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:
where 
is the thermal diffusivity and 
is the mass diffusivity.
is the thermal diffusivity and is the mass diffusivity.
The Lewis number can also be expressed in terms of the Schmidt number and the Prandtl number :
.
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