Forced Convection — Boundary Layers & Heat Transfer Coefficients
Nusselt correlations (Dittus-Boelter, Gnielinski), boundary layer development, turbulent heat transfer, entrance effects, and finned surfaces.
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Quick Explainer
Why is the Prandtl number so important for convective heat transfer?
The Prandtl number (Pr = mu*Cp/k) determines the relative thickness of velocity and thermal boundary layers. For liquid metals (Pr~0.01), heat diffuses much faster than momentum — thick thermal layer. For oils (Pr~100), the thermal layer is much thinner than the velocity layer and heat transfer is sensitive to conditions very close to the wall.
When do entrance effects matter in internal flow heat transfer?
Thermal entrance length is approximately 0.05 * Re * Pr * D. For water (Pr~7) at Re=10000, that is over 35 diameters before the Nusselt number stabilizes. In short heat exchangers or microchannels, local Nusselt near the inlet can be 2-5x the fully-developed value — using only fully-developed correlations significantly underestimates heat transfer.