Abstract
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.
Highlights
The phenomenon of the increase in mixing power when emptying the tanks with the impeller working occurs only in the case of impellers with axial action
The instantaneous increase in the mixing power for two working impellers placed on a common shaft and pumping the liquid toward the bottom of the mixer may be even more than 50% greater in relation to the calculated power
The increase in mixing power takes place when the direction of rotation of the vortices of the radial-axial circulation changes i.e., when the free surface of the liquid approaches the upper surface of the impeller blades
Summary
Mixing in tanks is one of the most common processes used in the technologies of the chemical, biochemical, pharmaceutical, and related industries. One of the more interesting solutions in the construction of mixing systems in tanks is the use of several impellers placed on one shaft. These solutions are often used when mixing gas–liquid, liquid–solid, or gas–liquid–solid systems [1,2,3,4,5], and in many cases, they have an advantage over solutions with one impeller [2,6,7], despite the fact that a construction with several impellers is more complicated [8,9,10]. Examples of industrial applications of systems with many impellers include the structures of fermenters, crystallizers, polymerizers, impellers with sewage aeration, and others [1,15,16,17,18]
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