can be eliminated by high rotational impeller speeds; however high shearing rates are undesirable for shear sensitive products in biological applications. This work investigates the impeller type and baffle effects using speed modulation protocols to destroy the IMRs in laminar mixing vessels. Glycerin (Newtonian fluid) was agitated in a 0.19 m diameter tank equipped with either a 6-blade Rushton turbine or pitched-blade turbine under baffled/non-baffled conditions. The mixing time was measured using direct visualization of an acid-base reaction with fluorescent green dye as passive tracer. The elliptical IMRs volume was calculated by the theorem of Pappus using digital images taken during the mixing process. The flow pattern and evolution of IMRs structures caused by each impeller were observed. The presence of baffles restricts the IMRs volume between the tank wall and the rotating impellers, while the speed modulation promotes more stretching and folding mechanism of the IMR structures. Therefore, the baffling effect and speed modulation enhance the chaotic mixing process. Between the two impellers used, the pitched-blade impeller leads to higher destruction rate of IMRs