Prandtl number

Summary

The Prandtl number $\mathrm{Pr} = \nu / \alpha$ is the dimensionless number that compares how fast a fluid diffuses momentum with how fast it diffuses heat. Unlike most dimensionless numbers it depends only on the fluid and its state, not on the flow. It controls the relative thickness of the velocity and thermal boundary layers, so it enters every problem of heat transfer by convection.