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Mass Transfer - Engineering

Q1:

Lewis number, which is important in problems involving simultaneous heat and mass transfer, is the ratio of

A mass diffusivity to momentum diffusivity.

B thermal diffusivity to mass diffusivity.

C momentum diffusivity to thermal diffusivity.

D none of these.

ANS:B - thermal diffusivity to mass diffusivity.

Lewis number (𝐿𝑒Le) is indeed an important dimensionless parameter in problems involving simultaneous heat and mass transfer. It is defined as the ratio of thermal diffusivity to mass diffusivity. So, the correct definition of Lewis number is: thermal diffusivity to mass diffusivity. The Lewis number is given by the equation: 𝐿𝑒=𝛼𝐷Le=Dα​ Where:

  • 𝛼α is the thermal diffusivity (m²/s),
  • 𝐷D is the mass diffusivity (m²/s).
This dimensionless number describes the relative importance of thermal diffusion to mass diffusion in a fluid medium. It helps in characterizing the rate of heat transfer compared to the rate of mass transfer in a given system.