Power consumption in uncovered-unbaffled stirred tanks: influence of viscosity and flow regime

Abstract

Notwithstanding the increasing industrial interest towards unbaffled tanks, available experimental information on their behaviour is still scant, even for basic quantities such as mechanical power drawn. In this work the influence of Reynolds and Froude numbers on power consumption characteristics is presented for unbaffled stirred tanks operating both in non-aerated conditions (sub-critical regime) and in aerated (super-critical) conditions, 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 Np 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 (super-critical regime), a steep reduction of power number is observed. A novel overall correlation for power number prediction, able to deal with both subcritical and super-critical regimes is finally proposed.