ANS:B - Lewis number

The Lewis number (Le) is a dimensionless parameter used in fluid mechanics and heat transfer to characterize the relative importance of molecular diffusion to heat conduction in a fluid flow. It is defined as the ratio of the thermal diffusivity to the mass diffusivity of a fluid. Mathematically, the Lewis number (Le) is given by: 𝐿𝑒=𝛼𝐷Le=Dα​ Where:

• 𝛼α is the thermal diffusivity of the fluid.
• 𝐷D is the mass diffusivity of the fluid.

The thermal diffusivity (𝛼α) represents how quickly heat can propagate through a material, while the mass diffusivity (𝐷D) represents how quickly mass or species can diffuse through a fluid. The Lewis number provides information about the relative rates of heat transfer and mass transfer in a fluid flow.

• When Le > 1: Heat transfer is dominant compared to mass transfer. This implies that thermal effects are more significant than mass diffusion effects.
• When Le < 1: Mass transfer is dominant compared to heat transfer. This implies that mass diffusion effects are more significant than thermal effects.
• When Le = 1: Heat transfer and mass transfer are equally important. This indicates that both thermal and mass diffusion effects have similar magnitudes.

The Lewis number is commonly used in the analysis of various fluid flow phenomena, including combustion, chemical reactions, and heat and mass transfer in convective flows. It helps engineers and researchers understand the relative importance of thermal and mass diffusion effects in different processes and systems.