Here the reader will find general information about what heat transfer is, and the phenomenon of radiant heat transfer, its subordination to certain laws, features of the process, the heat formula, human use of heat transfer and its occurrence in nature will be examined in detail.
Entry into heat transfer
To understand the essence of radiant heat transfer, you must first understand its essence and know what it is?
Heat transfer is a change in the energy indicator of the internal type without proceeding with work on an object or subject, as well as without performing work by the body. Such a process always proceeds in a specific direction, namely: heat transfers from a body with a large temperature index to a body with a smaller one. Upon reaching equalization of temperatures between the bodies, the process stops, and it is carried out using heat conduction, convection and radiation.
- Thermal conductivity is the process of transferring energy of an internal type from one fragment of a body to another or between bodies when they make contact.
- Convection is heat transfer resulting from the transfer of energy together with liquid or gas flows.
- The radiation is electromagnetic in nature, emitted due to the internal energy of a substance that is in a state of a certain temperature.
The heat formula allows you to make calculations to determine the amount of energy transferred, however, the measured values depend on the nature of the process:
- Q = cmΔt = cm (t 2 - t 1 ) - heating and cooling;
- Q = mλ — crystallization and melting;
- Q = mr — steam condensation, boiling and evaporation;
- Q = mq - fuel combustion.
The relationship of body and temperature
In order to understand what radiant heat transfer is, it is necessary to know the fundamentals of the laws of physics on infrared radiation. It is important to remember that any body whose temperature is above zero in the absolute mark always radiates thermal energy. It lies in the infrared spectrum of waves of an electromagnetic nature.
However, a variety of bodies, having the same temperature, will have different ability to emit radiant energy. This characteristic will depend on various factors, such as: body structure, nature, shape and surface condition. The nature of electromagnetic radiation refers to the dual, particle-wave. The electromagnetic field is of a quantum nature, and its quanta are represented by photons. Interacting with atoms, photons are absorbed and transfer their energy supply to electrons, the photon disappears. The energy of the thermal vibration of an atom in a molecule increases. In other words, the radiated energy turns into heat.
The radiated energy is considered the main quantity and is denoted by the sign W, measured by joules (J). In the radiation flux, the average value of power over a period of time is much greater than the periods of oscillations (the energy emitted during a unit of time). The unit radiated by the flux is expressed in joules divided by the second (J / s), watt (W) is considered the generally accepted option.
Introducing Radiant Heat Transfer
Now more about the phenomenon. Radiant heat transfer is the exchange of heat, the process of transferring it from one body to another, having a different temperature indicator. Occurs with the help of infrared radiation. It is electromagnetic and lies in the spectral regions of waves of an electromagnetic nature. The wavelength range is from 0.77 to 340 microns. Ranges from 340 to 100 microns are considered to be long-wavelengths, the medium-wave range is 100-15 microns, and from 15 to 0.77 microns are short-wavelengths.
The short-wavelength portion of the infrared spectrum is adjacent to visible light, and the long-wavelength portions of the waves leave in the region of the ultra-short radio wave. Infrared radiation is characterized by rectilinear propagation, it is able to refract, reflect and polarize. It is able to penetrate through a list of materials that are opaque to visible radiation.
In other words, radiant heat transfer can be characterized as heat transfer in the form of electromagnetic wave energy, while the process proceeds between surfaces that are in the process of mutual radiation.
The intensity indicator is determined by the relative position of the surfaces, the radiative and absorbent abilities of the bodies. Radiant heat transfer between bodies differs from convection and heat-conducting processes in that heat is able to be transmitted through vacuum. The similarity of this phenomenon with others is due to the transfer of heat between bodies with different temperature indicators.
Radiation flux
Radiant heat transfer between bodies has a certain number of radiation fluxes:
- The radiation flux of the intrinsic type is E, which depends on the temperature index T and the optical characteristics of the body.
- Streams of incident radiation.
- Absorbed, reflected and transmitted types of radiation fluxes. In total, they equal E pad .
The environment in which heat exchange occurs can absorb radiation and bring in its own.
Radiant heat transfer between a certain number of bodies is described by a stream of radiation of an effective nature:
E EF = E + E OTP = E + (1-A) E PAD .
Bodies with conditions of any temperature having indices A = 1, R = 0 and O = 0 are called “absolutely black”. Man created the concept of "black radiation". It corresponds with its temperature indicators to the balance of the body. The emitted radiation energy is calculated using the temperature of the subject or object, the nature of the body does not affect this.
Following the laws of Boltzmann
Ludwig Boltzmann, who lived on the territory of the Austrian Empire in 1844-1906, created the Stefan-Boltzmann law. It was he who allowed a person to better understand the essence of heat exchange and to operate with information, improving it over the years. Consider its wording.
The Stefan-Boltzmann law is an integral law that describes some of the features of absolutely black bodies. It allows you to determine the dependence of the radiation power density of a black body on its temperature index.
Obedience to law
The laws of radiant heat transfer obey the law of Stefan-Boltzmann. The level of heat transfer through heat conduction and convection is proportional to temperature. The radiant energy in the heat flux is proportional to the fourth degree of temperature. It looks like this:
q = σ A (T 1 4 - T 2 4 ).
In the formula, q is the heat flux, A is the surface area of the body radiating energy, T 1 and T 2 are the temperatures of the radiating bodies and the environment, which is involved in the absorption of this radiation.
The above law of heat radiation exactly describes only the ideal radiation created by a completely black body (a. H. T.). There are practically no such bodies in life. However, flat surfaces of black color are approaching the r.h. Light body radiation is relatively weak.
There is an emissivity coefficient introduced to account for deviations from the ideality of a large number of st. into the right compound expression explaining the law of Stefan-Boltzmann. The emissivity index is equal to a value less than unity. A flat black surface can bring this coefficient to 0.98, and a metal mirror will not exceed 0.05. Therefore, the absorption capacity is high for black bodies and low for SLRs.
About the gray body (s.t.)
In heat transfer, a term such as a gray body is often mentioned. This object is a body that has a spectral type coefficient of absorption of electromagnetic radiation less than one, which does not rely on wavelength (frequency).
The heat radiation is the same in accordance with the spectral composition of the radiation of a black body with the same temperature. A gray body differs from a black one in a lower indicator of energy compatibility. At the spectral level of black ST wavelength is not affected. In visible light, soot, coal, and platinum powder (black) are close to the gray body.
Applications for heat transfer knowledge
Radiation of heat occurs constantly around us. In residential and office premises, you can often find electric heaters that deal with heat radiation, and we see it in the form of a reddish glow of a spiral - this heat is apparently related to the fact that it “stands” at the edge of the infrared spectrum.
The heating of the room, in fact, deals with the invisible component of infrared radiation. The night vision device uses a heat radiation source and receivers that are sensitive to radiation of an infrared nature, which allows you to navigate well in the dark.
Energy of sun
The sun is rightfully a powerful emitter of energy of a thermal nature. It heats our planet from a distance of one hundred and fifty million kilometers. The indicator of the intensity of solar radiation, which has been recorded for many years and by various stations located in various corners of the earth, corresponds to approximately 1.37 W / m 2 .
It is the energy of the sun that 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 panels that can heat residential buildings and receive energy for the needs of everyday life.
Finally
Summing up, the reader can now define radiant heat transfer. Describe this phenomenon in life and nature. Radiant energy is the main characteristic of a wave of transmitted energy in such a phenomenon, and the above formulas show how to calculate it. In general position, the process itself obeys the Stefan-Boltzmann law and can have three forms depending on its nature: the flux of incident radiation, radiation of its own type and reflected, absorbed and transmitted.