Topic: Heat Transfer
What is the name of the dimensionless number that characterizes the ratio of thermal to mass diffusivity?
- A. Lewis number
- B. Prandtl number
- C. Reynolds number
- D. Schmidt number
Answer: A. Lewis number
Explanation: The Lewis number ($Le$) is defined as the ratio of thermal diffusivity ($\alpha$) to mass diffusivity ($D_{AB}$). It is used to characterize fluid flows where there is simultaneous heat and mass transfer. $Le = \frac{\alpha}{D_{AB}}$.