Theory

The mechanism of heat transfer wherein fluid motion is produced due to density changes resulting from temperature gradients is termed as Free or Natural convection heat transfer. The bulk fluid movement results due to the fact that the fluid particles in the immediate vicinity of the hot surface become warmer than the surrounding fluid resulting in a local change in density (thermal expansion). The warmer fluid would be replaced by colder fluid creating convection currents. These currents originate when a body force (gravity, centrifugal, electrostatic, etc.) acts on a fluid in which there are density gradients. The force which induces these convection currents is the buoyancy force.