Propeller agitators: which statements correctly characterize a typical propeller agitator used in liquid mixing?

Introduction / Context:
Agitator selection hinges on flow pattern and fluid rheology. Propeller agitators are high-speed, axial-flow impellers favored for blending, solids suspension, and heat transfer in low- to moderate-viscosity liquids.

Given Data / Assumptions:

• Newtonian or mildly non-Newtonian liquids, low viscosity.
• Typical propeller diameters small relative to tank.
• Speeds often hundreds of rpm, not a few rpm.

Concept / Approach:
Propellers generate strong axial pumping with relatively low power number and high flow number, ideal for bulk motion and turnover. They are not intended for pastes (very high viscosity), where anchors, helical ribbons, or double planetary mixers are preferred. Their speeds are high, not extremely slow; slow-speed operation is typical of large-diameter paddles or anchors.

Step-by-Step Solution:
Evaluate (a): axial flow—correct.Evaluate (b): high-viscosity pastes—incorrect for propellers.Evaluate (c): very slow speed—incorrect; propellers are high-speed.Thus only (a) is correct.

Verification / Alternative check:
Correlations of power number and flow number for marine propellers confirm axial predominance and high-speed operation ranges.

Why Other Options Are Wrong:
(b) and (c) contradict typical design practice and scaling.(d) aggregates incorrect statements.

Common Pitfalls:
Using propellers in viscous service leads to poor circulation and motor overload.

Final Answer:
Produces mainly axial flow.