Types of heat transfer: heat transfer coefficient

Any material body has such a characteristic as heat, which can increase and decrease. Heat is not a material substance: as part of the internal energy of matter, it arises from the motion and interaction of molecules. Since the heat of various substances can differ, a process of heat transfer from a warmer substance to a substance with less heat is carried out. This process is called heat transfer. The main types of heat transfer and the mechanisms of their action we will consider in this article.

Determination of heat transfer

Heat transfer, or the process of temperature transfer, can occur both within matter and from one substance to another. At the same time, the intensity of heat exchange largely depends on the physical properties of matter, the temperature of the substances (if several substances participate in heat exchange) and the laws of physics. Heat transfer is a process that always proceeds unilaterally. The main principle of heat exchange 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 trousers, and not vice versa. Heat transfer is a phenomenon dependent on the time index characterizing the irreversible propagation of heat in space.

Mechanisms of heat transfer

Mechanisms of thermal interaction of substances can take different forms. There are three types of heat exchange in nature:

1. Thermal conductivity is a mechanism of intermolecular heat transfer from one part of the body to another or to another object. The property is based on the temperature inhomogeneity in the substances under consideration.
2. Convection - heat exchange between fluids (liquid, air).
3. Radiation exposure is the transfer of heat from heated bodies heated by energy (sources) in the form of electromagnetic waves with a constant spectrum.

Consider the listed types of heat exchange in more detail.

Thermal conductivity

Most often, thermal conductivity is observed in solids. If, under the influence of some factors, regions with different temperatures appear on the same substance, the thermal energy from the more heated section will pass to the cold one. In some cases, this phenomenon can be observed even visually. For example, if you take a metal rod, say, a needle, and heat it on fire, then after a while you will see how the heat energy is transmitted through the needle, forming a glow in a certain area. At the same time 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 mug of hot tea and a hand)

The intensity of transmission of the heat flux depends on many factors, the ratio of which was revealed by the French mathematician Fourier. These factors include, first of all, 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 thermal conductivity coefficient (for all substances it is different, but the highest 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 amount of heat flux can be increased or decreased by changing one of these parameters.

Convection types of heat transfer

Convection, inherent mainly for gases and liquids, has two components: intermolecular thermal conductivity and motion (propagation) of the medium. The mechanism of action of convection occurs as follows: as the temperature of the flowing substance of its molecule increases, more active motion begins and, in the absence of spatial limitations, the volume of matter increases. The consequence of this process will be a decrease in the density of the substance and its upward movement. A vivid example of convection is the movement of the radiator-heated air from the battery to the ceiling.

There are free and forced convection heat exchange types. Heat transfer and mass movement in a free type occurs due to the heterogeneity of the substance, that is, the hot liquid rises above the cold in a natural way without exerting an influence of external forces (for example, heating the room through central heating). When forced convection, mass movement occurs under the influence of external forces, for example, stirring tea with a spoon.

Radiant heat transfer

Radiation or radiation heat transfer can occur without contact with another object or substance, so it is possible even in an 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 bright example is the sun's rays. The mechanism of action is as follows: the body continuously radiates a certain amount of heat into the surrounding space. When this energy hits another object or substance, 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.

Combined heat transfer mechanisms

In nature, the types of heat exchange processes are rarely encountered separately. Much more often they can be observed together. In thermodynamics these combinations even have names, say, thermal conduction + convection is convective heat transfer, and thermal conductivity + thermal radiation is called radiation-conductive heat transfer. In addition, there are such combined heat exchange types as:

• Heat transfer - the movement of heat energy between a gas or a liquid and a solid.
• Heat transfer is the transfer of t from one matter to another through a mechanical obstacle.
• Convective-radiant heat exchange is formed by combining convection and thermal radiation.

Types of heat transfer in nature (examples)

Heat transfer in nature plays a huge role and is not limited to the heating of the globe by the sun's rays. Extensive convection currents, such as the movement of air masses, largely determine the weather on our entire planet.

The thermal conductivity of the Earth's core leads to the appearance of geysers and the eruption of volcanic rocks. This is only a small part of the 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.

The use of heat exchange in anthropological activities

Heat is an important part of almost all production processes. It is difficult to say what type of heat exchange a person uses most in the national economy. Probably all three at the same time. Thanks to the processes of heat transfer, there is a smelting of metals, the production of a huge number of goods, from everyday items to space ships.

Extreme importance for civilization has thermal aggregates that can convert thermal energy into a useful force. Among them we can name gasoline, diesel, compressor, turbine units. For their work they use different types of heat exchange.