Let's talk about heat transfer. This term refers to the process of energy transfer in a substance. It has a complex mechanism, is described by the heat equation.
Varieties of heat transfer
How is heat transfer divided? Thermal conductivity, convection, radiation - three methods of energy transfer that exist in nature.
Each of them has its own distinctive characteristics, features, and applications in technology.
Thermal conductivity
By the amount of heat is understood the sum of the kinetic energy of the molecules. In a collision, they are able to transfer part of their heat to cold particles. Thermal conductivity is maximally manifested in solids, less characteristic of liquids, absolutely not characteristic of gaseous substances.
As an example, confirming the ability of solids to transfer heat from one site to another, consider the following experiment.
If you fix metal buttons on a steel wire, then bring the end of the wire to a burning spirit lamp, gradually the buttons from it will begin to fall off. When heated, the molecules begin to move at a faster speed, more often collide with each other. It is these particles that give their energy and heat to colder areas. If a sufficiently rapid outflow of heat is not provided in liquids and gases, this leads to a sharp increase in the temperature gradient in the hot region.
Thermal radiation
Answering the question of what type of heat transfer is accompanied by energy transfer, it is necessary to note this particular method. Radiant transfer involves the transfer of energy by electromagnetic radiation. This option is observed at a temperature of 4000K and is described by the heat equation. The absorption coefficient depends on the chemical composition, temperature, density of a particular gas.
The heat transfer of air has a certain limit, with an increase in the energy flow, an increase in the temperature gradient occurs, an increase in the absorption coefficient. After the temperature gradient exceeds the adiabatic gradient, convection will occur.
What is heat transfer? This is the physical process of transferring energy from a hot to a cold object when they are in direct contact or through a partition that separates materials.
If the bodies of one system have different temperatures, then the process of energy transfer occurs until a thermodynamic equilibrium is established between them.
Heat Transfer Features
What is heat transfer? What are the features of this phenomenon? It cannot be stopped completely, can it only be reduced in speed? Is heat transfer used in nature and technology? It is heat transfer that accompanies and characterizes many natural phenomena: the evolution of planets and stars, meteorological processes on the surface of our planet. For example, together with mass exchange, the heat transfer process allows you to analyze evaporative cooling, drying, diffusion. It is carried out between two carriers of thermal energy through a solid wall that acts as the interface between bodies.
Heat transfer in nature and technology is a way of characterizing the state of an individual body, analyzing the properties of a thermodynamic system.
Fourier law
It is called the law of heat conduction, since it relates the total power of heat loss, the temperature difference with the cross-sectional area of the parallelepiped, its length, and also with the coefficient of thermal conductivity. For example, for a vacuum, this indicator is practically zero. The reason for this phenomenon is the minimum concentration of material particles in a vacuum that can transfer heat. Despite this feature, in vacuum there is a variant of energy transfer by radiation. We will consider the use of heat transfer based on a thermos. Its walls are doubled in order to increase the reflection process. Between them pump out air, while reducing heat loss.
Convection
Answering the question of what is heat transfer, we consider the process of heat transfer in liquids or gases by spontaneous or forced mixing. In the case of forced convection, the movement of the substance is caused by the action of external forces: fan blades, pump. A similar option is used in situations where natural convection is not effective.
A natural process is observed in those cases when uneven heating leads to heating of the lower layers of the substance. Their density decreases, they rise up. The upper layers, on the contrary, cool, become heavier, fall down. Further, the process is repeated several times, and with stirring, self-organization into a vortex structure is observed, and a regular lattice is formed from convection cells.
Thanks to natural convection, clouds form, precipitation falls, and tectonic plates move. It is by convection on the Sun that granules form.
Proper use of heat transfer ensures minimal heat loss, maximum consumption.
The essence of convection
To explain convection, you can use the law of Archimedes, as well as the thermal expansion of solids and liquids. As the temperature rises, there is an increase in the volume of liquid, a decrease in density. Under the influence of the force of Archimedes, a lighter (heated) liquid tends to up, and cold (dense) layers fall down, gradually warm up.
In the case of heating the liquid from above, the warm liquid remains in its original position, so convection is not observed. This is exactly how the fluid cycle occurs, which is accompanied by the transfer of energy from heated areas to cold places. In gases, convection occurs by a similar mechanism.
From a thermodynamic point of view, convection is considered as a variant of heat transfer, in which the internal energy is transported by separate flows of substances heated non-uniformly. A similar phenomenon is found in nature and in everyday life. For example, heating radiators are installed at a minimum height from the floor, near the windowsill.
Cold air is warmed up by the battery, then gradually rises up, where it mixes with the cold air masses lowered from the window. Convection leads to the establishment of a uniform temperature in the room.
Among the common examples of atmospheric convection, we give winds: monsoons, breezes. Air, which is heated over some fragments of the Earth, is cooled over others, as a result of which it circulates, and moisture and energy are transferred.
Features of natural convection
It is influenced by several factors at once. For example, the daily movement of the Earth, sea currents, and surface topography affect the speed of natural convection. Convection is the basis for the exit from the craters of the volcano and smoke pipes, the formation of mountains, the soaring of various birds.
Finally
Thermal radiation is an electromagnetic process with a continuous spectrum, which is emitted by a substance, arises due to internal energy. In order to carry out calculations of thermal radiation, a blackbody model is used in physics. Thermal radiation is described using the Stefan-Boltzmann law. The radiation power of such a body is in direct proportion to the surface area and body temperature, taken to the fourth degree.
Thermal conductivity is possible in any bodies that have an inhomogeneous temperature distribution. The essence of the phenomenon is to change the kinetic energy of molecules and atoms, which determines the temperature of the body. In some cases, thermal conductivity is considered the quantitative ability of a particular substance to conduct heat.
Large-scale processes of heat energy exchange are not limited to heating the earth's surface with solar radiation.
Serious convection currents in the Earth's atmosphere are characterized by changes in weather conditions throughout the planet. With temperature differences in the atmosphere between polar and equatorial regions, convection flows arise: jet flows, trade winds, cold and warm fronts.
Heat transfer from the earth's core to the surface causes volcanic eruptions, the appearance of geysers. In many regions, geothermal energy is used to produce electrical energy, to heat residential and industrial premises.
It is heat that is becoming an indispensable participant in many production technologies. For example, metal processing and smelting, food production, oil refining, engine operation - all this is carried out only in the presence of thermal energy.