Mixing dynamics in uncovered unbaffled stirred tanks

• Mixing rates in uncovered unbaffled vessel are assessed by means of PLIF.
• Time evolution of spatial inhomogeneity was followed via PLIF maps standard deviation.
• Results show that two well-mixed, partially segregated regions exist.
• At near-critical conditions, mixing efficiency equals that of baffled tanks.
• A simple D=T/3D=T/3 PBT impeller is found to be the best choice.

The present work is aimed at providing experimental information on mixing rates in an unbaffled vessel under free surface vortexing conditions. The planar laser induced fluorescence (PLIF) technique was used for measuring the dispersion dynamics of a passive tracer over a vertical section of the vessel. In agreement with the quite scant literature information available for these systems, results confirm the existence of two well defined, partially segregated, zones that give rise to a double mixing dynamics behavior.A suitable mixing time definition is proposed and applied to a number of experimental runs with different stirrer geometries and agitation speeds.Results confirm that unbaffled vessels are indeed poorer mixers than baffled tanks. In fact, when compared on the basis of same power input, i.e.   in terms of mixing efficiency, mixing times are found to be 2–3 times larger than those pertaining to baffled tanks. However, in the 0.19m diameter vessel employed, the observed mixing times typically fell in the range between 10 and 100 s, which makes them fully compatible with many applications involving relatively slow processes. Moreover, when free-surface vortex bottom approached the impeller plane, mixing efficiencies became practically identical to those of baffled tanks. This is a remarkable finding, as it considerably widens the potential application field of unbaffled vessels.

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