Prandtl number

Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity and thermal diffusivity,

Related Topics:
Dimensionless number - Thermal diffusivity

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It is defined as:

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:mathit{Pr} = rac{ u}{lpha}

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where u is the kinematic viscosity and α is the thermal diffusivity.

Related Topics:
Kinematic viscosity - Thermal diffusivity

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Typical values for Pr are:

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• 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.

Related Topics:
Heat conduction - Convection

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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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