Любое материальное тело обладает такой characteristic as heat, which can increase and decrease. Heat is not a material substance: as part of the internal energy of a substance, it arises due to the movement and interaction of molecules. Since the heat of various substances may differ, the process of heat transfer from the more heated substance to the substance with less heat occurs. This process is called heat transfer. The main types of heat transfer and the mechanisms of their action will be discussed in this article.
Heat transfer, or temperature transfer,can occur both within matter and from one substance to another. In this case, the intensity of heat transfer largely depends on the physical properties of matter, the temperature of substances (if several substances are involved in heat exchange) and the laws of physics. Heat transfer is a process that always proceeds unilaterally. The main principle of heat transfer is that the most heated body always gives heat to an object with a lower temperature. For example, when ironing clothes, a hot iron gives warmth to the pants, and not vice versa. Heat transfer is a phenomenon dependent on a temporary indicator characterizing the irreversible distribution of heat in space.
The mechanisms of thermal interaction of substances can take different forms. There are three types of heat exchange in nature:
Consider these types of heat transfer in more detail.
Most often, thermal conductivity is observed in solidbodies. If, under the influence of any factors, areas with different temperatures appear in the same substance, the thermal energy from the more heated area will transfer to the cold one. A similar phenomenon in some cases can be observed even visually. For example, if we take a metal rod, say, a needle, and heat it on a fire, then after some time we will see how thermal energy is transmitted along the needle, forming a certain glow in a certain area. In this case, in a place where the temperature is higher, the glow is brighter and, conversely, where t is lower, it is darker. Thermal conductivity can also be observed between two bodies (a cup of hot tea and a hand)
The intensity of heat transfer dependsfrom many factors, the ratio of which revealed the French mathematician Fourier. These factors primarily include the temperature gradient (the ratio of the temperature difference at the ends of the rod to the distance from one end to the other), the cross-sectional area of the body, and the coefficient of thermal conductivity (for all substances it is different, but the highest is observed for metals). The most significant coefficient of thermal conductivity is observed in copper and aluminum. It is not surprising that these two metals are more often used in the manufacture of electrical wires. Following the Fourier law, the heat flux can be increased or decreased by changing one of these parameters.
Convection, peculiar mainly to gases andliquids, has two components: intermolecular thermal conductivity and movement (distribution) of the medium. The mechanism of action of convection occurs as follows: as the temperature of a flowing substance increases, its molecules begin to move more actively and, in the absence of spatial limitations, the volume of the substance increases. The consequence of this process will be a decrease in the density of the substance and its movement upwards. A striking example of convection is the movement of radiator-heated air from the battery to the ceiling.
Distinguish between free and forced convectivetypes of heat transfer. Heat transfer and mass movement in the free type occurs due to the heterogeneity of the substance, that is, the hot liquid rises above the cold naturally without exerting influence of external forces (for example, heating the room with central heating). In forced convection, the movement of mass occurs under the action of external forces, for example, stirring tea with a spoon.
Radiant or radiation heat transfer mayoccur without contact with another object or substance, therefore, it is possible even in airless space (vacuum). Radiation heat transfer is inherent in all bodies to a greater or lesser extent and manifests itself in the form of electromagnetic waves with a continuous spectrum. A vivid example of this is the sun's rays. The mechanism of action is as follows: the body continuously emits a certain amount of heat into the surrounding space. When this energy enters another object or substance, a part of it is absorbed, the second part passes through, and the third is reflected in the environment. Any object can both radiate heat and absorb, while dark substances are able to absorb more heat than light ones.
In nature, types of heat exchange processes rarelymeet separately. More often they can be observed in the aggregate. In thermodynamics, these combinations even have names, say, thermal conductivity + convection - this is convective heat transfer, and thermal conductivity + thermal radiation is called radiation-conductive heat transfer. In addition, they distinguish such combined types of heat exchange as:
Heat exchange in nature plays a huge role and notlimited to warming the globe with sunlight. Extensive convection currents, such as the movement of air masses, largely determine the weather on our planet.
The thermal conductivity of the core of the Earth leads togeysers and volcanic eruptions. This is only a small part of examples of heat exchange on a global scale. Together, they form the types of convective heat transfer and radiation-conductive types of heat transfer necessary to sustain life on our planet.
Heat is an important component of almost all.manufacturing processes. It is difficult to say what kind of heat exchange by man is used most in the national economy. Probably all three at the same time. Thanks to the processes of heat transfer, metals are smelted, a huge amount of goods are produced, starting with everyday objects and ending with space ships.
Extremely important for civilization arethermal units capable of converting thermal energy into useful force. Among them are gasoline, diesel, compressor, turbine installations. For their work, they use different types of heat transfer.
