Convection

Convection (from the Latin convectum, cosponsored ') is the entrainment by a flow. Thermal convection ( often simply convection) means both the entrainment of thermal energy ( one of the three modes of heat transfer ), and a flow as a result of heat flow in the presence of a gravitational field (right). Explains Simplified thermal convection is a change of location of readily mobile liquid or gaseous particles that lead to heat stored with it.

Also when carrying other physical quantities (e.g., pulse or vorticity ) or solute is called convection.

Mechanisms

Gravity and density differences

Flow is driven by gravity and density differences in the fluid. The flow thus caused is called natural convection or free convection. The differences in density resulting from temperature differences or differences in concentration. The different volumes of equal masses then leads to different buoyancy.

If the density differences caused by different material densities, this chemical convection in solutions solutale convection in salt solutions haline convection, or in conjunction with thermal convection and thermohaline convection is called.

In polar regions of the sea water freezes to ice cover. The sea salt contained in the sea water remains in the water under the ice, which increases in this area, the salt concentration. This highly concentrated salt water has a higher density and decreases in areas where the existing sea water has the same density as a result of lower temperature. The bottom displaced water flows up to under the ice and the process begins again. The result is a thermohaline convection and thermohaline circulation.

Surface tension ( Marangoni convection )

Marangoni convection is a flow arising from differences in the interfacial tension. This leads, inter alia, for the stabilization of foams. Cause of the difference in interfacial tension can be, for example, changes in temperature, the concentration of dissolved substances (eg detergents) or the charge density ( electrocapillarity ) along the interface. In this way, the fluid flows along the interface from the location in the direction of the lowered surface tension of the locally increased interfacial tension, which can be caused for example by a decrease in detergent concentration. Consequently, there is a reduction of the gradient of surface tension. A parameter for the Marangoni convection is the Marangoni number.

Watch can be the Marangoni convection when small soot particles floating in liquid wax of a candle. In the vicinity of the flame, the surface of the liquid wax is hotter than further outwards on the edge of the candle. Since, in general decreases the surface tension with increasing temperature, the interfacial tension is less dense than the flame at the edge of the candle. Characterized the liquid is torn outwardly near the surface and decreases with wax, which is thereby driven to a circular movement. This is visible through the soot particles.

Another good example are the so-called " tears " on the inner wall of a wine-filled glass. Particularly relevant for industrial applications, the Marangoni convection in processes with high gradients, such as in semiconductor manufacturing, or during welding.

External mechanical action

External mechanical action can be done for example by pumps or fans. There are generated differences in pressure, which in turn cause a flow of the fluid.

Further mechanisms

Other driving forces are pulses, magnetic and electric fields. They arise, for example, by electrical voltages or currents. The pulse is transferred to the fluid across the interface, or is applied to the volume of the fluid.

Transfer and exchange processes

In the convection physical quantities be transported and transferred via the interface layer to adjacent bodies or fluids or replaced with this. These processes are dependent on

• The material properties such as thermal conductivity or density,
• The shape of the body, such as tubular, flat plate, or irregular surface shape and
• Characterized the affected flow, which may be laminar or turbulent.

With the convection following transfer and exchange processes take place:

• Energy and entropy are transmitted by heat conduction from the flowing fluid and generated by the dissipation due to friction. Furthermore, energy and entropy can also be transmitted through phase transitions and mass transfer.

• Materials and electric charges are exchanged including through diffusion, phase transition (e.g., drying, sorption, evaporation, solidification ), dissociation, ionization, and friction.

If chemical reactions, the transported quantities are also affected. It additionally arise entropy, momentum, and chemical reaction products. In addition, the wall may act as a catalyst.

Some of the processes such as freezing and evaporation, found mostly or only when accompanied by convection instead.

Calculations and simulations

If sufficient information about the fluid -soaked body, the flow and other factors known, can be calculated all the streams of the physical quantities taking into account the multiple interactions via suitable equations, often this is just in turbulent currents only with the help of empirically determined models and approximations and only with some uncertainty possible. Carried out simulations of computer systems, graphical representations of the calculated fields (eg temperature fields, currents ) make the results graphically.

Described Basically, the exchange processes by balance equations are. These indicate for each volume, so each spatial area freely chosen, the existing value changes there. This value changes arising from the inflowing streams ( with sign) of the transported quantities. Thus, the balance equations allow the calculation of the fields. The different boundary layer equations are used to calculate the currents through the boundary layer between the fluid and one -soaked body. Crucial for a meaningful simulation of a convective transport process is the selection of appropriate models and equations appropriate to the specific problem. But decisions are particular to take what influences are negligible, since a complete calculation of real processes is usually not possible or at least difficult to install.

For the description and calculation of convective processes including various dimensionless parameters and equations of fluid mechanics, thermodynamics and other physics departments are used. Of central importance is the Navier -Stokes equations. The investigation of convective processes is multidisciplinary in fluid mechanics and other scientific fields.

The fluid is a mixture of substances, it requires a separate analysis of the individual components.

Examples

• A swimmer glides through cool water. The water flows over the body opposite to the advancing direction. The swimming strokes of the arms and legs lead to additional flows relative to these body parts. Occur on uneven laminar and turbulent flows. The release of heat energy to the water is mainly influenced by body temperature, the heating due to metabolism (exothermic chemical reaction), the line -like and convective transport of heat in the body and the heat exchange with and transport in the water. Replace the swimming strokes due to friction and pressure differences of momentum between water and body. The friction of the water in the boundary layer to the body surface produces heat energy and entropy and thus reduces heat loss from the body slightly. In addition, convective processes also occur between body and air ( including respiration and evaporation).
• A thin layer of a nematic liquid crystal is subjected to a temperature field, or an electric field. Under appropriate conditions are met, a by the temperature field or the electric field ( electric convection) a driven Konvektionströmung. With a medium strength of the field is convection rolls form in the anisotropic layer, high strength of the field, the pattern solved by the transition to turbulent flows on.
• In the growth of single crystals of metal alloys, the desired uniform crystal growth can be disturbed by convective processes during solidification of the melt, but also be influenced consciously. These processes, in addition to natural convection ( due to thermal and concentration differences ), the Marangoni convection (heat flow in the direction of high surface tension) and with inductive heating, or other moving magnetic fields and electromagnetic convection.