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Lewis number is the ratio of thermal diffusivity to species diffusivity.
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Formula
Le dimensionless; α and D in m2/s
| Symbol | Meaning | Unit |
|---|---|---|
| Lewis number | dimensionless | |
| Thermal diffusivity of the fluid | m2/s | |
| Molecular diffusivity of the species | m2/s |
\mathrm{Le}=\frac{\alpha}{D}The number compares thermal and molecular diffusion rates and is related to Pr and Sc by Le=Sc/Pr when properties are defined consistently. Approximate heat-mass transfer analogies often assume Le≈1; check that assumption for the actual mixture.
The number compares thermal and molecular diffusion rates and is related to Pr and Sc by Le=Sc/Pr when properties are defined consistently. Approximate heat-mass transfer analogies often assume Le≈1; check that assumption for the actual mixture.
Lewis number is the ratio of thermal diffusivity to species diffusivity.
Base equation for the target quantity
Preserve signs, boundary conditions and units when rearranging.
Work through the numerical example for “Lewis number for heat and mass diffusion” and check its unit and magnitude.
For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70.
Substituting a related quantity that shares the same symbol.
Use variable definitions and check: Le dimensionless; α and D in m2/s.
Formula cluster
Lewis number is the ratio of thermal diffusivity to species diffusivity.
For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70.
Compare heat and mass transport in mixtures and boundary layers.
Here is how to work through a typical Lewis number for heat and mass diffusion (Le = α/D) task step by step:
Task
Work through the numerical example for “Lewis number for heat and mass diffusion” and check its unit and magnitude.
Solution path
For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70.
Thermal and species diffusivities are similar. Some models can then use a heat-mass transfer analogy. This does not prove identical boundary layers; boundary conditions and properties still matter. Lewis number compares thermal and species diffusion for a stated mixture and state. Application: Compare heat and mass transport in mixtures and boundary layers. The numerical example For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70 anchors the idea in a concrete measurement case. The corresponding units are Le dimensionless; α and D in m2/s. For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70. This checks that the reference quantities are compatible.
Using Sc=ν/D and Pr=ν/α gives Le=α/D=Sc/Pr. This relation requires consistent kinematic viscosity and property evaluation. In mixtures, species diffusivity can depend on composition. Lewis number compares thermal and species diffusion for a stated mixture and state. The corresponding units are Le dimensionless; α and D in m2/s. For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70. This checks that the reference quantities are compatible. Scope: The number compares thermal and molecular diffusion rates and is related to Pr and Sc by Le=Sc/Pr when properties are defined consistently. Approximate heat-mass transfer analogies often assume Le≈1; check that assumption for the actual mixture. A practical application is Compare heat and mass transport in mixtures and boundary layers.
No. It compares diffusion scales but does not determine heat or mass-transfer coefficients without correlations. Equipment design also needs geometry, Reynolds number, inlet state, reaction kinetics and required transfer rate. Lewis number compares thermal and species diffusion for a stated mixture and state. Scope: The number compares thermal and molecular diffusion rates and is related to Pr and Sc by Le=Sc/Pr when properties are defined consistently. Approximate heat-mass transfer analogies often assume Le≈1; check that assumption for the actual mixture. A practical application is Compare heat and mass transport in mixtures and boundary layers.
Write the quantity and unit from the variable list next to every value: Le dimensionless; α and D in m2/s. Substitute only after converting to a consistent unit system, and state sign or reference direction explicitly. The example For α=1.4·10−5 m2/s and D=2.0·10−5 m2/s, Le=0.70. is a calculation check: result, unit and order of magnitude must fit the described situation. Round after intermediate calculations so small contributions are not lost prematurely. Lewis number compares thermal and species diffusion for a stated mixture and state.
The background states this starting condition: The number compares thermal and molecular diffusion rates and is related to Pr and Sc by Le=Sc/Pr when. That limits when the result is a valid model prediction. Compare the problem’s boundary conditions with this assumption before substituting values; real components, material properties or loads can introduce additional effects. The cited reference supports the relation used here but does not remove the need to check whether the specific case fits the model. Lewis number compares thermal and species diffusion for a stated mixture and state.
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Which formula describes Lewis number for heat and mass diffusion?
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How do you rearrange Lewis number for heat and mass diffusion for Base equation for the target quantity?
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Which common mistake happens with Lewis number for heat and mass diffusion?
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