How Does Matter Transfer Thermal Energy?

Heat transfer is a complex phenomenon, which is often divided into three basic ways, namely heat conduction, heat convection and heat radiation. The heat transfer phenomenon encountered in production and life is often a combination of different primary and secondary of these three basic methods. It should be pointed out that although there are only three basic ways of heat transfer, the heat transfer problems that are closely related to various fields of production and life are diverse, and further exploration is needed on the basis of understanding their basic laws Get more satisfactory results. ^[1]

Heat transfer is referred to as heat transfer. As long as there is a temperature difference within or between objects, heat is transferred spontaneously from high to low temperatures. There are temperature differences in daily life and industrial production in nature, so heat transfer has become a very common physical phenomenon. Studying the law of heat transfer means that the relationship between the heat transferred per unit time and the corresponding temperature difference can be obtained according to different heat transfer processes. Different heat transfer methods have different transfer laws, and the corresponding research and analysis methods are also different.

Heat transfer is widely used in various fields of science and technology. Although there are various forms of heat transfer problems encountered in various fields, the purpose of studying heat transfer can be summarized into three aspects.

(1) Strengthen the heat transfer and increase the heat transferred under certain conditions (such as a certain temperature difference, volume, weight, or pump work, etc.).

(2) Weaken heat transfer, or thermal insulation, that is, to minimize heat transfer under a certain temperature difference.

(3) Temperature or heat transfer control. In order to make some equipment operate safely and economically, or to obtain high-quality products and processes, it is necessary to control the temperature or heat transfer rate of key parts of the object during the heat transfer process. ^[2]

In the study of heat transfer, for the convenience of analyzing problems and mathematical processing, as in the study of fluid flow,

There are three basic modes of heat transfer: thermal conduction, thermal convection, and thermal radiation. Heat transfer can be performed in one of two ways, or in two or three ways simultaneously. According to the characteristics of the heat transfer medium, the heat transfer process can be divided into heat conduction, convective heat transfer and radiant heat transfer.

Heat transfer

Thermal conduction refers to the way in which heat is generated by the vibration, displacement and collision of molecules, atoms and electrons of matter. For example, the internal heat of a solid is transferred from the higher temperature part to the lower temperature part by conducting heat.

Heat transfer can occur in gaseous, liquid, and solid materials, but the mechanisms of heat transfer are different. The heat transfer of a gas is the result of collisions between gas molecules during irregular thermal motion. The kinetic energy of gas molecules is related to their temperature. Molecules in high temperature regions have greater kinetic energy, that is, higher speed. When they move to low temperature regions, they collide with molecules in low temperature regions. As a result, heat is transferred from high temperature to low temperature Area.

Solids transfer heat in two ways: lattice vibration and migration of free electrons. In non-conductive solids, energy is mainly transmitted through the vibration of molecules and atoms near the equilibrium position of the crystal structure; for good conductors such as metals, the movement of gas-like molecules, free electrons move between the lattices, and the heat is transferred from high Zone to low temperature zone. Due to the large number of free electrons, the heat transferred is greater than the heat transferred by the lattice vibration, so a good electrical conductor is generally a good thermal conductor.

The structure of a liquid is between a gas and a solid. Molecules can make small displacements. Heat transfer depends on both the vibration of the molecules and the collision between the molecules.

In the case where there is no relative displacement between various parts of the object, such as the heat transfer process that occurs in solid, stationary liquids and gases by thermal conduction, it is called heat conduction. ^[3]

Heat transfer convection

Thermal convection refers to the way in which heat transfer occurs due to the macroscopic movement of the fluid and the mixing of cold and hot fluids with each other. This heat transfer occurs only in liquids and gases. Since the molecules in the fluid are performing irregular thermal movements at the same time, convection is necessarily accompanied by heat conduction.

When a fluid flows through a solid wall surface, the heat transfer process that occurs is called convective heat transfer, and this process exists widely in engineering. In the process of convective heat transfer, according to the flow state of the fluid, heat may be transferred in a thermally conductive manner or in a convective manner.

According to the cause of fluid particle displacement (fluid flow), convective heat transfer can be divided into natural convective heat transfer and forced convective heat transfer. Natural convective heat transfer refers to the heat transfer process that occurs due to the uneven density distribution of the internal temperature of the fluid and the convection of the fluid under the action of buoyancy. Forced convection heat transfer refers to the heat transfer process that occurs due to fluid flow caused by water pumps, fans, or other external forces. While fluids perform forced convective heat transfer, they are often accompanied by natural convective heat transfer.

According to whether the fluid state changes during the heat transfer between the fluid and the wall, the convective heat transfer can be divided into convective heat transfer without phase change and convective heat transfer with phase change. Convective heat transfer without phase change means that the fluid does not undergo phase change during the heat transfer process; convective heat transfer with phase change means that the fluid undergoes a phase change during the heat transfer process, such as the condensation of gas into a liquid during the heat transfer process , Or the liquid boils and changes to a gas during the heat transfer process. ^[3]

Heat transfer heat radiation

The way objects transmit energy through electromagnetic waves is called radiation. There are many types of radiation, and the phenomenon of radiant energy due to heat is called thermal radiation.

Objects in nature constantly emit thermal radiation into space, while continuously absorbing the thermal radiation emitted by other objects. The combined result of the process of emitting thermal radiation and absorbing thermal radiation has resulted in the heat transfer between objects in a radiant way-radiant heat transfer. When the object is in thermal equilibrium with the object or the surrounding environment, the amount of radiative heat transfer is equal to zero, but this is a dynamic equilibrium. Studies have shown that the thermal radiation emitted by the object and the absorbed thermal radiation are equal, but the The process of radiation and absorption is still ongoing. The transfer of heat from a high-temperature object to a low-temperature object through radiative heat transfer is actually a comprehensive result that the radiant energy from a high-temperature object to a low-temperature object is greater than the radiant energy from a low-temperature object to a high-temperature object.

Compared with thermal and convective heat transfer, heat radiation has the following characteristics:

The radiant energy can be freely transmitted through a vacuum without any intermediate medium;

All objects whose temperature is higher than 0K can continuously emit radiant energy, and can also continuously absorb radiant energy from other objects;

Thermal radiation not only has the transfer of energy, but also has the conversion of energy form. Converted from thermal energy to radiant energy when emitted, and converted from radiant energy to thermal energy when absorbed. ^[2]