Which dimensionless number relates to diffusion processes in heat and mass transfer, being the ratio of thermal diffusivity to mass diffusivity?

• A. Prandtl Number
• B. Nusselt Number
• C. Lewis Number
• D. Peclet Number

Answer: (C)

The key idea is comparing how fast heat spreads versus how fast a substance diffuses. Thermal diffusivity, alpha, measures how quickly temperature disturbances travel through a medium, while mass diffusivity, D, measures how quickly concentration disturbances spread. The Lewis number, Le, uses their ratio: Le = alpha / D. This dimensionless group tells you which type of diffusion dominates in a problem with both heat and mass transfer. If Le is greater than one, heat diffuses faster than the species; if Le is less than one, the species diffuses faster; if Le is about one, they diffuse at comparable rates. In practical terms, Le appears in the equations governing coupled heat and mass transfer and helps predict the relative thickness of thermal versus concentration boundary layers.