Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity and thermal diffusivity,
It is defined as:
where ν is the kinematic viscosity and α is the thermal diffusivity.
Typical values for Pr are:
- around 0.7 for air and many other gases,
- between 100 and 40,000 for engine oil,
- between 4 and 5 for R-12 refrigerant
- around 0.015 for mercury
For mercury, heat conduction is very effective compared to convection: thermal diffusivity is dominant.
For engine oil, convection is very effective in transferring energy from an area, compared to pure conduction: momentum diffusivity is dominant.