Heat transfer

Heat transfer or heat transport is the transport of energy in the form of heat across at least one thermodynamic system boundary. There are three types of heat transport processes:

• Thermal conduction through mechanical contact,
• Convection , the entrainment of thermal energy in a flowing medium,
• Thermal radiation , i.e. electromagnetic waves.

The heat transfer by conduction takes place in the direction of the places with lower temperatures , with convection and heat radiation opposing processes are also possible.

The heat transfer between the surface of a solid and a fluid is called heat transfer . The heat transfer is described by the heat transfer coefficient .

Of heat transmission when the heat pipe is viewed through the wall together with the heat transfers to the both surfaces of a fluid through a wall on another fluid is spoken.

A physical quantity of heat transfer is the heat flow .

Technical devices for the transfer of heat, hot heat exchangers or heat exchangers .

species

With regard to the total energy , the heat is always transferred from "warm" to "cold" in three different ways:

1. With heat conduction or conduction , kinetic energy is transferred between neighboring atoms or molecules without material transport. This type of heat transfer is an irreversible process and transports the heat on a statistical mean from the higher energy level (with higher absolute temperature) to the lower level (with lower temperature). The heat transport through the movement of free electrons in the metal is also referred to as heat conduction. Typical examples:

   • With an electric soldering iron , the heat energy from the heating element is transferred a few centimeters to the soldering tip
   • Every refrigerator is "packed" by insulating materials in order to keep the heat conduction of the housing low
   • A radiator is usually made of metal so that the thermal energy of the hot water is well conducted from the inside to the outer surface
   • A metal doorknob feels cold because it dissipates body heat well

2. The thermal radiation according to the Stefan-Boltzmann law is part of the electromagnetic waves . Most of the time, the energy is transported by infrared waves , which are part of the electromagnetic spectrum . In the cosmic, but also in the submolecular range, other wavelengths or frequencies of the electromagnetic spectrum are also involved in the energy transport to a significant percentage. In the case of thermal radiation, there is - on closer examination - not only a heat transfer from warm to cold, but also from cold to warm, because there are no non-radiating surfaces (this would require emissivity  = 0). The heat flow from warm to cold is always greater than the other way around, so that the resultant of both heat flows always shows from warm to cold. In other words: the temperature difference is reduced further and further overall. Thermal radiation is the only type of heat transfer that can also penetrate the vacuum . Typical examples:

   • The earth is heated by the sun through the radiation that the sun emits as heat radiation.
   • A powerful carbon dioxide laser can melt metals with its very bright light. However, this is not about thermal radiation .
   • Thermos flasks are mirrored on the inside so that the contents lose little energy through thermal radiation.

3. With convection or heat flow , heat is carried along by a flowing fluid as internal energy or enthalpy . Convection occurs whenever a flowing fluid absorbs or gives off heat from a surface. In connection with convection, convective cells typically occur , within which the fluid circulates in a circuit between the heat source and heat sink. Convective cells can be very small or very large; large cells can contain many smaller cells. Typical examples:

   • An electric heating rod heats the water in a hot water tank. The heated water is distributed over the entire volume of the storage tank through free convection.
   • The metal of a boiler gives off thermal energy to the liquid flowing by. At the boundary layer between liquid and metal, heat conduction dominates, while the heat is distributed in the flowing liquid by convection. The amount of heat that can be transferred into the liquid by convection is determined by the turbulence of the flow and thus by the geometry of the surface and the speed of the flow. Depending on the internal volume of the boiler, the convective cells are millimeter to centimeter in size.
   • The heated liquid from the boiler can be transported to the radiators by natural convection alone . The cells are then the size of the heating circuits. Nowadays, pumps are mainly used because they allow smaller pipe cross-sections.
   • The wind and thus the weather are determined by the convective planetary circulation . The largest cells are thousands of kilometers long and wide and a few kilometers high.

   As soon as the fluid is forced to move, calculations are no longer based on free convection, but on mass transport, which depends on the delivery rate of the pump or fan and the heat capacity of the medium being transported. The externally induced heat transport is also conceivable with moving solid bodies.

   • An electric heating rod heats the water in a washing machine or dishwasher. The heated water is distributed throughout the interior of the machine by the circulation pump.
   • In a heating system with a pump, the heating water flows to the radiators at the speed specified by the pump. The flow speed in the radiator is greatly reduced, so that free convection also occurs as a result of the cooling on the inner surface of the radiator.
   • In the hair dryer , the heated air flow to the hair is carried out by the mass transport induced by the fan, while the cooling air flow is returned to the hair dryer by the convection in the room.

Usually several types of transmission work together in real systems. Mainly heat conduction, but possibly also heat radiation, takes place inside solids . Heat flow is also possible in liquids and gases . Thermal radiation takes place between surfaces when the medium in between absorbs little thermal energy , ideally in a vacuum. Gases are also largely permeable to thermal radiation ( diatherm ).

Systems in a state of equilibrium (same temperature) also exchange heat. However, the heat emitted and absorbed are the same, which is why the temperatures do not change.

Demarcation

Although objects are heated with dielectric heating and inductive heating , it is not a question of heat transfer because the respective "transmitters" neither emit the energy based on their respective temperature nor does it increase with increasing temperature.

Example: cooling system of an internal combustion engine

In water-cooled internal combustion engines , part of the heat generated during the combustion process is transferred to the wall, is transferred to the heat transport medium water by heat conduction, transported to the cooler by forced convection , where it is released into the air and with it from the engine compartment to the environment.

Under forced convection will be understood the heat transfer mechanism in liquids and gases, in which by macroscopic flow processes (mechanical z. B. driven by propeller pumps or fans) heat in the form of internal energy from one place transported to another.

The heat transfer to the fluid is largely dependent on the shape of the flow. In laminar flows , the lack of transverse movements of the particles means that the heat is transported mainly through heat conduction. In turbulent flow, on the other hand, the heat exchange through mixing movement significantly exceeds that through heat conduction. Since a laminar boundary layer always forms due to friction on the wetted surface of a solid body exposed to the flow, the heat transfer depends largely on the thickness of this boundary layer.

After the heat has been transferred to the fluid, it is transported by the material flow from the internal combustion engine to the coolant cooler. The heat transfer in the cooler takes place according to the same physical principle as in the internal combustion engine. The heat flows through the pipe walls to the cooling fins and from there is absorbed by the air mass flow and transported away.

A laminar boundary layer of the cooling air is also formed on the cooling fins of the cooler, the conduction of which significantly determines the heat transport.

Example: construction

The heating of a building, generally keeping the internal temperature constant at a reference value, is based on the energy balance between heat transfer from the building to the environment (heat loss) and heat transfer from the heating system to the heated room volume, and represents the required heating energy demand (thermal share of the energy demand calculation ).

The heat losses of the building are generally calculated using the heat transfer through components in accordance with EN ISO 6946 (for unheated parts of the building, among others, in accordance with EN 832 ).

The processes involved in room heating are complex, because heat conduction , convection and heat radiation overlap and are usually unsteady . Depending on which proportions predominate, one speaks, for example, of convection heating or radiant heating , whereby the heating surfaces can be integrated into the enclosures or arranged as free-standing radiators . For this purpose, thermal room models are used .

literature

• Norbert Elsner, Siegfried Fischer, Jörg Huhn: Fundamentals of technical thermodynamics. Volume 2, Heat Transfer . Akademie Verlag, Berlin 1993, ISBN 3-05-501389-1 .
• Walter Wagner: heat transfer . Vogel, Würzburg 1998, ISBN 3-8023-1703-3 .
• VDI-Gesellschaft process engineering and chemical engineering: VDI-Wärmeatlas: Calculation sheets for the heat transfer . 11th edition. Springer Vieweg, Berlin 2013, ISBN 978-3-642-19980-6 .

Web links

• John H. Lienhard IV, John H. Lienhard V: A Heat Transfer Textbook . mit.edu, 2000–2016 (PDF download; English)
• Heat transfers simply explained: Types of heat transfer (Document: German)

Individual evidence

1. ↑ Bernd Glück: Heat transfer; Heat dissipation from space heating surfaces and pipes . 2nd Edition. Verlag für Bauwesen, Berlin 1990, ISBN 3-345-00515-8 ( web link to scanned content (PDF) , dynamic spatial model , free download [accessed on January 22, 2010]).