Convection

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Convection is the internal movement of currents within fluids (i.e. liquids and gases). It cannot occur in solids due to the atoms not being able to flow freely. Convection may cause a related phenomenon called advection, in which mass or heat is transported by the currents or motion in the fluid. In the case where the advected substance is heat, the heat itself may cause fluid motion, so the problem of heat transport (and transport of other substances in the fluid due to it) may become quite complicated.

Differential heating of fluids may itself cause convection in a gravity field, due to variations in density due to a transfer of heat and subsequent fluid expansion, combined with differential buoyancy forces on the different parts of fluid which have differing densities. This type of purely heat-driven convection in gravity fields is sometimes referred to as "natural heat convection," in order to distinguish it from various types of forced heat convection (i.e., heat advection not due to buoyancy of heating). In forced heat convection, transfer of heat is due to movement in the fluid from forces other than heat, such as pumps or natural and artificially-driven mechanically-pushed fluid flows (i.e., flow from external motive forces, such as occurs in rivers or pumps). In addition, buoyancy forces in gravity fields which result from sources of density variations in fluids other than those produced by heat, such as variable composition (for example, salinity), are frequent convection causes.

Current movement during convection may be invisibly slow, or it may be as fast as a tornado or twister. Convection occurs in atmospheres, oceans, and planetary mantles.

[edit] Mantle convection

Main article: mantle convection

Convection within Earth's mantle is the driving force for plate tectonics. There are actually two convection currents occurring within the Earth. The outer core has an extremely rapid convective turnover of fluid metals (primarily iron and nickel) which are responsible for the Earth's magnetic field. The movement of metals forms electrical currents, which in turn generate magnetic fields.

As heat from the inner and outer core heat the lower portion of the mantle, a second set of convective currents form. This mantle convection is extremely slow, as the mantle is a thick semi-solid with the consistency of a very thick paste. This slow convection can take millions of years to complete one cycle.

The source of the heat in the inner core is the radioactive decay of ⁴⁰K and other heavy metals. This has allowed plate tectonics on Earth to continue far longer than it would have if it were simply driven by heat left over from Earth's formation.

[edit] Pattern formation

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Picture of the thermal field and its two-dimensional Fourier transform of a fluid under Rayleigh-Bénard convection [1]

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 the 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.

[edit] See also

• Bénard cells
• Fluid dynamics
• Advection
  • Vortex dynamics
  • Thermomagnetic convection
• Grashof number
• Heat transfer
  • Heat conduction
  • Thermal radiation
  • Heat convection

[edit] External links

• Correlations for Convective Heat Transfer
• Introduction to Mechanism of Free Convection