Preventing swirl and vortex — In agitated tanks, which design choices are effective at suppressing swirling and vortex formation?

Introduction:
Unbaffled, centrally agitated tanks often develop a central vortex, which reduces power efficiency and can entrain gas undesirably. Designers use internals and specialized hardware to disrupt swirl and improve mixing. This question highlights two common remedies.

Given Data / Assumptions:

• Cylindrical tank with central impeller.
• Moderate to high rotational speeds where vortexing can occur.
• Aerated systems using radial turbines benefit from controlled gas introduction.

Concept / Approach:
Baffles interrupt tangential flow, redirecting it into radial/axial components and suppressing large-scale swirl. Diffusion rings distribute gas beneath radial turbines to avoid gas accumulation at the shaft and reduce the tendency for vortex-induced gas ingress from the surface. Together, these measures curb vortex depth and stabilize hydrodynamics.

Step-by-Step Solution:
Add 4 equally spaced vertical baffles (typical) to break swirl.Provide a diffusion ring to introduce gas uniformly under the impeller, reducing shaft flooding and surface drawdown.Recognize that both methods effectively reduce vortex formation.

Verification / Alternative check:
Flow visualization and power draw measurements show decreased vortex depth and improved power number stability when baffles are installed and gas is introduced via diffusion rings.

Why Other Options Are Wrong:

• (a) Alone works but (c) acknowledges both effective solutions.
• (b) Helps specifically with gas dispersion and reduces shaft flooding but is most effective combined with baffles.
• (d) Incorrect because proven methods exist.
• (e) Tilting the tank aggravates swirl rather than suppressing it.

Common Pitfalls:
Using too-narrow baffles that are ineffective; placing gas spargers poorly so gas short-circuits to the surface and re-induces vortexing.

Final Answer:
Both (a) and (b)