Information about Convection
Convection in the most general terms refers to the movement of currents within fluids (i.e. liquids, gases and rheids).
Convection is one of the major modes of heat 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.[1]
A common use of the term convection relates to the special case in which the advected (carried) substance is heat. In this case, the heat itself often causes the fluid motion, while also being transported by it. In this case, the problem of heat transport (and related transport of other substances in the fluid due to it) may be more complicated.
Purely heat-driven convection in gravity fields, especially that which itself carries heat, is sometimes referred to as "natural heat convection." A familiar example is the process that carries heated air upward from a fire or hot object.
In the case of Earth's atmosphere, solar radiation heats the Earth's surface, and this heat is then transferred to the atmosphere by processes that are mostly convective. When a parcel of air is heated, it expands, becoming less dense and is pushed upward by buoyancy, carrying the heat energy upward with it. The air then cools, so it contracts, and sinks. The cycle then repeats with the cold air reheating and rising again. Since it cannot sink through the rising air beneath it, it moves laterally (sideways) and then begins to sink. 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 falling and rising current is called a convection cell.
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.
Neutrino flux measurements from the Earth's core (see kamLAND) show the source of about two-thirds of the heat in the inner core is the radioactive decay of 40K, uranium and thorium. 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; or with heat produced by rearrangement of denser portions to the centre of the earth.
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.
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.
When heat is fed into the system from one direction (usually below), at small values it merely diffuses (conducts) from below upward, without causing fluid flow. As the heat flow is increased, above a critical value of the Rayleigh number, the system undergoes a bifurcation from the stable conducting state to the convecting state, where bulk motion of the fluid due to heat begins. 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.
As the temperature difference between the top and bottom of the fluid becomes higher, significant differences in fluid parameters other than density may develop in the fluid due to temperature. An example of such a parameter is viscosity, which may begin to significantly vary horizontally across layers of fluid. 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. Such hexagons are one example of a convection cell.
As the Rayleigh number is increased even further above the value where convection cells first appear, the system may undergo other bifurcations, and other more complex patters, such as spirals, may begin to appear. These may be familiar as examples from systems in which viscosity is relatively low and heat through-put high, such as the spiraling upward flow of gases in a fire.
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
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A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere.
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Convection is one of the major modes of heat 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.[1]
A common use of the term convection relates to the special case in which the advected (carried) substance is heat. In this case, the heat itself often causes the fluid motion, while also being transported by it. In this case, the problem of heat transport (and related transport of other substances in the fluid due to it) may be more complicated.
Mechanism of the special case of heat-driven heat convection
The mechanism of heat-driven convection is that uneven heating of fluids may cause uneven densities due to temperature driven expansion or contraction. In a gravity field (or other equivalent acceleration situation), such differences cause forces due to buoyancy of the less-dense parcels of fluid.Purely heat-driven convection in gravity fields, especially that which itself carries heat, is sometimes referred to as "natural heat convection." A familiar example is the process that carries heated air upward from a fire or hot object.
Atmospheric heat-driven convection
)
In the case of Earth's atmosphere, solar radiation heats the Earth's surface, and this heat is then transferred to the atmosphere by processes that are mostly convective. When a parcel of air is heated, it expands, becoming less dense and is pushed upward by buoyancy, carrying the heat energy upward with it. The air then cools, so it contracts, and sinks. The cycle then repeats with the cold air reheating and rising again. Since it cannot sink through the rising air beneath it, it moves laterally (sideways) and then begins to sink. 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 falling and rising current is called a convection cell.
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.Buoyancy induced convection not due to heat
Buoyancy forces which cause convection in gravity fields may result from sources of density variations in fluids other than those produced by heat, such as variable composition. For example, 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 (see thermohaline circulation). Similarly variable composition within the Earth's interior which has not yet achieved maximal stability and minimal energy (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.Mantle convection
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.
Neutrino flux measurements from the Earth's core (see kamLAND) show the source of about two-thirds of the heat in the inner core is the radioactive decay of 40K, uranium and thorium. 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; or with heat produced by rearrangement of denser portions to the centre of the earth.
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.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.
When heat is fed into the system from one direction (usually below), at small values it merely diffuses (conducts) from below upward, without causing fluid flow. As the heat flow is increased, above a critical value of the Rayleigh number, the system undergoes a bifurcation from the stable conducting state to the convecting state, where bulk motion of the fluid due to heat begins. 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.
As the temperature difference between the top and bottom of the fluid becomes higher, significant differences in fluid parameters other than density may develop in the fluid due to temperature. An example of such a parameter is viscosity, which may begin to significantly vary horizontally across layers of fluid. 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. Such hexagons are one example of a convection cell.
As the Rayleigh number is increased even further above the value where convection cells first appear, the system may undergo other bifurcations, and other more complex patters, such as spirals, may begin to appear. These may be familiar as examples from systems in which viscosity is relatively low and heat through-put high, such as the spiraling upward flow of gases in a fire.
See also
- Bénard cells
- Fluid dynamics
- Advection
- Vortex dynamics
- Thermomagnetic convection
- Grashof number
- Heat transfer
- Heat conduction
- Thermal radiation
- Convective heat transfer
- Heat pipe
- Churchill-Bernstein Equation
References
1. ^ Frank P. Incropera; David P. De Witt (1990). Fundamentals of Heat and Mass Transfer, 3rd Ed., John Wiley & Sons. ISBN 0-471-51729-1.
External links
Meteorological data and variables |
|---|
| Atmospheric pressure Baroclinity Cloud Convection CAPE CIN Dew point Heat index Humidex Humidity Lifted index Lightning Pot T Precipitation Sea surface temperature Surface solar radiation Surface weather analysis Temperature Theta-e Visibility Vorticity Wind chill Water vapor Wind |
FLUID (Fast Light User Interface Designer) is a graphical editor that is used to produce FLTK source code. FLUID edits and saves its state in text .fl files, which can be edited in a text editor for finer control over display and behavior.
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Liquid is one of the four principal states of matter. A liquid is a fluid that can freely form a distinct surface at the boundaries of its bulk material.
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Characteristics
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Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape. Compared to the solid and liquid states of matter a gas has lower density and a lower viscosity.
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rheid is a solid material that deforms by viscous flow. To be considered a rheid, deformation by flow should exceed elastic deformation by at least a factor of three.
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Types of rheids
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In thermal physics, heat transfer is the passage of thermal energy from a hot to a cold body. When a physical body, e.g. an object or fluid, is at a different temperature than its surroundings or another body, transfer of thermal energy
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Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical systems. Mass transfer includes both fluid flow and separation unit operations.
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- This article is about the physical mechanism of diffusion. For alternative meanings, see diffusion (disambiguation).
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
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Brownian motion (named in honor of the botanist Robert Brown) is either the random movement of particles suspended in a fluid or the mathematical model used to describe such random movements, often called a Wiener process.
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Advection is transport in a fluid. The fluid is described mathematically for such processes as a vector field, and the material transported is described as a scalar concentration of substance, which is present in the fluid.
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Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion (observable movement) of fluids. This can be contrasted with conductive heat transfer, which is the transfer of energy molecule by molecule through a solid or fluid, and radiative heat
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Atmospheric convection or instability is the process by which heat vertically transfers air parcels between layers of the atmosphere. It is often responsible for the formation of clouds, precipitation, and even severe weather.
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EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
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Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.
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Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K.
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WIND (SOLARWIND) was a NASA spacecraft launched on November 1, 1994. It was deployed to study radio and plasma that occur in solar wind, in the Earth's magnetosphere. The spacecraft's original mission was to orbit the Sun at the L1
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- thermal, see thermal (disambiguation).
A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere.
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The CYCLONE, was an early computer built in 1959 by Iowa State University, was based on the Institute for Advanced Study (IAS) architecture developed by John von Neumann. As with all computers of its era, it was a one of a kind machine that could not exchange programs with other
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thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud.
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Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller ocean circulation) by which heat is distributed on the surface of the Earth.
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convection cell is a phenomenon of fluid dynamics which occurs in situations where there are temperature differences within a body of liquid or gas.
Fluids are materials which exhibit the property of flow.
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Fluids are materials which exhibit the property of flow.
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Convection in the most general terms refers to the movement of currents within fluids (i.e. liquids, gases and rheids).
Convection is one of the major modes of heat and mass transfer.
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Convection is one of the major modes of heat and mass transfer.
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Convection ovens or fan ovens augment a traditional oven by circulating heated air using a high temperature fan. Food warms faster in a convection oven since the moving air strips away the thin layer of air which otherwise would surround and insulate the food.
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Aerodynamic heating is the heating of a solid body produced by the passage of fluid (such as air) over the body. It is a form of forced convection in that the flow field is created by forces beyond those associated with the thermal processes.
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Salinity is the saltiness or dissolved salt content of a body of water. Salinity in Australian English and North American English may refer to salt in soil (see soil salination).
Water salinity
Fresh water Brackish water Saline water Brine
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Definition
Water salinity
Fresh water Brackish water Saline water Brine
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thermohaline circulation (THC) is the global density-driven circulation of the oceans. Derivation is from thermo- for heat and -haline for salt, which together determine the density of sea water.
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Earth's oceans
(World Ocean)
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(World Ocean)
- Arctic Ocean
- Atlantic Ocean
- Indian Ocean
- Pacific Ocean
- Southern Ocean
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equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
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Salinity is the saltiness or dissolved salt content of a body of water. Salinity in Australian English and North American English may refer to salt in soil (see soil salination).
Water salinity
Fresh water Brackish water Saline water Brine
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Definition
Water salinity
Fresh water Brackish water Saline water Brine
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