Inefficient mixing occurring in mixing operations carried out in stirred tanks at low to moderate Reynolds numbers has been studied extensively by several researchers in recent years. Using acid-base neutralization reactions and steady impeller rotations, previous studies have revealed the existence, location and size of isolated mixing regions (IMR) in laminar mixing flow. This paper investigates the mixing effects of unsteady stirring procedures in a Newtonian fluid using square wave modulation in the impeller rotation speed.

Glycerine was agitated inside an unbaffled tank using a standard six-bladed Rushton turbine impeller. The mixing time of the IMRs was determined using a non-intrusive technique based on direct visualization of an acid-base reaction with fluorescent green dye indicator. The size and the volume of the IMRs were determined using digital photographs taken during the mixing process.

Current results show that laminar mixing under Re < 100 could be enhanced by modulating the impeller rotational speed and period interval. It is also found that the IMRs could be easily destroyed in less than fifteen minutes using square wave modulation. Larger amplitude of time periodic fluctuations and shorter period intervals in the square wave modulation are found to enhance the degree of homogenization leading to better mixing performance.