The rate of energy transferred by convection to that by conduction is called
Nusselt number:- In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. Convection includes both advection (fluid motion) and diffusion (conduction).
Nusselt number is a non-dimensional heat transfer coefficient. It is used to determine whether the heat transfer is conduction or convection.