Impeller hydraulics in stirred tanks:\nWhich impeller type generates bulk liquid flow that is predominantly parallel to the agitator shaft (i.e., along the vessel’s axis)?

Introduction / Context:
Mixing impellers are broadly classified by the dominant direction of the liquid they discharge: axial, radial, or tangential. Correctly identifying which class sends flow parallel to the agitator shaft is essential for selecting equipment for suspension, blending, or gas dispersion in biochemical and chemical reactors.

Given Data / Assumptions:

• The question concerns the principal direction of bulk flow created by an impeller.
• Axial flow means along the tank’s vertical axis (parallel to the shaft).
• Radial flow means perpendicular to the shaft, toward the tank wall.
• Tangential flow largely follows circular motion around the wall.

Concept / Approach:
Axial-flow impellers (e.g., pitched-blade turbines, hydrofoils/propellers) pump fluid up or down along the vessel axis, creating top-to-bottom circulation loops that are excellent for blending, heat transfer, and solids suspension at moderate power numbers. Radial-flow impellers (e.g., Rushton turbines) eject fluid outward from the blade tips toward the wall, producing strong shear near the impeller for gas dispersion but less axial turnover unless baffles and multiple stages are used. Tangential flow dominates in anchor or helical ribbon mixers used for high-viscosity systems and is not parallel to the shaft.

Step-by-Step Solution:

Verification / Alternative check:
Vendor literature and mixing textbooks plot flow number vs. power number; hydrofoils/propellers are labeled axial-flow with high pumping efficiency and low power number compared to radial turbines.

Why Other Options Are Wrong:

• Radial flow: direction is perpendicular to the shaft.
• Both (a) and (b): mutually exclusive definitions.
• None of these: axial is valid.
• Tangential flow: mainly circumferential, not axial.

Common Pitfalls:
Assuming pitch angle creates radial flow; pitched blades primarily drive axial circulation despite some radial component.

Final Answer:
Axial flow