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Encyclopedia > Convection

Convection in the most general terms refers to the movement of currents within fluids (i.e. liquids, gases and rheids) A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. ... For other uses, see Liquid (disambiguation). ... Gas phase particles (atoms, molecules, or ions) move around freely Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape and without a definite volume. ... In geology, a rheid is a solid material that deforms by viscous flow. ...


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.[1] In thermal physics, heat transfer is the passage of thermal energy from a hot to a cold body. ... 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. ... diffusion (disambiguation). ... Three different views of Brownian motion, with 32 steps, 256 steps, and 2048 steps denoted by progressively lighter colors. ... Advection is the transport of a conserved scalar quantity that is transported in a vector field. ...


A common use of the term convection relates to the special case in which heat is advected (carried). 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. For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ...

Contents

Natural convective heat transfer

Main article: Convective heat transfer#Natural convective heat transfer

When part of a fluid expands due to heating, its density is reduced relative to the rest of the fluid, causing a net upward buoyant force due to the effect of gravity on the surrounding cooler fluid. When heat is carried by the circulation of fluids due to this effect, the process is known as natural convective heat transfer. This article or section does not cite any references or sources. ... In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i. ...


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.


Forced convection

Main article: Convective heat transfer#Forced convective heat transfer (advective heat transfer)

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. This article or section does not cite any references or sources. ... Convection is the transfer of heat by the motion of or within a fluid. ... Convection ovens use heated air that is forced into the oven by fans located in the back of the oven, generally for cooking food. ... Aerodynamic heating is the heating of a solid body produced by the passage of fluid (such as air) over the body. ...


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). Annual mean sea surface salinity for the World Ocean. ... A simplified summary of the path of the Thermohaline Circulation. ...


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. Animated map exhibiting the worlds oceanic waters. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe. ... Annual mean sea surface salinity for the World Ocean. ... The thermohaline circulation is a term for the global density-driven circulation of the oceans. ... A simplified summary of the path of the Thermohaline Circulation. ...


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 experiences 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. Mantle convection is the slow creeping motion of Earths rocky mantle in response to perpetual gravitationally unstable variations in its density. ... Earth cutaway from core to exosphere. ... The tectonic plates of the world were mapped in the second half of the 20th century. ...


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. Image:Kamland detector. ... Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ...


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. Earths atmosphere is the layer of gases surrounding the planet Earth and retained by the Earths gravity. ... Animated map exhibiting the worlds oceanic waters. ... This article is about the astronomical term. ... Earth cutaway from core to exosphere. ... This article is about weather phenomena. ... For other uses, see Black hole (disambiguation). ...


Pattern formation

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. Image File history File links Broom_icon. ... Image File history File links Convection1. ... Image File history File links Convection1. ... In mathematics, the Fourier transform is a certain linear operator that maps functions to other functions. ... Convection cells Bénard cells are convection cells that appear spontaneously in a liquid layer when heat is applied from below. ... The science of pattern formation deals with the visible, (statistically) orderly outcomes of self-organisation and the common principles behind similar patterns. ...


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. In fluid mechanics, the Rayleigh number for a fluid is a dimensionless number associated with the heat transfer within the fluid. ... In mathematics, specifically in the study of dynamical systems, a bifurcation occurs when a small smooth change made to the parameter values (the bifurcation parameters) of a system causes a sudden qualitative or topological change in the systems long-term dynamical behaviour. ... Sphere symmetry group o. ... In fluid dynamics, the Boussinesq approximation is used in the field of buoyancy-driven flow. ...


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. For other uses, see Viscosity (disambiguation). ... A convection cell is a phenomenon of fluid dynamics which occurs in situations where there are temperature differences within a body of liquid or gas. ...


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. In fluid mechanics, the Rayleigh number for a fluid is a dimensionless number associated with the heat transfer within the fluid. ... This article does not cite any references or sources. ...


See also

Bénard cells are obtained in a simple experiment that Bénard, a French physicist, conducted in 1900. ... Fluid dynamics is the sub-discipline of fluid mechanics dealing with fluids (liquids and gases) in motion. ... Advection is the transport of a conserved scalar quantity that is transported in a vector field. ... In 1858 Hermann von Helmholtz published his seminal paper entitled Ueber Integrale der hydrodynamischen Gleichungen, welche den Wirbelbewegungen entsprechen, in Journal für die reine und angewandte Mathematik, vol. ... Ferrofluids can be used to transfer heat, since heat and mass transport in such magnetic fluids can be controlled using an external magnetic field. ... The Grashof number is a dimensionless number in fluid dynamics which approximates the ratio of the buoyancy force to the viscous force acting on a fluid. ... In thermal physics, heat transfer is the passage of thermal energy from a hot to a cold body. ... Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and hence acts to even out temperature differences. ... Radiant heat redirects here. ... This article or section does not cite any references or sources. ... A heat sink (aluminium) with heat pipe (copper) A heat pipe is a heat transfer mechanism that can transport large quantities of heat with a very small difference in temperature between the hotter and colder interfaces. ... In convective heat transfer, the Churchill Bernstein equation is used to estimate the surface averaged Nusselt number for a cylinder in cross flow at various velocities. ...

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. 

John Wiley & Sons, Inc. ...

External links

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

  Results from FactBites:
 
Educator's Guide to Convection (567 words)
The convection that students are most likely to have observed is in cumulonimbus clouds or "thunderheads." These towering vertical clouds can be seen to evolve over a few minutes.
Thunderheads and granulation are large-scale examples of convection.
Convection acts as described in the examples above where gravity's effects are present (so that warm, low density fluids can rise and cool, high density fluids can fall).
  More results at FactBites »

 
 

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