On the assessment of power consumption and critical impeller speed in vortexing unbaffled stirred tanks

Unbaffled stirred tanks are increasingly recognized as a viable alternative to common baffled tanks for a number of processes and bio-processes where the presence of baffles is undesirable. Notwithstanding the increasing industrial interest towards unbaffled tanks, available experimental information on their behaviour is still very poor, even for important parameters such as mechanical power drawn and critical impeller speed (Ncr) at which the transition between non-aerated (sub-critical regime) and aerated (super-critical regime) conditions occurs. In this work the influence of Reynolds and Froude numbers on power consumption characteristics of unbaffled stirred tanks is presented for tanks operating both in non-aerated (sub-critical regime) and aerated (super-critical) conditions. Influence of scale-up, impeller to tank size, liquid height aspect-ratio and presence or not of a top-cover is investigated in order to provide a general correlation able to assess (i) the critical rotational speed Ncr and (ii) the power number Np under any fluid dynamic regime. Experimental results obtained show that the proposed general correlation is fully validated when scaling-up the system. Moreover, the variation of geometrical features such as impeller to tank size, liquid aspect ratio and the presence or not of a top-cover do not modify the functional dependencies of power number on Re and Fr and only different (dimensionless) multiplying shape factors must be adopted. Finally, an overall general correlation for critical rotational speed (Ncr) assessment is also proposed.