.png)
The Prandtl number of dry air at 20 °C and 1 atm is 0.7080. Across −100 to 1000 °C at 1 atm it stays between 0.698 and 0.740. A value of 0.7 is a good default for air in most heat transfer work. This page is for dry air.
The Prandtl number compares how fast momentum spreads (viscosity) with how fast heat spreads (conductivity):
Pr = μ·cp / k = ν / α
A Pr below 1 means heat spreads faster than momentum. The thermal boundary layer is then thicker than the velocity boundary layer. Use Pr in Nusselt number correlations for forced and natural convection. The inputs are on the viscosity, specific heat and thermal conductivity pages, and α is on the thermal diffusivity page.
Water at room temperature has Pr near 6 (see water Prandtl number).
Accuracy. Lemmon et al. (2000) state an uncertainty of about 0.1 % in density, 0.2 % in speed of sound and 1 % in heat capacity from the solidification point to 873 K (600 °C) and up to 70 MPa. Above 873 K there were no reliable air data, so nitrogen data were used and the paper gives about 0.5 % for density. Lemmon & Jacobsen (2004) state about 5 % for the viscosity and thermal conductivity of air, and more near the critical point. Viscosity and conductivity above about 600 °C, and the 1000 °C end of the range, are less certain than that. CoolProp uses a molar mass of 28.96546 g/mol, the U.S. Standard Atmosphere uses 28.9644 g/mol. That is the main reason CoolProp density at 15 °C and 1 atm is 0.04 % above the Standard Atmosphere value. Pr combines three properties, so its uncertainty is roughly 5 % or more.