Definition of Peclet Number
Definition
Advection and Diffusion: fluid transport of substance or heat.
- The macroscopic collective movement of a fluid is called advection.
- The microscopic motion of molecules that leads to homogenization is called diffusion.
The dimensionless quantity dimensionless quantity defined as the ratio of the transport rate due to advection to the transport rate due to diffusion is called the Péclet number. $$ \mathrm{Pe} = \frac{ \text{advetive transport rate} }{ \text{diffusive transport rate} } $$
Explanation
In simple terms, the Péclet number indicates whether a given transport is dominated by advection or by diffusion. Because transport problems commonly involve heat transfer and mass transfer, the Péclet number can be expressed as the product of the Reynolds number and another dimensionless number.
For heat transfer, it is expressed as the product with the Prandtl number $\mathrm{Pr}$. $$ \mathrm{Pe} = \mathrm{Re} \cdot \mathrm{Pr} $$
For mass transfer, it is expressed as the product with the Schmidt number $\mathrm{Sc}$. $$ \mathrm{Pe} = \mathrm{Re} \cdot \mathrm{Sc} $$