Nusselt number is a dimensionless number that represents the ratio of degree of heat transfer to thermal conductivity.

It can be expressed with the following equation:

where;
Nu : Nusselt number
h : Heat transfer coefficient [W/(m2·K)]
L : Characteristic length [m]
k : Thermal conductivity of fluid [W/(m·K)]

Nusselt number shows how much heat transfer capability of a fluid increases when the fluid convects compared to when the fluid is stationary. The state where fluid is stationary is expressed as Nu = 1.

Nusselt number is named after a German physicist Wilhelm Nusselt.