Convection
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Convection is the transfer of heat by the motion of or within a fluid. It may arise from temperature differences either within the fluid or between the fluid and its boundary, or from the application of an external motive force. It is one of the three primary mechanisms of heat transfer, the others being conduction and radiation. Convection occurs in atmospheres, oceans, and planetary mantles.
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Free and forced convection
In heat transfer, a distinction is made between free and forced convection.
Free convection is convection in which motion of the fluid arises solely due to the temperature differences existing within the fluid. Example: hot air rising off the surface of a radiator.
The basic premise behind free convection is that heated matter becomes more buoyant and "rises"; while cooler material "sinks". Free convection occurs in any liquid or gas which expands or contracts in response to changing temperatures when it is exposed to multiple temperatures in an acceleration field such as gravity or a centrifuge. The local changes in density results in buoyancy forces that cause currents in the fluid.
Forced convection happens when motion of the fluid is imposed externally (such as by a pump or fan). Example: a fan-powered heater, where a fan blows cool air past a heating element, heating the air. When a person blows on their food to cool it, he/she is using forced convection.
Convection at a surface
In both of the previous examples, an engineer would often be interested in the rate of heat transfer from the hot 'source' surface to the fluid medium.
The local convective heat flux of a fluid passing over a surface is expressed as
- q" = h (Ts - T∞);
- q" local heat flux (dq/dA)
- h local convection coefficient
- Ts surface temperature
- T∞ quiescent or ambient temperature
The total heat transfer over a surface is then calculated as the integral of q",
- q = ∫Asq" dAs
- As area of the surface
- q total heat transfer rate (units of energy/time)
This then leads to a definition of average convection coefficient, h-bar, defined from
- q = h-bar As (Ts - T∞)
Studies of forced convection lead to a close inspection of the flow in the boundary layer of the fluid.
- See also: Fluid dynamics, Nusselt number, Grashof number, and Heat transfer coefficient.
Atmospheric convection
In the case of Earth's atmosphere, solar radiation heats the Earth's surface, and this heat is then transferred to the air by conduction. When a layer of air receives enough heat from the Earth's surface, it expands, becomes less dense and is pushed upward by buoyancy. Colder, heavier air sinks under it and is then warmed, expands, and rises. The warm rising air cools as it reaches the higher, cooler regions of the atmosphere and becomes more dense. Since it cannot sink through the rising air beneath it, it moves laterally and then begins to sink. When it reaches the surface again it is heated, and is drawn back into the original rising column. These convection currents cause local breezes, winds, thermals, cyclones and thunderstorms, and at a larger scale, produce the global atmospheric circulation features.
A single region of air with a rising and falling current is called a convection cell.
Heat is lost from the rising air through radiation into space.
- See also: weather.
Oceanic convection
Solar radiation also affects the oceans. Warm water from the Equator tends to circulate toward the poles, while cold polar water heads towards the Equator.
Mantle convection
Convection, within a mantle, can cause continental drift.