Mass and thermal transport analogies need their own diffusivities, even for the same fluid.
Define the Interface or Transport Basis
NPTEL lecture 40 on concentration boundary layers distinguishes Schmidt number, the ratio of kinematic viscosity to mass diffusivity, from Prandtl number, its ratio to thermal diffusivity. These compare momentum spreading against two different transport processes; one fluid need not give similar values for both.
Review a Hypothetical Comparison
Assign hypothetical kinematic viscosity 1.2×10⁻⁶ m²/s, molecular diffusivity 2×10⁻⁹ m²/s and thermal diffusivity 1.6×10⁻⁷ m²/s. Then Sc=600 while Pr=7.5. The same numerator gives very different ratios because the species and heat transport denominators differ. Substituting 7.5 into a mass-transfer correlation that requires Sc would change the model input by a factor of 80.
Keep the Evidence with the Decision
List each property with its units, temperature and composition. Check whether viscosity is dynamic or kinematic before using it; converting dynamic viscosity requires the matching density. Record which species the molecular diffusivity represents. Read the actual correlation's definitions rather than assuming a thermal analogy permits substituting a heat property for a mass property.
These invented values illustrate the property check. The ratios alone do not establish a boundary-layer thickness, transfer coefficient or the suitability of a correlation in a particular geometry.
Does a proposed transport analogy use the relevant diffusivity and fluid properties?
This blank worksheet is for your own project. It contains no H M machine trial result.
| Review field | Reference or result | Responsible person |
|---|---|---|
| Kinematic viscosity or viscosity/density basis | To complete | To complete |
| Species-specific mass diffusivity | To complete | To complete |
| Thermal diffusivity at the same state | To complete | To complete |
| Correlation-required ratio and applicability | To complete | To complete |
Customer Questions
Does one fluid have only one Schmidt number?
The value depends on the molecular species and state used for its diffusivity.
Can dynamic viscosity be divided directly by D?
That quotient is not the dimensionless Sc defined here; use kinematic viscosity or the matching density conversion.
Can Pr replace Sc in a mass-transfer calculation?
Only if a justified model explicitly supports that transformation; the definitions differ.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
Discuss Your Machine Requirement
Share your product, container, required output and the evidence needed for your technical review.
Request a Technical Review