Prandtl Number

Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity and thermal diffusivity, It is defined as:
\mathit{Pr} = \frac{\nu}{\alpha}
where \nu 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, 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.

 

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