Impeller hydraulics:\nWhich of the following is best described as a radial-flow impeller used in stirred tank reactors?

Introduction / Context:
Impellers are classified by the predominant direction of bulk flow they induce: radial, axial, or mixed. Correct identification is essential for choosing appropriate mixers for gas dispersion, solid suspension, or viscous blending. This question asks you to identify the impeller that characteristically produces radial discharge flow.

Given Data / Assumptions:

• 45° pitched-blade impellers are common axial/mixed-flow devices.
• Rushton turbines are canonical radial-flow gas-dispersion impellers.
• Down-pumping designs and helical ribbons have axial or tangential characteristics.

Concept / Approach:
A Rushton turbine (flat-blade disc turbine) ejects liquid radially from the blade tips, generating strong shear near the impeller and effective bubble breakup when aerated. Pitched-blade impellers, by contrast, pump fluid axially (up or down depending on pitch), producing circulation loops that differ markedly from radial devices.

Step-by-Step Solution:

Verification / Alternative check:
Power and flow-number charts classify Rushton as radial with high power number; pitched blades show lower Np with significant axial flow components.

Why Other Options Are Wrong:

• 45° pitched blades: axial/mixed, not radial.
• Both (a) and (b): includes an axial device, so incorrect.
• Downward-directed axial impeller: by definition axial.
• Helical ribbon: used for high-viscosity, mainly tangential/axial circulation.

Common Pitfalls:
Assuming all turbines are radial; some modern high-efficiency turbines are mixed-flow—Rushton specifically is radial.

Final Answer:
A Rushton turbine (flat-blade disc turbine)