Radiation heat transfer photons

At temperatures above ca. 1000 K, heat transfer via radiation becomes significant, that is, the heat transfer can occur by optical energy waves (photons) as well as conduction (phonons), with the heat transfer equation expressed by... 

At a temperature between 0 and 1000°C it lies approximately between 10 and 2 pm. Based on considerations from the area of astrophysics, the energy transfer by radiation within a fine-grained material can be treated like a heat transfer through photons with a certain mean free path ... 

Here, h = (6.626 068 76 0.000 000 52)-10 34 Js is the Planck constant, also known as Planck s action quantum v is the frequency of the photons. Quantum theory is required to calculate the spectral distribution of the energy emitted by a body. Other aspects of heat transfer can, in contrast, be covered by classical theory, according to which the radiation is described as the emission and propagation of electromagnetic waves. 

Radiative heat transfer from one small volume or surface element to another is determined by accounting for the energies of photons of all wavelengths, emitted in all directions over a certain time interval. Depending on the location of each element and its orientation with respect to others, the amount of radiant energy exchange between elements will vary. In order to determine the contribution of each element to the radiation balance, we introduce a fundamental and mathematically convenient quantity termed radiation intensity. 

The transfer of heat in a fluid may be brought about by conduction, convection, diffusion, and radiation. In this section we shall consider the transfer of heat in fluids by conduction alone. The transfer of heat by convection does not give rise to any new transport property. It is discussed in Section 3.2 in connection with the equations of change and, in particular, in connection with the energy transport in a system resulting from work and heat added to the fluid system. Heat transfer can also take place because of the interdiffusion of various species. As with convection this phenomenon does not introduce any new transport property. It is present only in mixtures of fluids and is therefore properly discussed in connection with mass diffusion in multicomponent mixtures. The transport of heat by radiation may be ascribed to a photon gas, and a close analogy exists between such radiative transfer processes and molecular transport of heat, particularly in optically dense media. However, our primary concern is with liquid flows, so we do not consider radiative transfer because of its limited role in such systems. 

The measured apparent photon conductivity includes the boundary effect because photon mean free paths range from 0.1 to 10 cm. Common sample sizes are also in this range. Let us consider a situation where the electromagnetic radiation does not interact with the material, and photon conduction is the only energy transfer process. In this situation, the temperature gradient in the material is independent of the rate of heat transfer. Increasing the ratio of the distance between the boundaries and the photon mean free path (d/1,) increases the apparent conductivity. 

Microwave ovens emit microwave radiation that is absorbed by water. The absorbed radiation is converted to heat that is transferred to other components of the food. Suppose the microwave radiation has wavelength 12.5 cm. How many photons are required to increase the temperature of 100 mL of water (d = 1 g/mL) from 20°C to 100°C if all the energy of the photons is converted to heat ... 

The transfer or conversion of energy is always associated with the emission of electromagnetic waves. We met this concept in its simplest form in Chapter 2, when we looked at the transfer of infrared radiation (i.e. heat). This emission of photons occurs because all objects contain electrically charged particles and, whenever an electrically charged particles accelerates, it emits electromagnetic waves. 

Various theories have been proposed for horizontal transfer at the isoenergetic point. Gouterman considered a condensed system and tried to explain it in the same way as the radiative mechanism. In the radiative transfer, the two energy states are coupled by the photon or the radiation field. In the nonradiative transfer, the coupling is brought about by the phonon field of the crystalline matrix. But this theory is inconsistent with the observation that internal conversion occurs also in individual polyatomic molecules such as benzene. In such cases the medium does not actively participate except as a heat sink. This was taken into consideration in theories proposed by Robinson and Frosch, and Siebrand and has been further improved by Bixon and Jortner for isolated molecules, but the subject is still imperfectly understood. 

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