The Nusselt number is defined as Nu = where Lc is the characteristic length of the surface, h is the convective heat transfer coefficient, and k is the thermal conductivity of the fluid.

Nu = h Lc ÷ k

Explanation: The Nusselt number (Nu) can be defined as the ratio of convective to conductive heat transfer at a boundary in a fluid.

Nu =  Convective Heat Transfer               h Lc

              =      

         Conductive Heat Transfer                  k

Where h  = conductive hear transfer

           Lc = length of the surface

           k   = thermal conductivity of the fluid