Impeller taxonomy for mixing: which of the following is NOT classified as an axial-flow impeller in bioreactor applications?

Introduction / Context:
Impeller choice controls flow pattern, shear, mixing time, and gas dispersion. Recognizing whether an impeller generates axial flow (top-to-bottom circulation) or radial flow (outward from the blade) is core knowledge for reactor design.

Given Data / Assumptions:

• Standard, unshrouded impellers in baffled tanks.
• Marine/propeller and hydrofoils are designed for axial pumping.
• Rushton turbine is the canonical radial-flow, flat-blade disk turbine.

Concept / Approach:
Axial flow impellers (propellers, marine impellers, hydrofoils, pitched-blade turbines) pump fluid along the shaft axis, improving top-bottom circulation and often giving lower shear at comparable pumping. The Rushton turbine produces strong radial jets, high shear zones near the blade edges, and is preferred for gas dispersion in some regimes.

Step-by-Step Solution:

Verification / Alternative check:
Flow visualization and velocity profiles from particle image velocimetry confirm axial vs. radial patterns for these classes.

Why Other Options Are Wrong:

• Propeller, marine, hydrofoil, and pitched-blade: all designed to pump axially in baffled tanks.

Common Pitfalls:
Equating “turbine” with axial; many turbines (Rushton) are radial unless pitched for axial flow.

Final Answer:
A Rushton turbine.