Translate the following questions into English in writing and answer them:

For example:  Какие материалы хорошо проводят тепло? What materials conduct heat well? - Metals, glass, some plastics conduct heat well.

1. Посредством чего переносится значительная часть тепловой энергии? 2. Какие материалы плохо проводят тепло? 3. Как они часто называются? 4. От чего зависит cкорость перехода теплового потока с одной стороны предмета на другую? 5.Какой процесс называется тепловой диффузией?

 

Put the words in the correct order to make up questions in writing; answer the questions.

1. Occur/does/where/conduction? 2. Efficient method /is/ what /heat transfer/ the most/ of?3. Can/ the simplest/how/ described/be/ conduction heat transfer? 4. Resistance/ the thermal/ what/ depend on/does?

Try to find out what these outstanding scientists are famous for: Newton, Stefan-Boltzmann, Laplace, Fourier.

20. You are going to read Text 2С. Before reading try to answer these questions. Share your answers with your fellow students. 1. What do you know about electromagnetic spectrum? 2. How is thermal radiation defined? 3. What is called monochromatic radiation? 4. What does the word spectral denote? 5. Have you ever heardof the Stefan-Boltzmann law of thermal radiation and how it is applied?

Text 2 C

Heat Transfer by Radiation

 

Radiation heat transfer differs from that by convection and conduction because the driving potential is not the temperature, but the absolute temperature raised to the fourth power. Furthermore, heat can be transported by radiation without an intervening medium.

Radiation is the process of transferring heat by emitting electromagnetic energy in the form of waves or particles. Radiation can transfer heat through empty space, while the other two methods require some form of matter-on-matter contact for the transfer.

Radiant heat is simply heat energy in transit as electromagnetic radiation. All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum. In this case, heat moves through space as an electromagnetic radiation without the assistance of a physical substance. All objects that contain heat emit some level of radiant energy. The amount of radiation is inversely proportional to its wavelength (the shorter the wavelength the greater the energy content) which is, in turn, inversely proportional to its temperature (in °K).

When a body is placed in an enclosure whose walls are at a temperature below that of the body, the temperature of the body will decrease even if the enclosure is evacuated. The process by which heat is transferred from a body by virtue of its temperature, without the aid of any intervening medium, is called thermal radiation.

The physical mechanism of radiation is not completely understood yet. Radiant energy is envisioned sometimes as transported by electromagnetic waves, at other times as transported by photons. Neither viewpoint completely describes the nature of all observed phenomena. It is known, however, that radiation travels with the speed of light c, equal to about in a vacuum.

From the viewpoint of electromagnetic theory, the waves travel at the speed of light, while from the quantum point of view, energy is transported by photons that travel at that speed.Although all the photons have the same velocity, there is always a distribution of energy among them.

Radiation phenomena are usually classified by their characteristic wavelength. Electromagnetic phenomenon encompasses many types of radiation, from short-wavelength gamma-rays and x-rays to long-wavelength radio waves. The wavelength of radiation depends on how the radiation is produced. For example, a metal bombarded by high-frequency electrons emits x-rays, while certain crystals can be excited to emit long-wavelength radio waves. Thermal radiationis defined as radiant energy emitted by a medium by virtue of its temperature. In other words, the emission of thermal radiation is governed by the temperature of the emitting body.

The Sun’s heat is an example of thermal radiation that reaches the Earth. Radiative heat is transferred directly into the surface of any solid object it hits (unless it is highly reflective), but passes readily through transparent materials such as air and glass. An ideal thermal radiator or a blackbody, will emit energy at a rate proportional to the forth power of its absolute temperature and its surface area. Mathematically, that is

 

 

where σ is a proportionality constant (Stefan-Boltzmann constant = 5.669 × 10^-8 W/m².K⁴). The above equation is called the Stefan-Boltzmann law of thermal radiation and it applies only to the blackbodies. The fourth-power temperature dependence implies that the power emitted is very sensitive to temperature changes. If the absolute temperature of a body doubles, the energy emitted increases by a factor of 2⁴ = 16.

For bodies not behaving as a blackbody a factor known as emissivity e, which relates the radiation of a surface to that of an ideal black surface is introduced. The equation becomes

 

 

The emissivity ranges from 0 to 1; e = 1 for a perfect radiator and absorber (a blackbody) and e = 0 for a perfect radiator. Human skin, for example, no matter what the pigmentation, has an emissivity of about 0.97 in the infrared part of the spectrum. While a polished aluminum has an emissivity of about 0.05.

Thermal radiation from a body is used as a diagnostic tool in medicine. A thermogram shows whether one area is radiating more heat than it should, indicating a higher temperature due to abnormal cellular activity. Thermography or thermovision in medicine is based on the natural thermal radiation of the skin. Most advantage is the radiance free of the measuring principle.

Certain body regions have different temperature levels. If one exposes the body e.g. to a cooling attraction, then the body zones of the skin react, in order to repair the heat balance of the body. Thereby the thermal regulation of diseased body regions and organs is different to healthy one. The so-called "regulation thermography" is based on this principle.

Thermal radiation always encompasses a range of wavelengths. The amount of radiation emitted per unit wavelength is called monochromatic radiation; it varies with wavelength, and the word “spectral»is used to denote this dependence. The spectral distribution depends on the temperature and the surface characteristics of theemitting body. The sun, with an effective surface temperature of about 5800 K(10,400°R), emits most of its energy below 3 µm, whereas the earth, at a temperatureof about 290 K (520°R), emits over 99% of its radiation at wavelengths longerthan 3µm. The difference in the spectral ranges warms a greenhouse inside evenwhen the outside air is cool because glass permits radiation at the wavelength of thesun to pass, but it is almost opaqueto radiation in the wavelength range emitted bythe interior of the greenhouse. Thus, most of the solar energy that enters the greenhouseis trapped inside. In recent years, the combustion of fossil fuels has increased the amount of carbon dioxide in the atmosphere. Since carbon dioxide absorbs radiation in the solar spectrum, less energy escapes. This causes global warming, which is also called the “greenhouse effect.”

Consequently, the integration of radiation heat transfer into an overall thermal analysis presents considerable challenges, including the need for carefully stated boundary conditions and assumptions necessary for the appropriate inclusion in the thermal circuit of a system.