thumb|right|This figure shows a calculation for thermal convection. Colors closer to red are hot areas and colors closer to blue are cold areas. In this figure, a hot, less-dense lower boundary layer sends plumes of hot material upwards, and likewise, cold material from the top moves downwards. This figure is from a model of convection in the Earth's mantle.
Convection is the movement of molecules within fluids (i.e. liquids, gases and rheids). Convection is one of the major modes of heat transfer and mass transfer. In fluids, convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.

Convective heat transfer

A common use of the term convection leaves out the word "heat" but nevertheless refers to heat convection: that is, the case in which heat is the entity of interest being advected (carried), and diffused (dispersed). There are two major types of heat convection:

• Heat is carried passively by a fluid motion which would occur anyway without the heating process. This heat transfer process is often termed forced convection or occasionally heat advection.

• Heat itself causes the fluid motion (via expansion and buoyancy force), while at the same time also causing heat to be transported by this bulk motion of the fluid. This process is called natural convection, or free convection. With natural convection, heat transport (and related transport of other substances in the fluid due to it) is generally more complicated.

Both forced and natural types of heat convection may occur together (in that case being termed mixed convection).
Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion (observable movement) of fluids (see convection for concept details). This can be contrasted with conductive heat transfer, which is the transfer of energy by vibrations at a molecular level through a solid or fluid, and radiative heat transfer, the transfer of energy through electromagnetic waves.

As convection is dependent on the bulk movement of a fluid it can only occur in liquids, gases and multiphase mixtures.

Natural convection

Natural convective heat transfer

When heat is transferred by the circulation of fluids due to buoyancy from the density changes induced by heating itself, then the process is known as natural convection or free convection.

Familiar examples are the upward flow of air due to a fire or hot object and the circulation of water in a pot that is heated from below.

For a visual experience of natural convection, a glass that is full of hot water filled with red food dye may be placed inside a fish tank with cold, clear water. The convection currents of the red liquid will be seen to rise and also fall, then eventually settle, illustrating the process as heat gradients are dissipated.

Onset of natural convection

Natural convection occurs when a system becomes unstable and therefore begins to mix by the movement of mass. A common observation of convection is of thermal convection in a pot of boiling water, in which the hot and less-dense water on the bottom layer moves upwards in plumes, and the cool and more dense water near the top of the pot likewise sinks.

The onset of natural convection is determined by the Rayleigh number (Ra). This dimensionless number is given by

where

Natural convection will be more likely and/or more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection, and/or a larger distance through the convecting medium. Convection will be less likely and/or less rapid with more rapid diffusion (thereby diffusing away the gradient that is causing the convection) and/or a more viscous (sticky) fluid.

For thermal convection due to heating from below, as described in the boiling pot above, the equation is modified for thermal expansion and thermal diffusivity. Density variations due to thermal expansion are given by:

where

The general diffusivity, $D$, is redefined as a thermal diffusivity, $\backslash kappa$.

Inserting these substitutions produces a Rayleigh number that can be used to predict thermal convection.

Forced convection

Natural heat convection (also called "free convection") is distinguished from various types of forced heat convection, which refer to heat advection by a fluid which is not due to the natural forces of buoyancy induced by heating. In forced heat convection, transfer of heat is due to movement in the fluid which results from many other forces, such as (for example) a fan or pump. A convection oven thus works by forced convection, as a fan which rapidly circulates hot air forces heat into food faster than would naturally happen due to simple heating without the fan. Aerodynamic heating is a form of forced convection. Common fluid heat-radiator systems, and also heating and cooling of parts of the body by blood circulation, are other familiar examples of forced convection.

Flames and convection

In a zero-gravity environment, there can be no buoyancy forces, and thus no natural (free) convection possible, so flames in many circumstances without gravity, smother in their own waste gases. However, flames may be maintained with any type of forced convection (breeze); or (in high oxygen environments in "still" gas environments) entirely from the minimal forced convection that occurs as heat-induced expansion (not buoyancy) of gases allows for ventilation of the flame, as waste gases move outward and cool, and fresh high-oxygen gas moves in to take up the low pressure zones created when flame-exhaust water condenses.

Buoyancy induced convection not due to heat

The general term for this is gravitational convection. Natural heat convection is only one form of gravitational heat convection. Differential buoyancy forces producing convection in gravity fields may result from non-heat sources of density variations such as variable composition. For example, gravitational convection can be seen in the diffusion of a source of dry salt downward into wet soil due to the buoyancy of fresh water in saline. Variable salinity in water and variable water content in air masses, are frequent causes of convection in the oceans and atmosphere, which do not involve heat, or else involve additional compositional density factors other than the density changes from thermal expansion (see thermohaline circulation). Similarly, variable composition within the Earth's interior which has not yet achieved maximal stability and minimal energy (in other words, with densest parts deepest) continues to cause a fraction of the convection of fluid rock and molten metal within the Earth's interior (see below).

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.

Vibration convection in gravity fields

Vibration-induced convection occurs in powders and granulated materials in containers subject to vibration, in a gravity field. When the container accelerates upward, the bottom of the container pushes the entire contents upward. In contrast, when the container accelerates downward, the sides of the container push the adjacent material downward by friction, but the material more remote from the sides is less affected. The net result is a slow circulation of particles downward at the sides, and upward in the middle.

If the container contains particles of different sizes, the downward-moving region at the sides is often narrower than the larger particles. Thus, larger particles tend to become sorted to the top of such a mixture.

Scale and rate of convection

Convection may happen in fluids at all scales larger than a few atoms. Convection occurs on a large scale in atmospheres, oceans, and planetary mantles. Current movement during convection may be invisibly slow, or it may be obvious and rapid, as in a hurricane. On astronomical scales, convection of gas and dust is thought to occur in the accretion disks of black holes, at speeds which may closely approach that of light.

See also

• Advection

• * Vortex dynamics

• * Thermomagnetic convection

• Atmospheric convection

• Bénard cells

• Churchill-Bernstein Equation

• Double diffusive convection

• Fluid dynamics

• Grashof number

• Heat transfer

• * Heat conduction

• * Thermal radiation

• Heat pipe

• Laser-heated pedestal growth

• Nusselt number