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GATE 2025 CH – Question 21

Mass Transfer · Mass transfer theories and momentum, heat and mass transfer analogies · 1 mark · Multiple choice

Schmidt number is defined as

  1. $\frac{\text{Mass Diffusivity}}{\text{Thermal Diffusivity}}$
  2. $\frac{\text{Momentum Diffusivity}}{\text{Mass Diffusivity}}$
  3. $\frac{\text{Momentum Diffusivity}}{\text{Thermal Diffusivity}}$
  4. $\frac{\text{Thermal Diffusivity}}{\text{Mass Diffusivity}}$

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Show answer and explanation

Correct answer: (B) $\frac{\text{Momentum Diffusivity}}{\text{Mass Diffusivity}}$

Explanation

The Schmidt number is $Sc = \frac{\nu}{D_{AB}} = \frac{\mu}{\rho D_{AB}}$, the ratio of momentum diffusivity (kinematic viscosity) to mass diffusivity. The ratio with thermal diffusivity is the Prandtl number, and mass over thermal diffusivity is the Lewis number (inverted).