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 impeller is examined computationally. Results demonstrate that the flow generated by constant impeller speed is partially chaotic. Under these conditions, the stretching experienced by fluid elements located within segregated toroidal regions of the flow increases slowly at a linear rate characteristic of regular (non-chaotic) flows. Within the bulk flow region, since the flow is chaotic, stretching increases at the expected exponential rate. The use of dynamic flow perturbations enhances mixing; when time-dependent RPM are applied, a globally chaotic flow is generated. We investigate computationally the effect on mixing performance of protocols in which the agitation speed oscillates between two steady values. Different frequencies of speed change and several RPM settings are compared. Under variable RPM schemes, the segregated torii are periodically relocated and fluid elements have the opportunity to abandon the regular regions. Under these conditions stretching increases exponentially throughout the entire flow domain. In general, mixing protocols with higher frequency of speed fluctuation produce the largest increase in stretching rates. Counter-intuitively, at a given RPM fluctuation frequency, stretching rates were higher for the lower RPM settings, either per revolutions or per unit of energy spent.