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1.2 Thermal convection

Convection is a heat transferred by motion of fluids. In a typical convective heat

transfer, a hot surface heats the surrounding fluid, which is then carried away by fluid

movement such as wind. The warm fluid is replaced by cooler fluid, which can draw

more heat away from the surface. Since the heated fluid is constantly replaced by cooler

fluid, the rate of heat transfer is enhanced.

Heat transfer by thermal convection means energy transfer:

• from liquids and gases to the surface of a body or a wall, or

• from the surface of a body to the liquid.

Convection is divided into two categories:

- free convection, natural convection - the flow is generated by nonhomogeneous

densities caused temperature difference;

- forced convection - the flow is produced by external sources (pump, fan).

Natural convection (or free convection) refers to a case where the fluid movement is

created by the warm fluid itself. The density of fluid decrease as it is heated; thus, hot

fluids are lighter than cool fluids. Warm fluid surrounding a hot objects rises, and is

replaced by cooler fluid. The result is a circulation of air above the warm surface, as

shown in Figure 8.

Figure 8 - Natural convection

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