Course No Course Title Credits

μ = absolute or dynamic viscosity (kg/m. s, cP)
c p = specific heat capacity (J/kg K,)
k = thermal conductivity (W/m K)
The Prandtl Number is often used in heat transfer and free and forced
convection calculations.
Note that whereas the Reynolds number and Grashof number are
subscripted with a length scale variable, Prandtl number contains no such
length scale in its definition and is dependent only on the fluid and the fluid
state. As such, Prandtl number is often found in property tables alongside
other properties such as viscosity and thermal conductivity.
Typical values for Pr are:
• around 0.015 for mercury
• around 0.16-0.7 for mixtures of noble gases or noble gases with
hydrogen
• around 0.7-0.8 for air and many other gases,
• between 4 and 5 for R-12 refrigerant
• around 7 for water (At 20 degrees Celsius)
• between 100 and 40,000 for engine oil
• around 1 × 10 25 for Earth's mantle.
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.
In heat transfer problems, the Prandtl number controls the relative
thickness of the momentum and thermal boundary layers. When Pr is small, it
means that the heat diffuses very quickly compared to the velocity
(momentum). This means that for liquid metals the thickness of the thermal
boundary layer is much bigger than the velocity boundary layer.