What is Prandtl Number? Heat Transfer Operations

The Prandtl number is a dimensionless number, named after its inventor, a German engineer Ludwig Prandtl, who also identified the boundary layer. The Prandtl number is defined as the ratio of momentum diffusivity to thermal diffusivity. The momentum diffusivity, or as it is formally called, kinematic viscosity, tells us the material’s resistance to shear-flows (different layers of the flow travel with different velocities due to e.g. different speeds of adjacent walls) in relation to density.

Small values of the Prandtl number, Pr very very less than 1, means the thermal diffusivity dominates. Whereas with large values, Pr very very greater than 1, the momentum diffusivity dominates the behaviour. For example, the typical value for liquid mercury, which is about 0.025, indicates that the heat conduction is more significant compared to convection, so thermal diffusivity is dominant.
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