Convection

Convection is the transfer of potential energy, for example heat, by currents within a fluid. Most fluids are liquids, gases, and plasmas, although large solid bodies such as Earth's mantle also behave like a fluid on long time scales and at high pressure and temperature. Thermal convection can arise from temperature differences either within the fluid or between the fluid and its boundary, which maintains a gravitationally unstable density gradient if the temperature gradient increases in the direction of gravity. Other sources of density variations, such as variable composition (for example, salinity), or from the application of an external motive force are also often causes. 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.

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 unstable density gradients (for example, the temperature differences existing within the fluid) that can be maintained in the fluid. Example: hot air rising off the surface of a radiator.

The basic premise behind free convection is that heated fluid becomes more buoyant and "rises," while cooler fluid "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. In zero gravity, because buoyancy no longer becomes a factor, free convection does not occur.

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. A person blowing on their food to cool it 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

$q'' = h(T_s - T_\infty)$

where:

$q''$ - local heat flux rated
$h$ - local convection coefficient
$T_s$ - surface temperature
$T_\infty$ - ambient temperature

The total heat transfer $q$ is then calculated as the integral of $q''$ over the surface area,

$q=\int_A q''\, dA$

This then leads to a definition of average convection coefficient, $\overline{h}$ , defined from

$q= \overline{h}(T_s - T_\infty)$

Studies of forced convection lead to a close inspection of the flow in the boundary layer of the fluid.

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 convection. 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 denser. 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 when it radiates 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. Oceanic convection is also frequently driven by density differences due to varying salinity, known as thermohaline convection, and is of crucial importance in the global thermohaline circulation. In this case it is quite possible for relatively warm, saline water to sink, and colder, fresher water to rise, reversing the normal transport of heat.

Mantle convection

Convection within Earth's mantle is the driving force for plate tectonics. However, unlike familiar examples of convection like boiling soup, most of the heat flow comes from within the mantle itself. The source of this heat is radioactive decay of ⁴⁰K. This has allowed plate tectonics on Earth to continue far longer than it would have if simply driven by heat left over from Earth's formation.

Pattern formation

Convection, especially Rayleigh-Bénard convection, where the convecting fluid is contained by two rigid horizontal plates, is a convenient example of a pattern forming system. Above a critical value of the Rayleigh number, the system undergoes a bifurcation from the stable conducting state to the convecting state. If fluid parameters other than density do not depend significantly on temperature, the flow profile is symmetric, with the same volume of fluid rising as falling. This is known as Boussinesq convection. If the temperature difference between the top and bottom of the fluid is higher, parameters like viscosity begin to vary across the layer. This breaks the symmetry of the system, and generally changes the pattern of up- and down-moving fluid from stripes to hexagons, as seen at right.

As Rayleigh number is increased further above the value where convection first appears, the system may undergo other bifurcations, where patterns such as spirals begin to appear.