Convection (heat transfer)
Convection (from the Latin convehere , 'to gather ', 'to bring together') or heat flow is one of the three mechanisms for heat transfer of energy from one place to another , along with heat conduction and heat radiation . Convection is always linked to the transport of particles that carry their energy with them, which is why the term heat transfer is also used. Consequently, there can be no convection in non- permeable solids or in a vacuum . Convection can hardly be avoided in gases or liquids .
Solid particles in fluids can also be involved in convection, see e.g. B. fluidized bed . Solid bodies can therefore also transport heat energy through movement if they absorb it in one place and later release it in another, but this is not convection in itself. Only the flow of a fluid enables convection.
In connection with flows, in addition to the heat transfer by convection (heat flow) described here, other convective processes take place, which transfer other physical quantities in addition to energy.
General
Convection is caused by a current that carries particles. The cause of the transporting flow can be different forces, such as B. gravity or forces resulting from pressure , density , temperature or concentration differences.
One differentiates between the
- Forced convection , in which the particle transport is caused by external influences, for example a fan or a pump , and
- free or natural convection , in which the particle transport is caused exclusively by the effects of the temperature gradient .
Free convection due to differences in thermal density: When heated, substances usually expand (exception, e.g. the density anomaly of water ). Under the action of gravitational force, areas with lower density rise against the gravitational field ( static 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.
Convection without mass transfer
The picture shows the temperature profile in a solid wall with convective heat transfer on both sides. No atoms are moved in the wall, so there is heat conduction there.
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, as they cannot be measured or are difficult to determine and often change at a high frequency .
The heat flow is described by the heat transfer coefficient α or the dimensionless Nusselt number Nu .
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 should therefore be aimed for for optimal use. In the case of forced convection, on the other hand, the orientation in the room is arbitrary, as the flow is normally dimensioned structurally so that the proportion of unavoidable free convection is irrelevant.
Numerical simulation of convection over a horizontal plate - stream filaments ...
Same for a vertical plate - vertical speed component
... and temperature field
... and temperature field
Since the parameters that characterize 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 room heaters for each type and size under different operating conditions with fixed boundary conditions must be determined individually by measurement. A computational simulation, on the other hand, is even more complex and, above all, less precise, even with today's high-performance computers.
The advantage of free convection is that the heat is transported without additional drive energy and apparatus, however, 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 heat transport with fluids over long distances is disadvantageous for both types of convection because of the thermal losses, for example in district heating .
A circulation system is also possible with free convection 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 with increasing temperature difference and vice versa.
The heat transfer can be much 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 ). There is also 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.
Convection in a horizontal layer
A fluid standing over a heated horizontal surface (example: air over a heated surface of the earth, water in a saucepan) does not flow over the surface with a very small temperature difference and no external influences. There is only heat conduction and heat diffusion. With a higher temperature difference, convection currents form in the form of roll-shaped or hexagonal structures, the convection cells or Bénard cells. If the temperature difference increases further, the structures become turbulent, see granulation (astronomy) .
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 mixture of substances 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 ( see evaporative cooling ).
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 buoyancy or downforce if the temperature difference is too small.
The process is characterized in that the heat is superimposed by a mass transfer. Both follow roughly the same laws, which is called the "analogy between heat and mass transfer". This is also expressed in the mathematical description: the heat transport is described by Fourier 's law , the mass transport by Fick's law , which are formally the same, differ only in the variables temperature or concentration and the respective transition resistances.
In the case of liquids that are immiscible with one another, for example water and oil , the processes at low flow velocity 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.
If both fluids are miscible with each other, as is always the case with gases, there is no interface that could stabilize the interface. 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 . There is a strong temperature gradient from the flame core to the outside, through which the flame gases rise, "suck in" the surrounding air and "carry it along" upwards. Even with relatively small differences in flow velocity, turbulence and consequent mixing takes place.
Large differences in density of gases can stabilize a boundary layer despite a large temperature gradient, for example the sulfuric acid clouds of Venus have a mostly structureless surface and printed circuit boards are visibly immersed in the hot Galden ™ vapor during vapor phase soldering.
Examples
Free convection
- Gulf Stream : From the Caribbean, warm surface water is first transported along the east coast of the USA, then further in a north-easterly direction across the Atlantic past Ireland. By evaporative losses and increasing the salt concentration involved is the water specific gravity and falls in Iceland in depth. Without this "hot water heating", the temperatures in Europe would be as low as in central Canada.
- The earth's atmosphere and the oceans or seas form a 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 heat sinks (the sun remote side of the earth and regions near the pole), circulation. Air is warmed up on the warm ground and rises, a decisive factor for the formation of wind , clouds and thunderstorms. Large-scale horizontal heat transport is also known as advection .
- In the temperature-related density stratification of lakes , at times of cooling on the surface (at night and in autumn) vertical convection currents occur between the upper and lower water layers .
- In the interior of the earth , rocks are conditionally fluid and transport heat over a long period of time. The mantle and the outer core of the earth also form convection systems when viewed over geological time periods. These are the cause of plate tectonics and thus earthquakes and volcanoes . 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 .
- In stars and cooling planets , convection transports thermal energy from within to the outside.
- The granular structure of the sun's surface is created by ascending and descending material in the outer areas of the sun. Hotter and 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.
- 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 up into the radiators by convection, where it cools down and flows down again.
- Free convection of air occurs on the outside of radiators , underfloor heating and other components: Air expands when it is heated and is forced upwards due to the increased static buoyancy . The cooler air flows from below over the floor and walls.
- Solar tower , updraft power plant : generation of electrical energy from free convection currents.
- When gliding , flight energy is u. a. obtained from thermal updraft , the so-called thermals .
- In the chimney ( chimney ), convection ensures that the hot combustion exhaust gases are always carried to the outside by the buoyancy ( chimney effect ). The chimney must be dimensioned in such a way that a sufficient buoyancy flow is maintained despite the heat dissipation via the inner wall. This is achieved through a suitable height and suitable diameter.
- In residential buildings, the joint ventilation effect ensures that warm air escapes through the upper joints and cold air flows in through the lower gaps.
- Laundry drying on a line: like hair drying, but free convection (evaporation cools, air flows downwards)
- When a refrigerator is opened, cold air flows out from below. In return, warm air flows into the upper part of the door opening.
- With a heat pipe , large amounts of energy can be transported with little effort and in a small space. This enables effective cooling.
Forced convection
- Cooling of computer processors with fans.
- Water cooling of motor vehicle engines
- When drying hair with a hairdryer , convection is forced by a fan.
- Hot water heating : Here, circulation pumps ensure that the hot water is also distributed to the remote components of the heating system.
- The coils of large generators have to be cooled. The coils in the stator are cooled with water. The coils in the rotor, on the other hand, use hydrogen , which circulates through the generator housing under a pressure of up to 10 bar and gives off its heat in a downstream heat exchanger .