Lewis number is a dimensionless number defined as the ratio of thermal diffusivity

Thermal diffusivity

In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It has the SI unit of m²/s...

 to mass diffusivity. It is used to characterize fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 flows where there is simultaneous heat and mass transfer by convection.

It is defined as:

where is the thermal diffusivity and is the mass diffusivity.

The Lewis number can also be expressed in terms of the Schmidt number

Schmidt number

Schmidt number is a dimensionless number defined as the ratio of momentum diffusivity and mass diffusivity, and is used to characterize fluid flows in which there are simultaneous momentum and mass diffusion convection processes...

 and the Prandtl number :.

It is named after Warren K. Lewis

Warren K. Lewis

Warren Kendall Lewis was an MIT professor who has been called the father of modern chemical engineering. He co-authored an early major textbook on the subject which essentially introduced the concept of unit operations...

 (1882–1975), who was the first head of the Chemical Engineering Department at MIT. Some workers in the field of combustion assume (incorrectly) that the Lewis number was named for Bernard Lewis (1899–1993), who for many years was a major figure in the field of combustion research.