Free and Forced Convection

When a hot body simply sits in contact with still air (say, air around a burning log), the air removes heat from it through spontaneously-arising currents -- this is FREE (or NATURAL) convection; land and sea breezes are large-scale examples of free convection currents set up in the atmosphere.

When the convection process is instead deliberately sped up -- for example, by a fan driving a rapid circulation of fresh air -- this is called FORCED convection. The heater/convector, air conditioner, and IC-engine radiator examples of §7.9.2.1 are, more precisely, all examples of forced convection, since a fan o …