Computational analysis of regular and chaotic mixing in a stirred tank reactor

Mixing in an unbaffled stirred tank equipped with a 6-blade radial flow imp eller is examined computationally. Results demonstrate that the flow genera ted by constant impeller speed is partially chaotic. Under these conditions , the stretching experienced by fluid elements located within segregated to roidal regions of the flow increases slowly at a linear rate characteristic of regular (non-chaotic) flows. Within the bulk flow region, since the flo w is chaotic, stretching increases at the expected exponential rate. The us e of dynamic flow perturbations enhances mixing; when time-dependent RPM ar e applied, a globally chaotic flow is generated. We investigate computation ally the effect on mixing performance of protocols in which the agitation s peed oscillates between two steady values. Different frequencies of speed c hange and several RPM settings are compared. Under variable RPM schemes, th e segregated torii are periodically relocated and fluid elements have the o pportunity to abandon the regular regions. Under these conditions stretchin g increases exponentially throughout the entire flow domain. In general, mi xing protocols with higher frequency of speed fluctuation produce the large st increase in stretching rates. Counter-intuitively, at a given RPM fluctu ation frequency, stretching rates were higher for the lower RPM settings, e ither per revolutions or per unit of energy spent. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.