Dimensionless quantity calculated as the ratio of momentum diffusivity (kinematic viscosity) \nu to thermal diffusivity a:
(1) | {\rm Pr} = \frac{\nu}{a} |
For most gases over a wide range of temperature and pressure, Prandtl number is approximately constant at around {\rm Pr} = 0.71.
For water {\rm Pr} = 7.56.