convection

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Convection (from latin convehere , induce bring ') or flow transport the transportation is physical state variables in flowing gases or liquids . Physical state variables are, for example, entrained heat , matter or momentum . The convective transport of thermal energy is a mechanism of heat transport and is also called heat transfer .

If a static buoyancy occurs as the cause of the flow due to temperature differences , this is called thermal convection, natural convection , free convection or heat flow . In addition, the flow can e.g. B. caused by pumps or fans or arise from thermodynamic imbalances, this is called forced convection .

Convection cells in a vessel heated from below

In the Anglo-Saxon language area, convection denotes any movement of molecules within a fluid and therefore includes not only pure advection through flow but also diffusion through movement at the atomic level (i.e. within fluids or solid matter).

Mechanisms

Static buoyancy

→ Main article : Natural convection

Differences in density in the fluid lead to static buoyancy in the gravitational field . The density differences can be caused by a temperature difference or different material densities . The movement driven in this way is called natural or free convection.

If the density differences are caused by different material densities , this is called chemical convection, in solutions also solute convection, in salt solutions also haline convection or in connection with thermal convection also called thermohaline convection. If the differences in density are caused by an accumulation of microorganisms on the surface of the liquid, we speak of bioconvection .

Examples

Water is heated in a saucepan on the stove. The floor is heated, the side walls are insulated and the water cools down on the surface through evaporation or the outside temperature. As a result of the warming, water rises with a lower density, water cooled above sinks. Convection cells form, as shown schematically in the picture above. Such an arrangement is called Rayleigh-Bénard convection .

In metorology, numerous phenomena are related to natural convection:

• In thermal conditions , the air on the ground is heated and rises.
• Rising moist air can lead to cloud formation (especially cumulus and cumulonimbus ) and thunderstorms .

External mechanical impact

Centrifugal fan

→ Main article : Forced convection

When the flow is driven by forces outside the fluid, it is called forced convection. This occurs, for example, with pumps or fans .

If there are temperature and thus density differences with forced convection, the same forces act as with free convection. The Archimedes number then indicates the ratio of free to forced convection.

example

A circulation pump transports warm water from the heating system to the radiators.

Magnetohydrodynamics

Magnetic and electric fields can act as additional driving forces . This is formulated mathematically in magnetohydrodynamics .

Examples

• The earth's magnetic field is generated by the dynamo effect .
• The structure of the sun's corona , especially sunspots , is explained with the MHD.

Surface tension (Marangoni convection)

The Marangoni effect causes the wine tears, which can be seen here in the shadow of the wine glass.

As Marangoni convection is called a flow generated by the gradient of the interfacial tension arises. The cause of the different interfacial tension can e.g. B. be a temperature gradient or concentration gradient of dissolved substances along the interface. The fluid flows along the interface in the direction of the greater stress. The Marangoni number , which can be understood as the ratio of interfacial tension to viscosity, is suitable as a key figure for characterizing Marangoni convection .

Examples

The Marangoni convection can be observed when small soot particles float in the liquid wax of a candle. The surface of the liquid wax is hotter near the flame than further out on the edge of the candle. Since the interfacial tension generally decreases with increasing temperature, the interfacial tension close to the flame is lower than at the edge of the candle. This tears the surface outwards and takes wax close to the surface with it, which is driven into a circular motion. This becomes visible through the soot particles.

Another well-known example are the so-called wine tears. Due to the adhesion , a thin film of liquid creeps up the surface of the glass. Since alcohol evaporates faster than water, the alcohol concentration decreases towards the top and therefore the surface tension increases, further liquid flows in until gravity prevails. Running down liquid with high surface tension contracts into narrow rivulets as it traverses the zone with low surface tension.

The Marangoni effect plays a key role in stabilizing liquid foams . The surface tension gradient induced by a disruption of the foam film surface causes a convective flow of the interlamellar fluid that heals the disorder.

The Marangoni effect is also important for processes in metal processing with high temperature gradients, such as B. in semiconductor manufacture or welding.

Mathematical description

The substantial discharge in a fluid consists of the local and the convective discharge. In Cartesian coordinates, the following applies to a fluid property : ${\displaystyle \Phi }$

${\displaystyle {\frac {{\text{d}}\Phi ({\vec {x}},t)}{{\text{d}}t}}=\underbrace {\frac {\partial \Phi }{\partial t}} _{\mbox{lokal}}+\underbrace {({\vec {v}}\cdot {\vec {\nabla )}}\Phi } _{\mbox{konvektiv}}}$

The convection term occurs in this form, especially in the convection-diffusion equation .

There is special in the Navier-Stokes or Euler equations with the fluid velocity . So the term is convective acceleration . ${\displaystyle \Phi ={\vec {v}}}$ ${\displaystyle {\vec {v}}={\vec {v}}(x,y,z)}$ ${\displaystyle ({\vec {v}}\cdot {\vec {\nabla }}){\vec {v}}}$

Transfer and exchange operations

With convection, physical quantities are transported. Some of these variables can be transferred to or exchanged with adjacent bodies or fluids via the boundary layer (in particular the temperature). These operations depend on

• the material properties, such as B. the thermal conductivity or the density,
• the shape of the body, such as B. tube, flat plate or irregular surface shapes and
• the flow influenced by it, which can be laminar or turbulent .
• possibly other influences (e.g. gravitation)

The following transmission and exchange processes can take place with convection:

• Energy and entropy are transferred from the flowing fluid by conduction or generated by dissipation as a result of friction. Furthermore, energy and entropy can also be transferred through phase transitions and mass transfer.
• Substances and electrical charges are exchanged u. a. through diffusion , phase transition (e.g. drying , sorption , evaporation , solidification ), dissociation , ionization and friction .

If chemical reactions occur, the transported parameters are also influenced. Entropy, momentum and chemical reaction products also arise. The wall can also act as a catalyst.

Some of the processes mentioned, such as solidification and evaporation, take place mostly or only when convection occurs at the same time.

Further examples

• A swimmer slides through cool water. The water flows over the body against the direction of movement. The strokes of the arms and legs create additional currents relative to these body parts. Uneven laminar and turbulent flows occur. The transfer of heat energy to the water is primarily influenced by body temperature, heating through metabolism (exothermic chemical reaction), the conduction-like and convective transport of heat in the body and the heat exchange with and transport in the water. The strokes exchange momentum between the water and the body due to friction and pressure differences. The friction of the water in the boundary layer to the body surface produces thermal energy and entropy and thus slightly reduces the body's heat emission. In addition, convective processes also occur between the body and air (including breathing and evaporation).
• A thin layer of a nematic liquid crystal is exposed to a temperature field or an electric field. Under suitable conditions, a convection flow driven by the temperature field or the electric field (electric convection) occurs. With medium strength of the field, convection rollers form in the anisotropic layer; with high strength of the field, the patterns dissolve through the transition into turbulent flows.
• When growing single crystals from metal alloys, the desired uniform crystal growth when the melt solidifies can be disturbed by convective processes, but it can also be deliberately influenced. In addition to natural convection (thermal and as a result of concentration differences), these processes are also Marangoni convection (melt flows in the direction of high surface tension) and, with inductive heating or other moving magnetic fields, also electromagnetic convection.

literature

• Michael Jischa: Convective impulse, heat and material exchange . Vieweg, Braunschweig / Wiesbaden 1982, ISBN 3-528-08144-9 .
• Ulrich Kilian, Christine Weber [Red.]: Lexicon of Physics in six volumes . tape 3 . Spectrum, Akademischer Verlag, Heidelberg 1999, ISBN 3-86025-293-3 .

Web links

Wiktionary: convection  - explanations of meanings, word origins, synonyms, translations

Commons : Convection  - collection of images, videos and audio files

Individual evidence

1. ↑ convection. In: Duden online. Retrieved June 11, 2019 . 
2. ^ Script by M. Fraaß, Beuth University
3. ^ Ingo Rehberg: Pattern formation in hydrodynamic systems. (PDF, 3.28 MB) 1994, accessed on January 6, 2015 (25th IFF Jülich holiday course: Complex systems between atom and solid). 
4. ↑ Holger Bitterlich: Breeding and physical properties of rare earth transition metal single crystals. Dissertation, Technical University of Dresden. 2000, accessed January 6, 2015 .