ANS:A - 1

The Lewis number (Le) is defined as the ratio of the molecular diffusivity of a species to its thermal diffusivity. In the case of the air-water vapor system, the Lewis number can be approximated as: 𝐿𝑒=𝑆𝑐𝑃𝑟Le=PrSc​ where:

• 𝑆𝑐Sc is the Schmidt number, representing the ratio of momentum diffusivity (kinematic viscosity) to mass diffusivity.
• 𝑃𝑟Pr is the Prandtl number, representing the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity.

For air-water vapor system, the Schmidt number (Sc) for water vapor in air is approximately 0.60, and the Prandtl number (Pr) for air is approximately 0.71. So, 𝐿𝑒≈0.600.71Le≈0.710.60​ 𝐿𝑒≈0.845Le≈0.845 Therefore, the value of the Lewis number (Le) for the air-water vapor system is approximately 0.845. Given the options provided, none of them match the calculated value. It seems there might be a mistake in the options. If we consider options within the typical range for the Lewis number for air-water vapor system, it should be around 0.8 to 0.9.