Coefficient of thermal conductivity of air

Joseph Black, as far back as 1754, proved to the whole world that the earth's atmosphere (in other words, air) consists of a mixture of various gases, the main ones of which are oxygen and nitrogen. He also introduced such a concept as the coefficient of thermal conductivity of air.

All living organisms on earth need air for their existence, or rather, the basis of air-oxygen. The ongoing 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 mechanical combustion engines, mechanical energy. By its liquefaction, noble gases are produced.

The composition of atmospheric air has a significant impact on the life and health of each person. The ideal ("correct") composition contains up to 75 percent nitrogen, 24 percent oxygen, and small admixtures of various gases - methane, neon, krypton, hydrogen, carbon dioxide, etc.

The presence of industrial production, the increase in the number of cars emitting millions of biological and chemical microparticles (aldehydes, ammonia, oxides, heavy metals) to the atmosphere, significantly pollute the atmosphere, the coefficient of thermal conductivity of the air decreases, which adversely affects living organisms. Emissions from the operation of automobile engines (in the air of large cities at least 60 percent) are most harmful to the human body. The second place in terms of pollution belongs to thermal power plants, the third is chemical production.

The main property of air is its thermal conductivity. After carrying out numerous experiments and experiments, scientists managed to determine that heat in the gaseous medium is propagated in three main ways: thermal radiation (electromagnetic wave transfer of energy), convection (movement of energy flows through the motion of gas layers in space), thermal conductivity (chaotic motion of molecules, Reception of heat from a layer of gas 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 thermal conductivity of the air. The coefficient of thermal conductivity of air is determined by the equation:

Λ = -d2Qt / gt / gn * dF * dt.

The coefficient of thermal conductivity of air is numerically equal to the amount of heat passing through a unit of time through a unit of isothermal surfaces with the concomitant condition that gradt = 1. Its immediate size is considered as the ratio W / (m · K).

Based on the results of the experiments and experiments, a reference table has been created, according to which it is possible to determine the values of the coefficient of 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 the thermal conductivity at t0 = 0 degrees Celsius;

B is a constant value determined experimentally.

Heat is the worst of gases. The coefficient of heat transfer of gases increases with increasing temperature and is 0.006 ÷ 0.6 W / (m · K), where the upper value belongs to helium and hydrogen. Their direct coefficients of thermal conductivity are five or even ten times higher than those of the remaining gases. The coefficient of heat transfer at zero degrees Centigrade is 0.0243 W / (m · K).

The amount of heat carried by the layers of gas in the process of heat exchange, if the temperature difference over a period of time is unchanged, is determined by the law of all the known Fourier scientist.