## Lewis number $\mathrm{Le} = \frac{\alpha}{D} = \frac{\textrm{thermal diffusion rate}}{\textrm{mass diffusion rate}} = \frac{\lambda}{\rho Dc_{p} }$ $\mathrm{Le}$ is the ratio of thermal diffusivity and mass diffusivity. If write $\alpha$ in the form of thermal conductivity and specific heat, we get the latter expression. Here $\rho$ is density, $\alpha$ is [thermal diffusivity](https://en.wikipedia.org/wiki/Thermal_diffusivity), $k$ is [thermal conductivity](https://en.wikipedia.org/wiki/Thermal_conductivity), $c_{p}$ is specific heat. Similarly concepts are [[Prandtl number]] and [[Schmidt number]]. >[!Info] >See more on wiki: https://en.wikipedia.org/wiki/Lewis_number.