Mixing regimes in stirred tanks — Which statement about flow regimes and the role of speed and baffles is most accurate?
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AMixing in stirred tanks can be classified as laminar, transitional, or turbulent depending on Reynolds number.
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BIncreasing impeller speed will always guarantee turbulence regardless of scale and fluid properties.
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CAdding baffles induces laminar flow by suppressing vortices.
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DAll of the above statements are correct.
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ENone of the above statements is correct.
Answer
Correct Answer: Mixing in stirred tanks can be classified as laminar, transitional, or turbulent depending on Reynolds number.
Explanation
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.