|  9| Heat Transfer around Cylinders and Spheres

Free convection occurring around cylinders and spheres is a slow process meaning that that RaD < 1012 and the flow remains laminar in the localised area around the cylinder or sphere.

In terms of application, cylindrical rods are frequently used in different chemical reactors to regulate the temperature inside the reactor by adding or extracting heat.    

For flow around a long horizontal cylinder, the local Nusselt Number is given by:

For flow around a sphere, the local Nusselt Number is given by:

Note: The Nusselt Number calculation is the same whether the cylinder/sphere is adding or extracting heat from the system.