Let's talk about what heat transfer is. By this term we mean the process of energy transfer in matter. It is characterized by a complex mechanism, described by the equation of heat conductivity.
Types of heat transfer
How is heat transfer divided? Thermal conductivity, convection, radiation are three modes of energy transfer that exist in nature.
Each of them has its own distinctive characteristics, features, applications in technology.
Thermal conductivity
By the amount of heat we mean the sumkinetic energy of molecules. They can transmit part of their heat to cold particles in a collision. Thermal conductivity is maximum manifested in solids, less characteristic of liquids, absolutely not characteristic for gaseous substances.
As an example, confirming the ability of solids to transfer heat from one site to another, consider the following experiment.
If the steel wire is fixedmetal buttons, then bring the end of the wire to the burning spirit lamp, gradually the buttons from it begin to fall off. When heated, the molecules begin to move at a faster rate, often colliding with each other. It is these particles that give up their energy and heat to colder regions. If liquid and gas do not provide a sufficiently fast heat outflow, this leads to a sharp increase in the temperature gradient in the hot region.
Thermal radiation
Answering the question about what kind of heat transferaccompanied by the transfer of energy, it is necessary to note this method. Radiation transfer involves the transmission of energy by electromagnetic radiation. This variant is observed at a temperature of 4000 K, described by the equation of thermal conductivity. The absorption coefficient depends on the chemical composition, temperature, and density of a specific gas.
Heat transfer of air has a certain limit,With an increase in the energy flux, the temperature gradient increases, the absorption coefficient increases. After the value of 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 object to a cold object by direct contact or through a partition that separates materials.
If the bodies of one system have different temperatures, then the process of energy transfer takes place until thermodynamic equilibrium is established between them.
Heat Transfer Features
What is heat transfer?What are the features of this phenomenon? It can not be stopped completely, can we only reduce its rate of flow? Is heat transfer used in nature and technology? It is the heat exchange 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 process of heat transfer allows analyzing evaporation cooling, drying, diffusion. It is carried out between two carriers of thermal energy through a solid wall acting as the interface of 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.
The Fourier Law
It is called the law of heat conduction, becauseit relates the total power of heat loss, the temperature difference with the 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 carry heat. Despite a similar feature, in a vacuum there is a variant of energy transfer by radiation. The application of heat transfer will be considered on the basis of thermos. The walls of it are made double in order to increase the process of reflection. Between them pumped out the air, while reducing the heat loss.
Convection
Answering the question about what is heat transfer,Let us 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: the fan blades, the pump. A similar option is used in those situations where natural convection is not effective.
A natural process is observed in those cases,when, with uneven heating, the lower layers of matter are heated. Their density decreases, they go up. The upper layers, on the other hand, are cooled, heavier, lower down. Further, the process is repeatedly repeated, and with mixing, self-organization is observed in the structure of the vortices, a regular grating is formed from the convection cells.
Due to natural convection, clouds form, precipitation of atmospheric precipitation, motion of tectonic plates is carried out. It is by convection on the sun that granules form.
Proper use of heat transfer guarantees minimum heat loss, maximum consumption.
Essence of convection
To explain convection, one can use the lawArchimedes, as well as the thermal expansion of solids and liquids. As the temperature rises, the volume of the liquid increases, the density decreases. Under the influence of Archimedes' force, a lighter (heated) liquid tends upward, and cold (dense) layers fall down, gradually warming up.
In the case of heating the liquid from above, a warm liquidremains in the initial position, so convection is not observed. This is how the fluid flows, which is accompanied by the transfer of energy from heated areas to cold places. In gases, convection occurs by a similar mechanism.
From the thermodynamic point of view, convectionare considered as a variant of heat transfer, in which the transfer of internal energy goes by separate streams of substances heated unevenly. A similar phenomenon occurs in nature and in everyday life. For example, heating radiators are installed at a minimum height from the floor, near the window sill.
Cold air is warmed up by the battery, thengradually rises, where it mixes with cold air masses, which are lowered from the window. Convection leads to the establishment of a uniform temperature in the room.
Among the common examples of atmosphericconvection winds: monsoons, breezes. The air that heats over certain fragments of the Earth cools over others, as a result of which its circulation occurs, and moisture and energy are transferred.
Features of natural convection
Several factors affect it at once.For example, the daily movement of the Earth, sea currents, and the surface relief affect the rate of natural convection. It is convection that is the basis of the exit from the craters of the volcano and the pipes of smoke, the formation of mountains, the floating of various birds.
Finally
Thermal radiation is electromagneticprocess with a continuous spectrum, which is emitted by matter, arises from internal energy. In order to conduct calculations of thermal radiation, a black body model is used in physics. Thermal radiation is described using the Stefan-Boltzmann law. The radiation power of such a body is directly proportional to the surface area and body temperature taken to the fourth power.
Thermal conductivity is possible in any bodies thathave an inhomogeneous temperature distribution. The essence of the phenomenon consists in changing the kinetic energy of molecules and atoms, which determines the temperature of the body. In some cases, heat conduction is considered a quantitative ability of a certain substance to conduct heat.
The scale processes of heat energy exchange are not limited to the heating of the earth's surface by solar radiation.
Serious convection currents in the Earth's atmosphereare characterized by changes on the whole planet of weather conditions. When temperature drops in the atmosphere between the polar and equatorial regions convective currents arise: jet currents, trade winds, cold and warm fronts.
The transfer of heat from the earth's core to the surfacecauses volcanic eruptions, the emergence of geysers. In many regions, geothermal energy is used to generate electricity, to heat residential and industrial premises.
It is the heat that becomes an obligatory participantmany production technologies. For example, processing and smelting of metals, food production, oil refining, engine operation - all this is done only in the presence of thermal energy.
