If the impeller speed is set too low in an aerated stirred tank, bubbles may accumulate and coalesce beneath the impeller. What is this condition called?

Introduction / Context:
Correct impeller speed is vital for keeping bubbles dispersed. At too low a speed for a given gas rate, gas accumulates beneath the blades and forms a cavity that rotates with the impeller. This phenomenon is known as a flooded impeller and compromises oxygen transfer and mixing uniformity.

Given Data / Assumptions:

• Aerated, baffled stirred tank with mechanical agitation.
• Gas flow rate is held while impeller speed is reduced.
• Liquid properties are water-like.

Concept / Approach:
When flooding occurs, the impeller loses “bite” on the liquid because gas occupies the blade region. The effective power draw falls, and bubble breakup is poor. The cure is to increase impeller speed, reduce gas rate, or change to an impeller less prone to flooding at the operating gas load.

Step-by-Step Solution:

Verification / Alternative check:
Torque and power measurements show a dip in gassed power when flooding initiates; visual inspection confirms coalesced gas under blades.

Why Other Options Are Wrong:

• Coalesced impeller/river/dry: nonstandard terms.
• Choked: used in other contexts, not standard for this hydrodynamic state.

Common Pitfalls:
Confusing flooding with complete loss of aeration; gas still enters but is not effectively dispersed.

Final Answer:
Flooded impeller