Joseph Black back in 1754 experimentally proved to the whole world that the atmosphere of the earth (in other words, air) consists of a mixture of various gases, the main of which are oxygen and nitrogen. He also introduced such a concept as the coefficient of thermal conductivity of air.
For all living organisms on earth, air is needed for existence, or rather, the basis of air is oxygen. The process of oxidation of oxygen entering the body from the surrounding air produces energy, without which there is no continuation of life.
Oxygen is widely used in production and in everyday life - during combustion, fuel is released, and in internal combustion engines - mechanical energy. By liquefying it, noble gases are produced.
The composition of atmospheric air has a significant impact on the life and health of every person. An ideal (“correct”) composition contains up to 75 percent nitrogen, 24 percent oxygen and small impurities of various gases - methane, neon, krypton, hydrogen, carbon dioxide, etc.
The presence of industrial production, an increase in the number of cars that emit millions of biological and chemical microparticles (aldehydes, ammonia, oxides, heavy metals) into the atmosphere, significantly pollute the atmosphere, and the coefficient of thermal conductivity of air decreases, which adversely affects living organisms. Emissions from the operation of automobile engines (at least 60 percent of them in the air of large cities) are most harmful to the human body. The second place in pollution belongs to thermal power plants, the third is chemical production.
The main property of air is its thermal conductivity. After conducting numerous experiments and experiments, scientists were able to determine that heat in a gaseous medium is distributed in three main ways: thermal radiation (electromagnetic wave energy transfer), convection (the movement of energy flows through the movement of gas layers in space), and thermal conductivity (random motion of molecules, contributing to heat transfer from a gas layer with a higher temperature to a less “warm” gas layer). In the process of heat transfer, molecules containing more energy transfer it to molecules with a lower energy content. The characteristic ability to conduct heat is a physical parameter of the coefficient of thermal conductivity of air. The thermal conductivity of air is determined by the equation:
λ = -d2Qt / gt / gn * dF * dt.
The thermal conductivity coefficient of air is numerically equal to the amount of heat passing over a period of time through a unit of isothermal surfaces under the concomitant condition when gradt = 1. Its direct dimensional value is considered as the ratio of W / (m · K).
Based on the results of the experiments and experiments, a look-up table has been created, from which it is possible to determine the values of the thermal conductivity of air and other substances. For most substances, the heat transfer coefficient can be represented as a linear function
λ = λ0 * [1 + b * (t-to)],
where λ0 is the value of the coefficient acting on thermal conductivity at t0 = 0 degrees Celsius;
b is a constant value determined experimentally.
The worst conduct gases are heat. The heat transfer coefficient of gases increases with increasing temperature and amounts to 0.006 ÷ 0.6 W / (m · K), where the upper value belongs to helium and hydrogen. Their direct thermal conductivity coefficients are five, or even ten times higher than other gases. The heat transfer coefficient of air at zero degrees Celsius is 0.0243 W / (m · K).
The amount of heat carried by the gas layers during the heat exchange process, if the temperature difference over the period of time is unchanged, is determined by the law of the well-known scientist Fourier.