Péclet Number

The Péclet number (Pe) is a dimensionless number used in thermodynamics and fluid mechanics to compare the rate of advection of a physical quantity by the flow to the rate of diffusion of the same quantity driven by an appropriate gradient. It is named after the French physicist Jean Claude Eugène Péclet. The Péclet number for mass transfer is defined as the product of the velocity (v) and the characteristic length (L) divided by the thermal diffusivity (α).

v represents the velocity of the flow,
L represents the characteristic length which could be the hydraulic diameter of the system (for internal flow),
α represents the thermal diffusivity of the fluid.
The higher the Péclet number, the more advection dominates over diffusion.