Radiative heat transfer: concept, calculation

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2019-04-10 09:20:33

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Here the reader will find General information about what heat transfer is and will be considered in detail the phenomenon of radiative heat transfer and subordination to certain laws, peculiarities of the process, the formula of heat, the use of human heat transfer and flow in nature.

Introduction to heat transfer

radiative heat transfer

To understand the essence of radiant heat transfer, it is necessary to begin to understand its essence and to know what it is?

Heat – variation of internal energy without the flow of work on the object or the subject and without work body. Such a process always proceeds in a particular direction, namely, heat moves from the body with a higher temperature to the body with less. To achieve equalization of temperature between the bodies process is terminated, and it is carried out using conduction, convection, and radiation.

  1. Thermal Conductivity – is the process of transferring internal energy from one fragment to another body or between bodies in the committing of a contact.
  2. Convection-is heat transfer, carried out as a result of energy transfer with the liquid or gas flows.
  3. The Radiation is electromagnetic in nature, emitted due to the internal energy of a substance which is in a certain temperature.

Formula heat allows you to make calculations to determine the amount of energy, but the measured values depend on the nature of processes:

  1. Q = cm ∆ t = cm(t2 – t1) – heating and cooling;
  2. Q = mλ – crystallization and melting;
  3. Q = mr-steam condensation, boiling and evaporation;
  4. Q = mq – the combustion of fuel.

The Relationship of the body and temperature

To understand what is radiant heat exchange, it is necessary to know the basics of the laws of physics about infrared radiation. It is important to remember that any body whose temperature is above zero in the absolute level always emits the energy of a thermal nature. It lies in the infrared range of the spectrum of waves of electromagnetic nature.

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However, the various bodies having the same temperature, will have different ability to emit radiant energy. This feature will depend on various factors, such as body structure, nature, form and surface condition. The nature of electromagnetic radiation refers to the dual, wave-particle. The electromagnetic field has quantum character and its rays represented by photons. Interacting with atoms, photons are absorbed and transfer their energy to the electron, the photon disappears. Energy increased thermal vibrations of the atom in the molecule increases. In other words, the radiated energy is converted into heat.

The Radiated energy is considered the main value and denoted by the symbol W, measured in joules (j). The radiation flux is expressed by the average value of power over a period of time far exceeding the periods of the fluctuations (the energy emitted during the time unit). Emitted by the stream unit Joule divided by second (j/s), common variants is considered to be watts (W).

Stefan Boltzmann

Familiarizing yourself with the radiant heat exchange

Now more about the phenomenon. Radiant heat – heat exchange, the process of transferring it from one body to another having a different temperature. Using infrared radiation. It is a solenoid and lies in the areas of the spectra of waves of electromagnetic nature. The wave band is in the range from 0.77 to 340 microns. The ranges from 340 to 100 µm are considered long-wave, medium-wave range are 100 - 15 µm and from 15 to 0.77 microns are to high.

Two-way plot of the infrared spectrum adjacent to visible light type, and long-wave parts of the wave go in the field of ultrashort radio waves. Infrared radiation characteristic of rectilinear propagation, it can be refracted, reflected and polarized. Able to penetrate through some list of materials that are opaque to visible radiation.

gray body

In Other words, radiative heat transfer can be described as heat transfer in the form of energy of an electromagnetic wave, the process then flows between the surfaces that are in the process of mutual radiation.

The Increased intensity is determined by the relative position of the surfaces radiating and absorbing powers of bodies. radiative heat transfer between bodies is different from convection and heat transfer processes that is able to heat transferred through the vacuum. The similarity of this phenomenon with other due to the transfer of heat between bodies of different temperature indicator.

Irradiance

The radiative heat transfer between bodies has a number of streams of radiation:

  1. The Flux is a private type-E, which depends on the exponent of temperature T and optical characteristics of the body.
  2. Threads of the incident radiation.
  3. Absorbed, reflected and noise-types of fluxes. In sum, they are equal to EPAP.

The Environment in which the heat exchange takes place, may engage in the absorption of radiation and to bring their own.

The radiative forcingbetween a number of bodies is described by the radiation flux effective:

EFUS=E+ENEG=E+(1-A)EPAP.
Body, in any temperature with indices L=1, R=0 and O=0, referred to as “completely black”. Man has created the concept of "black light”. It corresponds to its temperature performance of the balance of the body. The emitted energy of radiation is calculated using a temperature of the subject or object, the nature of the body is not affected.

Following the laws of Boltzmann

radiant energy

Ludwig Boltzmann, who lived on the territory of the Austrian Empire in 1844-1906 years, created the Stefan-Boltzmann law. It was he who enabled man to better understand heat exchange and to use information, fine-tuning it for many years. Consider its wording.

The Stefan-Boltzmann Law-is the law of integral type, describing some of the features of absolute black bodies. It allows to determine the dependence of the power density of blackbody radiation from its temperature target.

Compliance

The Laws of radiant heat transfer is subject to the Stefan-Boltzmann law. The intensity of heat transfer through conduction and convection is proportional to the temperature. The radiant energy in the thermal flow is proportional to temperature to the fourth power. It looks like this:

Q = σ A (T14 – T24).

In the formula, q-is heat flow, A-surface area of the body radiating energy, T1 And T2 – the value of temperatures of the emitting bodies and the environment, which deals with the absorption of this radiation.

The law of heat radiation exactly describes only ideal radiation produced by a blackbody (and. CH. t). Such bodies in the life of almost no. However, the flat surface of the black approach.h.t. The emission of the light bodies is relatively weak.

There is a coefficient of emissivity introduced to account for deviations from ideality large number of S. T. in the right compound expressions, explaining the Stefan-Boltzmann law. Indicator emissivity equal to the value smaller units. Flat black surface can bring this ratio to 0.98, and the metallic mirror does not exceed 0.05. Therefore, lucaogou ability high for black bodies and low for a mirror.

formula Tepla

About the gray body (S. T.)

Heat exchange In the common reference to such terms as gray body. This object represents the body, which has a coefficient spectral type of electromagnetic radiation absorption of less than one, which is not based on wave length (frequency).

The heat Radiation is the same in accordance with the spectral composition of blackbody radiation with the same temperature. Has the grey body of the black lower index energy compatibility. On the spectral level black S. T. the wavelength is not affected. In visible light to the gray body close to soot, coal and platinum powder (black).

Applications of knowledge about the heat

heat radiation

The heat Radiation is constantly around us. In residential and office buildings often you can find electric heaters that deal with radiation, and we see it in the form of a reddish glow spiral – such heat refers to appears to be, it is “worth” at the border of the infrared spectrum.

The Heating, in fact, is a compound invisible infrared radiation. Night vision device uses a source of heat radiation and detectors sensitive to infrared radiation of nature, which allow to navigate in the dark.

Solar Energy

radiative heat transfer between bodies

The Sun is a powerful emitter of energy, wearing a thermal nature. It heats our planet from a hundred and fifty million kilometers. The indicator of intensity of solar radiation that was registered for many years and at different stations, located in various corners of the earth, or approximately 1.37 W/m2.

The energy of the sun is the source of life on planet Earth. Currently, many minds are trying to find the most effective way to use it. Now we know solar is able to heat residential buildings and to obtain energy for the needs of everyday life.

In conclusion

Summing up, you now know how to define the radiant heat transfer. To describe this phenomenon in life and nature. Radiant energy is the main characteristic of the transmitted wave energy in the phenomenon, and these formulas show how to calculate it. In General position the process obeys the Stefan-Boltzmann law and can have three forms depending on its nature: the flow of the incident radiation, radiation own type and the reflected, absorbed and ignored.


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Alin Trodden - author of the article, editor
"Hi, I'm Alin Trodden. I write texts, read books, and look for impressions. And I'm not bad at telling you about it. I am always happy to participate in interesting projects."

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