Lewis Number

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.  The Lewis number is often used to characterize heat and mass transfer processes, particularly in situations involving simultaneous heat and mass transfer, such as in combustion, chemical reactions, or natural convection.  It provides information about the relative importance of thermal and mass diffusion in a system.

Lewis Number Interpretation

• Lewis Number (Le  >  1)  -  Thermal diffusion outpaces mass diffusion.  Heat spreads faster than the substance does.  For example, in a flame, the temperature might rise quickly across a region before the fuel or oxygen fully mixes in.  This is common in gases where thermal conductivity is relatively high compared to species diffusion.
• Lewis Number (Le  <  1)  -  Mass diffusion is faster than thermal diffusion.  The substance spreads out more quickly than heat does.  This might happen in liquids where molecules diffuse rapidly but heat transfer is slower due to lower thermal conductivity. In combustion, this could mean fuel mixes with air before the temperature fully stabilizes.
• Lewis Number (Le  ≈  1)  -  Thermal and mass diffusion occur at roughly the same rate.  This is an idealized case often assumed in simplified models (some combustion theories) because it simplifies calculations.  It’s roughly true for many gases under certain conditions, like air at moderate temperatures.

The Lewis number is particularly relevant in fields such as combustion, chemical engineering, and atmospheric science, where the interaction between heat and mass transfer plays a significant role.  It helps researchers and engineers understand the interplay between diffusion processes and how they affect the overall behavior of fluid flows with coupled heat and mass transfer.