The thermal conductivity of gases

At room temperature, the thermal conductivity of most gases and vapors is 0.01 to 0.03 W / mK. The notable exceptions are helium (0.15) and hydrogen (0.18).

The most general theoretical explanation for the heat transfer in the gas is provided by dynamic gas theory, which considers atomic or intermolecular collisions as the main mode of energy transfer. Radiant heat transfer is ignored in this way

According to this theory, the thermal conductivity is proportional to the heat capacity per unit volume, the average gas velocity, and the mean degree of freedom. Dynamic viscosity is proportional to the product of density, velocity and mean free path, so thermal conductivity is also proportional to viscosity. In practice, this relationship is much more complex, and high precision can not be expected by deriving the thermal conductivity from the above more readily measured physical properties.

Regarding the temperature dependence of the thermal conductivity, it should be noted that the value increases at least in the range of "normal" pressure in proportion to the absolute temperature. As pressure increases, thermal conductivity also increases. At about 0.001 bar, the mean free path is comparable to that of the confinement gas and this value linearly increases with pressure. Above 0.001 bar, the increase in thermal conductivity is an increase of about 1% of the pressure per bar. From these figures, for example, it can be concluded that the change in thermal conductivity due to atmospheric changes can be neglected in most cases.

In the case of gas, the theoretical prediction and experiment of gas kinetics confirms that the thermal conductivity of the gas is proportional to the square root of the temperature T and inversely proportional to the square root of the molar mass M. However, thermal conductivity is independent of the various pressures actually encountered. When the system heats up, it will store some heat and transfer the remaining heat to the other system. As we have seen, the ability of a material to transmit thermal energy is called thermal conductivity. The amount of heat stored in the material is called the heat capacity of the material. The heat capacity of a material is represented by Cp.

Light gases such as hydrogen and helium generally have high thermal conductivity. Dense gases such as helium and dichlorodifluoromethane have low thermal conductivity. The exception is sulfur hexafluoride (high density gas) having a relatively high thermal conductivity due to its high heat capacity. Argon and helium are more dense than air and are commonly used in insulating glass (double glazing) to improve heat insulation. The thermal conductivity of the bulk material in the form of porous or particulate forms depends on the type of gas in the gas phase and its pressure. At lower pressures the thermal conductivity of the gas phase decreases This behavior is controlled by the number of Knudsen defined as where the mean free path of the gas molecules is located and the typical gap size of the space filled with the gas is. The particulate material corresponds to the characteristic size of the gas phase in the pores or intergranular spaces.