Calculate the dimensionless Prandtl number (Pr) to characterize fluid flow and heat transfer.
Last updated: March 2026 | By ForgeCalc Engineering
The Prandtl number (Pr) is a dimensionless number that represents the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity. It relates the thickness of the velocity boundary layer to the thermal boundary layer.
When Pr is small (e.g., in liquid metals), heat diffuses very quickly compared to momentum. When Pr is large (e.g., in heavy oils), momentum diffuses much faster than heat. For air at standard conditions, Pr is approximately 0.7.
At 20°C, the Prandtl number for water is approximately 7.0.
For most liquids, Pr decreases as temperature increases because viscosity drops significantly. For gases, Pr is relatively independent of temperature.
If Pr = 1, the momentum and thermal boundary layers have the same thickness, and heat and momentum diffuse at the same rate.
Yes, Pr is a key parameter in heat transfer correlations for both laminar and turbulent flows (e.g., the Dittus-Boelter equation).
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