Subtopic: Simultaneous heat and mass transfer in agricultural processing operations
Question Statement:
The Lewis relation $\frac{h_c}{k_y C_s} \approx 1$ holds true for air-water vapour mixtures under normal ambient conditions primarily because:
Options:
(A) The Lewis number $Le = \frac{Sc}{Pr} = \frac{\alpha}{D_{AB}}$ is approximately equal to unity ($1.0$)
(B) The latent heat of vaporization of water is strictly zero
(C) Air has a higher thermal conductivity than pure liquid water
(D) Convective heat transfer occurs without any boundary layer formation
Answer Key:
A
Step-by-Step Solution & Explanation
The Lewis relation arises from the Chilton-Colburn analogy for simultaneous heat and mass transfer. When $Le = \frac{\alpha}{D_{AB}} \approx 1.0$, the thermal boundary layer and concentration boundary layer develop at identical rates, causing the wet-bulb temperature to coincide almost exactly with the adiabatic saturation temperature.