Convection in the most general terms refers to the movement of molecules within fluids (i. FLUID ( F ast L ight '''U'''ser '''I'''nterface D esigner is a graphical editor that is used to produce FLTK Source code e. liquids, gases and rheids). Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of This page is about the physical properties of gas as a state of matter 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 thermal physics, heat transfer is the passage of Thermal energy from a hot to a colder body Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of Atoms and Molecules 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. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement This article is about the physical phenomenon for the stochastic process see Wiener process. Advection, in mechanical and chemical engineering is a transport mechanism of a substance or a conserved property with a moving 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 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). In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature In one of two major types of heat convection, the heat may be carried passively by fluid motion which would occur anyway without the heating process (a heat transfer process termed loosely as "forced convection"). Convection in the most general terms refers to the movement of molecules within Fluids (i In the other major type of heat convection, heating itself may cause the fluid motion (via expansion and buoyancy force), while at the same time also causing heat to be transported by this motion of the fluid (a process known loosely as natural convection, or "free convection"). Convective heat transfer is a mechanism of Heat transfer occurring because of bulk motion (observable movement of fluids In the latter case, the problem of 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.

Natural convective heat transfer

Papers blowing due to air current from radiator

Main article: Convective heat transfer

When heat is carried by the circulation of fluids due to buoyancy from density changes induced by heating itself, then the process is known as free or natural convective heat transfer. Convective heat transfer is a mechanism of Heat transfer occurring because of bulk motion (observable movement of fluids Convective heat transfer is a mechanism of Heat transfer occurring because of bulk motion (observable movement of fluids

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 full of hot water with red food dye may be placed in a fish tank with cold, clear water. The convection currents of the red liquid will be seen to rise and fall, then eventually settle, illustrating the process as heat gradients are dissipated.

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. Convective heat transfer is a mechanism of Heat transfer occurring because of bulk motion (observable movement of fluids Convective heat transfer is a mechanism of Heat transfer occurring because of bulk motion (observable movement of fluids 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. Convection ovens or fan ovens or turbo ovens augment a traditional Oven by circulating heated air using a fan. Aerodynamic heating is a form of forced convection. Aerodynamic heating is the heating of a solid body produced by the passage of fluid (such as air over a body such as a Meteor, Missile, or Airplane. 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.

In zero-g environments 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. Weightlessness is a phenomenon experienced by people during Free-fall. 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. ^[2]

Buoyancy induced convection not due to heat

The general term for this phenomenon is gravitational convection. Gravitational convection is a general term for buoyancy-induced convection in a gravity field especially that not due directly to heat Gravitational heat convection is the same as free convection. However, differential 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, diffusion of a source of dry salt downward into wet soil assisted by the mechanism of the fact that saline is heavier than fresh water, is a type of gravitational convection ^[3] 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). Salinity is the Saltiness or dissolved salt content of a body of Water. The term thermohaline circulation (THC refers to the part of the large-scale ocean circulation that is thought to be driven by global density gradients created by surface heat and 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. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. Warm water from the Equator tends to circulate toward the poles, while cold polar water heads towards the Equator. The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the A geographical pole, or geographic pole, is either of two fixed points on the surface of a spinning body or Planet, at 90 degrees from the Equator, based 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. Salinity is the Saltiness or dissolved salt content of a body of Water. The term thermohaline circulation (THC refers to the part of the large-scale ocean circulation that is thought to be driven by global density gradients created by surface heat and The term thermohaline circulation (THC refers to the part of the large-scale ocean circulation that is thought to be driven by global density gradients created by surface heat and 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

Main article: mantle convection

Convection within Earth's mantle is the driving force for plate tectonics. Mantle convection is the slow creeping motion of Earth's rocky mantle in response to perpetual gravitationally unstable variations in its density The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere 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.

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. The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND is an experiment at the Kamioka Observatory, an underground Neutrino observatory near Toyama Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 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 center 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. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided Current movement during convection may be invisibly slow, or it may be obvious and rapid, as in a hurricane. A tropical cyclone is a storm system characterized by a low pressure center and numerous Thunderstorms that produce strong winds and Flooding 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. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e

Pattern formation

The left picture represents the thermal field and the right picture is its two-dimensional Fourier transform of a fluid under Rayleigh-Bénard convection. This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and Bénard cells are Convection cells that appear spontaneously in a liquid layer when heat is applied from below ^[4]

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. The science of pattern formation deals with the visible ( statistically) orderly outcomes of Self-organisation and the common principles behind similar

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. In Fluid mechanics, the Rayleigh number for a fluid is a Dimensionless number associated with buoyancy driven flow (also known as Free convection or natural Bifurcation theory is the mathematical study of changes in the qualitative or Topological structure of a given family 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. Symmetry generally conveys two primary meanings The first is an imprecise sense of harmonious or aesthetically-pleasing proportionality and balance such that it reflects beauty or This is known as Boussinesq convection. In Fluid dynamics, the Boussinesq approximation (named for Joseph Valentin Boussinesq) is used in the field of buoyancy-driven flow (also known as Natural

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. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. 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. A convection cell is a phenomenon of Fluid dynamics that occurs in situations where there are Temperature differences within a body of Liquid or

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. In Fluid mechanics, the Rayleigh number for a fluid is a Dimensionless number associated with buoyancy driven flow (also known as Free convection or natural In Mathematics, a spiral is a Curve which emanates from a central point getting progressively farther away as it revolves around the point

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

External links

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