Impeller Reynolds number (Rei):\nWhat numerical range corresponds to laminar mixing in agitated tanks based on the impeller Reynolds number definition?

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:

  • Standard definition of Rei with density ρ, speed N, impeller diameter D, and viscosity μ.
  • Newtonian fluid assumption for regime boundaries.
  • Baffled tank unless otherwise specified.


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:

Recall Rei expression and regime thresholds used in design charts.Match “laminar” with Rei below approximately 10.Confirm transitional and turbulent ranges exceed this value.Select Rei < 10 as the laminar regime.


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:

  • 10–10^4: transitional.
  • >10^4: turbulent.
  • “None”: recognized boundaries exist.
  • 100–1000: sits well inside transitional for mixing practice.


Common Pitfalls:
Using pipe-flow Reynolds thresholds; mixing thresholds differ because geometry and rotating flow modify transitions.


Final Answer:
Rei < 10

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