# Lewis Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Lewis number, abbreviated Le is a dimensionless number which is the ratio of thermal diffusivity to mass diffusivity. It is used to characterize fluid flows where there is simultaneous heat and mass transfer.

### 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

Solve for:

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

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