Lewis Number Formula |
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\( Le \;=\; \dfrac{ \alpha }{ D_m }\) (Lewis Number) \( \alpha \;=\; Le \cdot D_m \) \( D_m \;=\; \dfrac{ \alpha }{ Le }\) |
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| Symbol | English | Metric |
| \( Le \) = Lewis number | \(dimensionless\) | \(dimensionless\) |
| \( \alpha \) (Greek symbol alpha) = thermal diffusivity | \(ft^2\;/\;sec\) | \(m^2\;/\;s\) |
| \( D_m \) = mass diffusivity | \(ft^2\;/\;sec\) | \(m^2\;/\;s\) |
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
Low 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.
Intermediate 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.
High 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.
