Agitation regimes in mixing:\nDuring liquid agitation in a tank, the flow is said to be laminar when the agitator Reynolds number (Re) is in which range?

Difficulty: Easy

Correct Answer: < 10

Explanation:


Introduction / Context:
In mixing, the agitator Reynolds number Re = (rho * N * D^2) / mu indicates the flow regime, where rho is fluid density, N is rotational speed, D is impeller diameter, and mu is viscosity. Recognising the thresholds between laminar, transitional, and turbulent regimes guides impeller selection, power draw estimates, and scale-up strategies for reactors and blend tanks.


Given Data / Assumptions:

  • Re is defined for mechanically agitated vessels.
  • We refer to the typical textbook thresholds.


Concept / Approach:
For mixing, laminar flow generally occurs for Re less than about 10; transitional behavior spans roughly Re 10 to 10^4 (depending on geometry), and turbulent flow dominates beyond that range. In the laminar regime, viscous forces dominate, streamlines are ordered, and power numbers vary inversely with Re. Thus, “Re < 10” is the accepted indicator of laminar mixing conditions for common impellers.


Step-by-Step Solution:

Recall definition of Re for mixing: Re = rho * N * D^2 / mu.Map standard thresholds: laminar < 10; transitional ~ 10–10^4; turbulent > 10^4.Select “< 10” as the laminar condition.


Verification / Alternative check:
Mixing handbooks and design correlations (e.g., power number vs. Re) show distinct asymptotes for Re below 10, consistent with laminar flow and predictable shear fields.


Why Other Options Are Wrong:

  • > 10 or > 100: move into transitional/turbulent domains.
  • < 100: too loose; includes transitional region; the accepted laminar threshold is much lower (~10).


Common Pitfalls:
Applying pipe-flow Re thresholds directly to mixing; geometry and rotating flow change the numerical ranges.


Final Answer:
< 10

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