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:
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:
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:
Common Pitfalls:
Applying pipe-flow Re thresholds directly to mixing; geometry and rotating flow change the numerical ranges.
Final Answer:
< 10
Discussion & Comments