Lewis Number Calculator
Enter thermal and mass diffusivities to compute the Lewis number Le = α/D (ratio of thermal to mass diffusion).
Input Data
Results
At a glance:The Lewis number Le = α/D is a dimensionless group comparing thermal and mass diffusion. α = k/(ρ·c_p) is thermal diffusivity (how fast heat spreads) and D is the mass (species) diffusivity (how fast a solute spreads). Le = 1 means heat and species diffuse at the same rate; Le > 1 means thermal diffusion dominates (heat spreads faster than mass); Le < 1 (typical of gases, where D is relatively large) means mass diffuses faster. It appears in coupled heat-and-mass-transfer problems such as evaporation, combustion and drying, often alongside the Prandtl and Schmidt numbers (Le = Sc/Pr).
Formula
Lewis number: Le = α/D
Also: Le = Sc/Pr
$$\mathrm{Le} = \dfrac{\alpha}{D} = \dfrac{\mathrm{Sc}}{\mathrm{Pr}}$$How to Use
- Enter the thermal diffusivity α (m²/s).
- Enter the mass diffusivity D (m²/s).
- The calculator returns Le.
Case Studies
Air-water vapor
α ≈ 2.2×10⁻⁵ m²/s, D ≈ 2.6×10⁻⁵ m²/s.
Le ≈ 0.85.
Mass (vapor) diffuses slightly faster than heat — typical humid-air evaporation.
FAQ
What does Le = 1 mean?
Heat and mass diffuse at the same rate, so thermal and concentration boundary layers have the same thickness — a simplifying assumption in some evaporation models.
How is it related to Prandtl and Schmidt numbers?
Le = Sc/Pr, combining the momentum, thermal and mass transport dimensionless numbers.
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References
Content review: Calculatorism Editorial Team. Results are for reference only; please refer to the relevant authorities for the official figures.