Power Consumption in Uncovered Unbaffled Stirred Tanks: Influence of the Viscosity and Flow Regime

Notwithstanding the increasing industrial interest toward unbaffled tanks, available experimental information on their behavior is still scant, even for basic quantities such as the mechanical power drawn. In this work, the influence of the Reynolds and Froude numbers on the power consumption characteristics is presented for unbaffled stirred tanks operating both in nonaerated conditions (subcritical regime) and in aerated conditions (supercritical regime), i.e., when the free surface vortex has reached the impeller and the gas phase is ingested and dispersed inside the reactor. Experimental results obtained at various liquid viscosities show that power numbers obtained in subcritical conditions do line up quite well on a smooth N-p versus Re function, with no need to involve the Froude number in the correlation. At rotational speeds involving air entrapment and dispersion inside the reactor (supercritical regime), a steep reduction of the power number is observed. A novel overall correlation for power number prediction, able to deal with both the subcritical and supercritical regimes, is finally proposed.