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Boundary-layer thickness equality: condition on the Prandtl number (Pr). When do the thermal and hydrodynamic boundary-layer thicknesses become approximately equal over a surface in external flow? Choose the correct value of Pr that ensures δᵗ ≈ δ.

Difficulty: Easy

equal to one
greater than one
less than one
equal to Nusselt number

Correct Answer: equal to one

Explanation:

ConceptThe Prandtl number Pr = ν/α compares momentum and thermal diffusivities. When Pr ≈ 1, momentum and heat diffuse at similar rates, so the thermal (δᵗ) and velocity (δ) boundary layers have comparable thicknesses.

Final Answerequal to one.

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