Rushton turbine design feature: Why is a flat disc used with a radial-flow (Rushton) turbine impeller in bioreactors?

Introduction / Context:
The Rushton turbine is a classic radial-flow impeller for gas–liquid contacting. Its geometry includes a central flat disc with six flat blades. Understanding the role of the disc helps explain gas dispersion, shear fields, and energy distribution in aerated tanks.

Given Data / Assumptions:

• Flat disc with radial blades in a baffled tank.
• Often used below a sparger for bubble dispersion.
• Operation in turbulent regimes for microbial fermentations.

Concept / Approach:
The disc reduces flow through the impeller hub region and forces liquid to accelerate around blade tips, where strong shear is created. In aerated systems, the high-shear zones efficiently fragment incoming bubbles, improving interfacial area. The disc also helps maintain a uniform discharge pattern and prevents energy loss in the low-shear core near the shaft.

Step-by-Step Solution:
Recognize that shear and turbulence primarily arise near blade edges.The flat disc blocks short-circuit flow through the hub to concentrate energy at the blades.High shear at the blade tips assists bubble breakup and dispersion.Therefore, the disc simultaneously promotes shear, bubble breakup, and efficient energy use at the blades.

Verification / Alternative check:
Power distribution and laser Doppler velocimetry studies show intense turbulent kinetic energy near blade tips and reduced recirculation through the central region when a disc is present.

Why Other Options Are Wrong:
Each individual statement captures only part of the function; in practice the disc achieves all of these outcomes together.

Common Pitfalls:

• Assuming the disc is only a mounting hub; its hydrodynamic role is significant.
• Underestimating the disc’s importance in gas dispersion performance.

Final Answer:
All of the above