The Nusselt number ($Nu$) is a dimensionless number used to characterize the ratio of convective to conductive heat transfer across a fluid boundary layer. It provides insight into the efficiency of convective heat transfer compared to conductive heat transfer. The Nusselt number is defined as the ratio of convective heat transfer to conductive heat transfer at a boundary layer interface. The formula for the Nusselt number ,

Where:

• $Nu$ is the Nusselt number,
• $h$ is the convective heat transfer coefficient,
• $L$ is a characteristic length of the system,
• $k$ is the thermal conductivity of the fluid.