# Lewis Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Lewis number, abbreviated as Le, a dimensionless number, is the ratio of thermal diffusivity to mass diffusivity.  It is used to characterize fluid flows where there is simultaneous heat and mass transfer.  Some common values of non-metallic thermal diffusivity values can be found here.

## Lewis number formula

 $$\large{ Le = \frac{ \alpha }{ D_m } }$$

### Where:

$$\large{ Le }$$ = Lewis number

$$\large{ D_m }$$ = mass diffusivity

$$\large{ \alpha }$$  (Greek symbol alpha) = thermal diffusivity