# Lewis number

Lewis number (Le) is a dimensionless number defined as the ratio of thermal diffusivity to mass diffusivity. It is used to characterize fluid flows where there is simultaneous heat and mass transfer by convection.

It is defined as:

$\mathrm{Le} = \frac{\alpha}{D}$

where $\alpha$ is the thermal diffusivity and $D$ is the mass diffusivity.

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

$\mathrm{Le} = \frac{\mathrm{Sc}}{\mathrm{Pr}}$.

It is named after Warren K. Lewis (1882–1975),[1][2] 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.

## References

1. ^ W. K. Lewis: The Evaporation of a Liquid Into a Gas In: Transactions of the American Society of Mechanical Engineers, No. 1849, 1922, p. 325-340.
2. ^ A. Klinkenberg, H. H. Mooy: Dimensionless Groups in Fluid Friction, Heat, and Material Transfer In: Chemical Engineering Progress, Vol. 44, No. 1, 1948, p. 17-36.