The Nusselt number is defined as the ratio of convective to conductive heat transfer across a boundary.

• A. The ratio of turbulent to laminar heat transfer across a boundary
• B. The ratio of conductive to convective heat transfer across a boundary
• C. The ratio of convective heat transfer to conductive heat transfer across a boundary
• D. The ratio of the diffusion coefficient to the thermal conductivity

Answer: (C)

The Nusselt number compares how much heat is carried by convection to how much would be carried by conduction across the boundary. It’s defined as Nu = hL/k, where h is the convective heat transfer coefficient, L is a characteristic length, and k is the thermal conductivity. This makes Nu a dimensionless measure of the effectiveness of convection relative to conduction for a given geometry and temperature difference. If convection greatly enhances transfer, Nu is large; if conduction dominates, Nu is near or around 1.

So the statement that best matches this idea is that the Nusselt number is the ratio of convective heat transfer to conductive heat transfer across a boundary. The other notions—turbulent versus laminar flow, an inverted ratio, or using a diffusion coefficient instead of thermal conductivity—do not capture the fundamental comparison between convective and conductive mechanisms that Nu expresses.