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Mass Transfer - Engineering

Q1:

Lewis number (Le) is

A Sc / Pr

B Pr x St

C Sh x Pr

D St x Sh

ANS:A - Sc / Pr

The Lewis number (𝐿𝑒Le) is a dimensionless number used in fluid mechanics and heat transfer to characterize the relative importance of molecular diffusivity to thermal diffusivity. It is defined as the ratio of the thermal diffusivity to the mass diffusivity. The Lewis number (𝐿𝑒Le) is given by: 𝐿𝑒=𝐷𝛼Le=αD​ Where:

  • 𝐷D is the molecular diffusivity (often represented by the Schmidt number 𝑆𝑐Sc),
  • 𝛼α is the thermal diffusivity, and
  • π‘ƒπ‘ŸPr is the Prandtl number.
Therefore, the correct expression for the Lewis number (𝐿𝑒Le) is: 𝐿𝑒=π‘†π‘π‘ƒπ‘ŸLe=PrSc​ So, the correct option is: 𝑆𝑐/π‘ƒπ‘ŸSc/Pr.