Question

“Rotational stirrers are advantageous for mixing”?

(a) True

(b) False

This question was posed to me in semester exam.

I would like to ask this question from Mixing topic in chapter Fluid Flow and Mixing of Bioprocess Engineering

Answer 1

Right option is (b) False

Explanation: Simple circular flow of liquid around the shaft is generally disadvantageous and should be avoided. In circular flow, liquid moves in a streamline fashion and there is little mixing between fluid at different heights in the tank. Circular flow also leads to vortex development. At high impeller speeds, the vortex may reach down to the impeller so that gas from the surrounding atmosphere is drawn into the liquid; this is generally undesirable as it produces very high mechanical stresses in the stirrer shaft, bearings and seal. Prevention of circular flow has a high priority in design of mixing systems, and is usually achieved by installing baffles which interrupt the circular flow pattern and create turbulence in the fluid.