Mixing regimes in stirred tanks — Which statement about flow regimes and the role of speed and baffles is most accurate?

Introduction:
Flow regime identification is fundamental to predicting power draw, blending times, and mass/heat-transfer coefficients in agitated vessels. This question distinguishes correct statements about regime classification from common misconceptions about speed and baffles.

Given Data / Assumptions:

• Cylindrical, baffled vessels with typical impellers.
• Reynolds number for mixing defined as Re = rho * N * D^2 / mu.
• Fluids may be Newtonian or non-Newtonian, but the concept of regime via Re remains instructive.

Concept / Approach:
Regimes are often categorized as laminar (Re ≲ 10), transitional (10 ≲ Re ≲ 10^4), and turbulent (Re ≳ 10^4) for Newtonian systems. Baffles help prevent vortex formation and promote radial/axial flow; they do not create laminar flow. Increasing speed raises Re, but turbulence is not guaranteed if viscosity or scale constraints keep Re low (e.g., very viscous polymer solutions, large D with low N limits, or non-Newtonian effects).

Step-by-Step Solution:
Assess statement (a): Regime classification by Re is standard practice — correct.Assess statement (b): Speed helps, but high viscosity or small impellers can keep Re low; thus, “always” is incorrect.Assess statement (c): Baffles reduce swirl and increase turbulence intensity for a given N; they do not make flow laminar.Since (a) is true and (b)/(c) are false, “all of the above” cannot be right.

Verification / Alternative check:
Correlations for power number vs Re show regime transitions; photographic flow visualization confirms that baffles suppress vortex yet enhance bulk turbulence at sufficient Re.

Why Other Options Are Wrong:

• (b) Overgeneralizes; turbulence depends on Re, not speed alone.
• (c) Misstates baffle function; they encourage turbulent, not laminar, flow.
• (d) Cannot be right since (b) and (c) are wrong.
• (e) Cannot be right since (a) is correct.

Common Pitfalls:
Misusing speed as a surrogate for Re; assuming baffles “calm” flow into laminar; ignoring non-Newtonian effective viscosity shifts.

Final Answer:
Mixing in stirred tanks can be classified as laminar, transitional, or turbulent depending on Reynolds number.