Gas radiation

Gas radiation describes the thermal radiation of gases . To simplify matters, the influence of gases between two bodies in the radiation exchange is almost always neglected.

Gases absorb and emit electromagnetic radiation , e.g. B. visible light , unlike other aggregate states of matter: with gases these effects depend on the wavelength of the radiation ("radiation band"): ${\displaystyle \lambda }$

• Degree of absorption ${\displaystyle \alpha =\alpha (\lambda )}$
• Transmittance ${\displaystyle \tau =\tau (\lambda )}$
• Reflectance ${\displaystyle \rho =\rho (\lambda )}$

It applies

${\displaystyle \alpha (\lambda )+\tau (\lambda )+\rho (\lambda )=1}$

Without reflection, it follows: ${\displaystyle \rho (\lambda )=0}$

${\displaystyle \alpha (\lambda )+\tau (\lambda )=1}$

A gas can not be regarded as a gray emitter , and an averaged emissivity can not be given for a gas .

Gases can also reflect at interfaces .

Influence in the earth's atmosphere

The radiation behavior of the gas mixture air is not trivial to determine. Air essentially consists of the five gases nitrogen  (N ₂ ), oxygen  (O ₂ ), water vapor  (H ₂ O), argon (Ar) and carbon dioxide  (CO ₂ ), so the radiation behavior of air is made up of contributions from these five gases together. Despite their large proportion, nitrogen and oxygen hardly influence the radiation band of air, because due to the symmetrical structure of these molecules they are not IR-active, i.e. that is, they do not absorb or emit infrared radiation. As a noble gas, argon is also not IR-active. Ultimately, air has a radiation band that is mainly composed of a combination of CO ₂ and H ₂ O. It lies precisely in the range of infrared radiation with wavelengths greater than 1 µm that is relevant for the greenhouse effect . Thus, these two gases contribute significantly to the warming of the earth's atmosphere , as they do not release the radiation unhindered into space ( greenhouse gases ). At the same time they allow the atmosphere, energy per broadcasting deliver into space.

Gas radiation also plays an important role in cooling the earth's surface at night: Since there is no solar radiation at night , the earth cools down through heat radiation into the night sky . However, the layer of air that is located directly above the earth's surface also sends heat radiation back to the earth's surface due to the gas radiation, which reduces the cooling of the soil by a certain amount. Since this reflection occurs against the normal direction of radiation, it is called atmospheric counter-radiation .

literature

• Heinz Herwig: Heat transfer - physical basics . Vieweg + Teubner Verlag, Hamburg 2006, ISBN 3-8348-0755-9 .
• Thomas Hertel: A model for the calculation of gas radiation and its application in the development of a simple combustion chamber calculation method . Dissertation, Darmstadt 1992.
• Carl Kramer, Alfred Mühlbauer (Hrsg.): Practical Guide Thermoprocess Technology. Vol. 1: Basics - Procedure . Vulkan Verlag, Essen 2002, ISBN 3-8027-2922-6 .