# Can flow air through solids

convection (from lat. convehere = carry along, take with you) is, in addition to thermal conduction and thermal radiation, a mechanism for transporting thermal energy. Convection is characterized by the fact that the heat transfer is brought about by the transport of particles that carry thermal energy with them. Consequently, there can be no convection in solids or in a vacuum. Convection occurs e.g. in fluids, i.e. in gases or liquids. Solid particles in fluids can also be involved in convection, see e.g. fluidized bed.

Instead of heat you can convection also transport other properties or quantities such as electrical charge.

### General

Convection is caused by a current that carries particles. The cause of the transporting flow can be different forces, such as gravity or forces that result from pressure, density, temperature or concentration differences.

A distinction is made between the

• free or natural convection, in which the particle transport is brought about exclusively by the effects of the temperature gradient, for example by the upward or downward force of the fluid as a result of the density differences caused by the temperature change, and
• forced convectionin which the particle transport is caused by external influences, for example a fan or a pump.

Free convection due to thermal density differences: When heated, substances usually expand (exception, e.g. the density anomaly of water). Under the action of the gravitational force, areas with lower density rise against the gravitational field (buoyancy) within a fluid, while areas with higher density sink in it.

If heat is supplied on the underside and there is the possibility of cooling on the upper side, a continuous flow is created: the fluid is heated, expands and rises upwards. Once there, it cools down, contracts again and sinks to be heated again below.

Examples of natural convection:

• If the central heating boiler is installed at the lowest point of the heating system, it can work without a circulation pump (gravity heating); the warm water rises through convection into the radiators, where it cools down and flows down again. However, this leads to an uneven heat supply in branched heating systems. In most cases, therefore, circulation pumps ensure that the hot water is also distributed to the more remote parts (forced convection).
• Air is heated on the warm ground and rises - a decisive factor for the development of wind and thunderstorms.
• Convection of molten rocks (magma) in the earth's interior (high viscosity and low flow velocities), responsible for plate tectonics and thus for earthquakes and volcanoes. Ascending convection centers in the earth's mantle are called plumes or hot spots.
• In stars, convection transports thermal energy from within to the outside.

### Convection without mass transfer

Convection occurs, for example, when the fluid flows over the surface of another volume and the temperature is equalized in the process. In the simplest case, the “other” volume is a solid, the boundary surface of which is static and consequently the convection represents a pure heat exchange. This is a heat transfer from a solid surface to, for example, air, water or other fluids.

The picture shows the temperature profile in a solid wall with convective heat transfer on both sides.

While pure heat conduction with a linear temperature profile takes place in a solid body, heat transport in the fluid takes place within a thermal boundary layer. Due to the local flow velocity, which must be zero directly on the wall, there is initially also heat conduction in the fluid close to the wall, which is continuously superimposed by mixing processes, so that the linear temperature profile close to the wall changes into a non-linear one, regardless of whether in which direction the heat flows.

The convection is determined here by the "boundary layer", the layer between the two volumes in which the physical parameters differ from those of the two volumes. The essential parameters are the temperature and the composition of the substances, as well as the flow rate. Each of these parameters forms its own boundary layer. In the case of convection between fluids, the determination of the boundary layers is usually very difficult or even impossible, since they cannot be measured or are difficult to determine and often change at a high frequency.

The heat flow is determined by the heat transfer coefficientα or the dimensionless Nusselt numberNu described.

Naturally, with free convection, the direction of the flow is given by gravity, because the flow is caused by differences in density and thus weight. A vertical alignment of the surface of the solid body is therefore desirable for optimal use. In the case of forced convection, on the other hand, the orientation in the room is arbitrary, since the flow is normally dimensioned structurally in such a way that the proportion of unavoidable free convection is irrelevant.

Since the parameters characterizing the heat flow (temperature differences, density differences, lift / downforce, flow velocities) influence each other in the latter, the determination of the heat transfer of technical components is very complicated. For example, the power measurement on space heaters for each type and each size under different operating conditions under fixed boundary conditions must be determined individually by measurement. A computational simulation, on the other hand, is even more complex and, above all, less accurate, even with today's high-performance computers.

The advantage of free convection is that the heat is transported without additional drive energy and apparatus, but gravity sets limits in the local distribution, since the flow is preferably oriented vertically. The disadvantage is the poor heat transfer, which has to be compensated for by large areas. The transport of heat with fluids over long distances is disadvantageous for both types of convection because of the thermal losses (for example for district heating).

With free convection, a circulation system is also possible if there is a heat source and a heat sink in a closed room (example: room heating, heat pipe), which has a self-regulating effect within certain limits (negative feedback), since the circulation increases as the temperature difference increases and vice versa.

The heat transfer can be considerably more effective, even with free convection, if the fluid has a boiling point in the working temperature range, for example the condenser of a refrigeration machine (the pipe coil on the outside of the back of a household refrigerator, in which the refrigerant condenses on the inside). In addition, there is the advantage that the heat transfer on this side is almost completely isothermal, i.e. the temperature difference to the room air is almost the same throughout the pipe.

### Special case of free convection on a horizontal surface (Rayleigh-Bénard convection)

A fluid standing over a temperature-controlled horizontal surface (example: air over a heated earth's surface, water in a saucepan) does not normally flow over the surface and does not form a boundary layer, because the buoyancy forces are perpendicular to the surface. One can also say that the entire fluid consists of a boundary layer, as the temperature changes upwards to the surface, whereby in the case of the earth's atmosphere a boundary layer only about one millimeter thick develops above the ground and almost all of the heat exchange between air and Floor is realized. This causes warmed molecules to rise and colder ones to sink. Mixing takes place with simultaneous heat exchange until a stable temperature stratification is achieved. With targeted flow guidance, the surface can also be flowed over horizontally and the convection accelerated (example: underfloor heating, thermal power plant), which leads to a circulation flow. Horizontal mass flows such as wind can also lead to the formation of boundary layers.

### Convection with mass transfer

Often the “other” volume is also a fluid itself, which means that the interfaces flow smoothly into one another and, in many cases, there is an exchange of substances in addition to the heat exchange, which means that the substance composition is also brought into line. If the fluid flows over a solid or a substance mixture with a lower saturation vapor or sublimation pressure, this leads to a mass transfer in that the substance whose vapor or sublimation pressure is exceeded diffuses into the fluid (example: drying). A temperature difference is not absolutely necessary for this, but it is beneficial. As a rule, this occurs because the substance that is evaporated or sublimed draws the heat of evaporation from its own solid or liquid phase and thus cools it down, which is also the case with evaporation.

In this case, natural convection can also arise from the fact that the fluid changes its density as a result of the material transport and thus receives the uplift or downforce if the temperature difference is too small.

The process is characterized in that the heat is superimposed by a material transport. Both follow roughly the same law, which is called the "analogy between heat and material exchange". This is also expressed in the mathematical description: the heat transport is controlled by the fourier, the mass transport through the fuckin law which are formally the same, differ only in terms of the variables temperature or concentration and the respective contact resistances.

This then also means that, analogous to the temperature profile in the image, a concentration profile including a corresponding boundary layer is established within the fluid.

### Convection between fluids

Strictly speaking, convective processes between two fluids are always associated with an exchange of substances, since a liquid has a finite saturation vapor pressure and its vapors thus diffuse into a gaseous or liquid boundary layer. Diffusion takes place solely through partial pressure differences. It can be overlaid by mixing or intermixing if there is also a current or is emerging. In contrast to a solid wall, the flow velocity at the interface is not necessarily zero, so that pure heat conduction can be excluded here.

A typical case is a flame, for example a candle or a lighter. Due to the convection of the gases flowing up, their own combustion air flows in from below due to the negative pressure generated. From the flame core to the outside, there is a strong temperature gradient through which the flame gases rise, “suck in” the surrounding air and “carry it along” upwards. This effect also continues above the flame, although it subsides sharply since no further temperature differences are generated here. In this way, a natural chimney is created, i.e. without a fixed limit, which draws in air vertically from below and horizontally from all sides and transports it vertically upwards.

If both fluids are in the same state of aggregation as with the flame, then even with relatively small flow velocity differences in the boundary layer, turbulence and consequent mixing takes place. The interface is then no longer clearly defined and the heat transfer is dominated by the mixing, especially in the case of gases and vapors, which are often miscible with one another or soluble in one another in any ratio.

The swirl is clearly visible when you extinguish a burning candle. The steam flowing up from the now unburned candle tallow condenses quickly and is visible as a stream of very fine droplets, which swirl strongly in contact with the air and ultimately spread widely, making them invisible again.

If a gas flows around or over a liquid, as long as the vapor pressure of the gas is below its saturation vapor pressure, i.e. the gas is not yet saturated, the liquid diffuses into the gas phase. Even if the gas is warmer than the liquid, the liquid cools down because the heat of evaporation is withdrawn from it. Example: air and water. In this case one also speaks of evaporationbecause the gas phase does not consist of pure vapor of the liquid.

In the case of immiscible liquids, such as water and oil, the processes at low flow rate differences are comparable to those on a solid wall; at higher flow rates, droplets can form, which leads to an emulsion. This in turn leads to an increased heat transfer due to an enlargement of the interfaces on the droplets.

### Special case of convection on a surface caused by surface tension (Marangoni convection)

A fluid with a surface, for example a horizontal boundary layer to another fluid that is heated from below, can Marangoni effect or the Marangoni convection demonstrate. If the surface experiences a wave-like disturbance, then wave troughs and wave crests form. Since the wave troughs are closer to the heat source, they have a higher temperature. However, at higher temperatures the surface tension decreases. The difference in surface tension between mountains and valleys is the force that drives convection between them. Friction, heat conduction and possibly Rayleigh-Bénard convection counteract this effect. The dimensionless Marangoni number indicates the relationship between Marangoni convection and heat conduction, while the dimensionless Bond number indicates the relationship between Marangoni and Bénard convection.

The Marangoni convection plays a role in weather phenomena, but also poses a problem, for example, in the manufacture of single crystals for semiconductor production.

The Marangoni effect plays a key role in stabilizing liquid foams. Here, 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.

### Examples of convection

• The earth's atmosphere and the oceans or seas form a gigantic system of free convection with a two-phase system air / water, with evaporation / condensation and mixing / segregation (clouds / rain) as well as heat sources (solar heated surfaces on the mainland and the seas) and sinks ( side of the earth facing away from the sun or regions close to the poles), circulation (Gulf Stream) etc. Large-scale horizontal heat transport is also referred to as advection.
• In the temperature-related density stratification of lakes, at times of superficial cooling (at night and in autumn) convection currents occur in the epilimnion, which, due to their energy content, can involve the upper layers of the metalimnion and thus lead to an increasing thickness of the mixed area, ultimately down to the bottom of the lake ("full circulation").
• In the interior of the earth, solids, in this case rocks, are also conditionally fluid and lead to heat transport processes over a long period of time. Both the mantle and the outer core of the earth form convection systems of planetary dimensions. The rock convects in the earth's mantle and, due to the high temperatures on geological time scales, can flow like a liquid (solid body creep). One speaks of a jacket convection through the so-called plumes. In the outer core, the convection of the liquid iron alloy creates the earth's magnetic field.
• The granulation of the sun's surface is caused by ascending and descending gases in the outer areas of the sun. Hotter and therefore brighter material rises in the granules, gives off heat as radiation and sinks again in the darker zones between the granules. In contrast, the sunspots and prominences are a magnetic phenomenon.
• Warm water heating: on the air side with free convection with circulation, on the water side with forced convection or in very simple systems with free convection
• Solar tower, updraft power plant: generation of electrical energy from free convection currents
• Glider flight: Flight energy from updraft, the so-called thermals, on inclined solar-heated earth surfaces (e.g. slopes).
• Chimney (chimney): ensures that as long as the fire is burning, the combustion exhaust gases are always discharged to the outside as a result of the buoyancy that this creates and fresh air flows in (chimney effect).The chimney must be dimensioned in such a way that, in spite of the heat being given off to the inner wall, a sufficient buoyancy flow is maintained, which is achieved by appropriate height and clearance.
• the effect of joint ventilation in residential buildings, in which warm air escapes through the upper joints and cold air flows in through the lower gaps.
• Hair drying (hair dryer): forced convection with evaporation (here more precisely: evaporation)
• Laundry drying (leash): like hair drying, but free convection (evaporation cools, air flows downwards)
• Cooling of processors: Shows the power density for dissipating the power loss:
1. Performance class Intel 8086 to Intel 80486/40: horizontally aligned smooth surface, free convection
2. from performance class Intel 80486/66: Heat sink with vertical flow surfaces without fan, free convection
3. Intel 80486/100, Intel Pentium: how Intel 80486/66, but with an additional fan, forced convection
4. various newer processors or mainframes: water-cooled instead of air-cooled, heat transfer favored due to the increased specific heat capacity of water, forced convection