Convection

     Heat transfer between a surface and a liquid or gas in motion is known as convection. As the fluid or gas travels faster, the convective heat transfer increases.

     The mode of energy transfer between a solid surface and the adjacent liquid/gas
that is in motion, and it involves the combined effects of conduction and fluid motion.

     The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction.

Heat transfer from a hot surface to
air by convection.

Types of Convection

     Two types of convection are natural convection and forced convection.

     Natural convection is when fluid motion results from the hot atoms in the fluid, where the hot atoms move upwards toward the cooler atoms in the air—the fluid moves under the influence of gravity. Examples of this include the rising clouds of cigarette smoke, heat from the hood of a car that rises upwards.

     Forced convection is where the fluid is forced to travel over the surface by a fan or pump or some other external source.

Forced convection:  If the fluid is forced to flow over the surface by external means
such as a fan, pump, or the wind.

Natural (or free) convection: If the fluid motion is caused by buoyancy forces that
are induced by density differences due to the variation of temperature in the fluid.

     Heat transfer processes that involve change of phase of a fluid are also considered to be convection because of the fluid motion induced during the process, such as the rise of the vapor bubbles during boiling or the fall of the liquid droplets during condensation.