ANS:C - Lewis

The Lewis number (𝐿𝑒Le) is a dimensionless number used in heat and mass transfer correlations, particularly in the context of mass transfer accompanied by simultaneous heat transfer. It expresses the ratio of thermal diffusivity to mass diffusivity within a fluid. Mathematically, the Lewis number (𝐿𝑒Le) is defined as: 𝐿𝑒=Thermal diffusivityMass diffusivityLe=Mass diffusivityThermal diffusivity​ or 𝐿𝑒=𝛼𝐷Le=Dα​ where:

• 𝛼α is the thermal diffusivity of the fluid,
• 𝐷D is the mass diffusivity of the species undergoing mass transfer.

The Lewis number characterizes the relative rates of heat and mass transfer in a fluid. It is particularly relevant in situations where heat and mass transfer occur simultaneously, such as in convective heat transfer accompanied by species diffusion, as seen in cooling tower applications, combustion processes, and chemical reaction engineering. The Lewis number provides insight into the relative dominance of thermal and concentration boundary layer effects. A Lewis number greater than unity (𝐿𝑒>1Le>1) indicates that thermal boundary layer effects are dominant, meaning that heat transfer occurs more rapidly than mass transfer. Conversely, a Lewis number less than unity (𝐿𝑒<1Le<1) suggests that concentration boundary layer effects dominate, implying that mass transfer is more rapid than heat transfer. In summary, the Lewis number is a crucial parameter in understanding and analyzing heat and mass transfer processes, providing insights into the relative importance of thermal and concentration boundary layer effects in different systems.