## Abstract

The simplest case of thermal radiation is the condition of radiative equilibrium at which the temperature of the medium must be a constant throughout the medium. Under the thermodynamic equilibrium, there is a definite relation between the absorption coefficient and the emission coefficient. We are going to discuss the laws of radiation under equilibrium condition in §§ 2 to 4. When there is a radiation field in the flow field, the thermodynamic relations change according to the relative strength of the radiation pressure and the gas pressure. We shall discuss these relations in §§ 4 and 5. For the case of complete thermodynamic equilibrium, the temperature of the whole region must be constant. In the flow field of a gas, the temperature is usually different at different points. Hence we usually will not have the thermodynamic equilibrium condition. However, since in ordinary flow problem, we usually may define a temperature at each point in the flow field, it is a good approximation to assume that the radiative equilibrium is reached locally according to the local temperature. In other words, we may assume that the emission of radiation is according to the local temperature as if the thermodynamic equilibrium condition is reached. Of course, we can not assume that the radiation absorbed is also according to the local temperature, because the radiation to be absorbed comes from other points in the flow field which has different temperature from the local temperature. We shall discuss the local thermodynamic equilibrium condition in § 7 which is very useful in the analysis of radiation gasdynamics.

## Keywords

Thermodynamic Equilibrium Radiation Pressure Black Body Radiation Local Thermodynamic Equilibrium Thermodynamic Equilibrium Condition## Preview

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