Nusselt number is the ratio of convective heat transport to conductive heat transport normal to the fluid boundary. It is a dimensionless number used in heat transfer in boundary of fluid.
where,
h = heat transfer coefficient (Btu / hr.m2.K)
L = characteristic length (m) (see below)
k = thermal conductivity (Btu.m / hr.m2.K)
Nusselt no. greater than 1 shows that heat transfer by convection is dominant as compared to that by conduction. And Nusselt no. less than 1 shows the opposite.
Selection of the characteristic length should be in the direction of growth (or thickness) of the boundary layer; some examples of characteristic length are: the outer diameter of a cylinder in (external) cross flow (perpendicular to the cylinder axis), the length of a vertical plate undergoing natural convection, or the diameter of a sphere. For complex shapes, the length may be defined as the volume of the fluid body divided by the surface area.
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