Change in Mixing Power of a Two-PBT Impeller When Emptying a Tank

The paper presents research on the phenomenon of an increase in mixing power during the emptying of a tank with two 6-PBT45° axial impellers in operation, located on a common shaft, pumping the liquid to the bottom of the mixing tank. A large increase in mixing power took place when the free surface of the liquid was just above the upper edge of one of the impellers (hp/D < 0.1). This increase was even more than 50% compared to the design power for a fully filled mixing vessel. Admittedly, high motor overload, while not very long, may damage it. The study investigated the instantaneous torques acting on the impeller shaft during the emptying of the tank and the velocity distributions in planes r-z. On their basis, the mechanism of the phenomenon observed was determined and correlation relationships were given that permitted the calculation of the numerical values of the power increase factors.