Nusselt number

The Nusselt number is a dimensionless number which measures the enhancement of heat transfer from a surface which occurs in a 'real' situation, compared to the heat transfer that would be measured if only conduction could occur. Typically it is used to measure the enhancement of heat transfer when convection takes place.

where

• L = characteristic length
• k_f = thermal conductivity of the fluid
• h = convection heat transfer coefficient

The Nusselt number can also be viewed as being a dimensionless temperature gradient at the surface.