The ratio of thermal diffusivity to mass diffusivity is designated by which dimensionless number?

• A. Prandtl Number
• B. Rayleigh Number
• C. Prandtl Number
• D. Lewis Number

Answer: (C)

In transport phenomena, you compare how fast different things diffuse with a dimensionless number. The ratio of thermal diffusivity to mass diffusivity is called the Lewis number. It is defined as Le = α / D, where α is the thermal diffusivity and D is the mass diffusivity. Thermal diffusivity α = k/(ρ c_p) describes how quickly temperature fields even out, while D measures how fast species concentrations spread. So Le tells you whether heat diffuses faster or slower than mass. If Le is greater than one, heat diffuses faster than mass; if Le is less than one, mass diffuses faster than heat; and if Le is about one, they diffuse at similar rates. This is distinct from the Prandtl number, which compares momentum diffusivity to thermal diffusivity, and from the Rayleigh number, which combines buoyancy, viscous, and thermal effects rather than a direct heat-to-mass diffusion ratio.