Difficulty: Easy
Correct Answer: Rei < 10
Explanation:
Introduction / Context:
The impeller Reynolds number Rei = ρ * N * D^2 / μ is used to classify flow regimes in mixing: laminar, transitional, or turbulent. Correctly identifying the laminar range helps choose impeller types and predict power and blending times, especially for viscous or polymer-containing fluids.
Given Data / Assumptions:
Concept / Approach:
Typical regime cutoffs in mixing practice are: laminar for Rei less than about 10, transitional for roughly 10 to 10^4, and fully turbulent above about 10^4. In the laminar regime, power number varies inversely with Rei and flow is dominated by viscous effects, making axial/radial distinctions less meaningful than in turbulent flow.
Step-by-Step Solution:
Verification / Alternative check:
Power-number correlations (Np vs Rei) show linear laminar scaling for Rei < 10; deviations begin as inertia grows in the transitional regime.
Why Other Options Are Wrong:
Common Pitfalls:
Using pipe-flow Reynolds thresholds; mixing thresholds differ because geometry and rotating flow modify transitions.
Final Answer:
Rei < 10
Discussion & Comments