Nusselt number, Nu, is the dimensionless parameter characterizing convective heat transfer. It is defined as
where α is convective heat transfer coefficient, L is representative dimension (e.g., diameter for pipes), and λ is the thermal conductivity of the fluid. Nusselt number is a measure of the ratio between heat transfer by convection (α) and heat transfer by conduction alone (λ/L).
Hewitt, G. F., Shires, G. L. and Bott, T. R. (1994) Process Heat Transfer, CRC Press, Boca Raton, FL.