Here the reader will find general information about what heat transfer is, and will also consider in detail the phenomenon of radiant heat transfer, its obedience to certain laws, the features of the process, the formula of heat, the use of heat transfer by man and its flow in nature.
Entry into heat exchange
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 index of the internal type without work on the object or subject, and also without work done by the body. Such a process always proceeds in a specific direction, namely: heat passes from a body with a higher temperature index to a body with a lower one. Upon reaching the equalization of temperatures between the bodies, the process stops, and it is carried out with the help of heat conduction, convection and radiation.
- Thermal conduction is the process of transferring internal energy from one body fragment to another or between bodies when they make contact.
- Convection is heat transfer resulting fromenergy transfer along with liquid or gas flows.
- 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 \u200b\u200bare dependent on the nature of the ongoing process:
- Q=cmΔt=cm(t2 – t1) – heating and cooling;
- Q=mλ – crystallization and melting;
- Q=mr - steam condensation, boiling and evaporation;
- Q=mq – fuel combustion.
Relationship between body and temperature
To understand what radiant heat transfer is, you need to know the basic laws of physics about infrared radiation. It is important to remember that any body whose temperature is above zero in absolute terms always radiates thermal energy. It lies in the infrared spectrum of waves of electromagnetic nature.
However, different 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, corpuscular-wave. The field of the electromagnetic type has a quantum character, and its quanta are represented by photons. Interacting with atoms, photons are absorbed and transfer their energy to electrons, the photon disappears. Energy exponent thermal fluctuationatom in a molecule increases. In other words, the radiated energy is converted into heat.
Radiated energy is considered the main quantity and is denoted by the sign W, measured in joules (J). The radiation flux expresses the average value of power over a period of time that is much greater than the periods of oscillations (the energy emitted during a unit of time). The unit emitted by the stream is expressed in joules per second (J / s), the watt (W) is considered the generally accepted option.
Introduction to 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, which has a different temperature index. Occurs with the help of infrared radiation. It is electromagnetic and lies in the regions of the wave spectra of an electromagnetic nature. The wave range lies in the range from 0.77 to 340 µm. Ranges from 340 to 100 µm are considered longwave, 100 - 15 µm belong to the medium wave range, and short wavelengths from 15 to 0.77 µm.
The short-wave portion of the infrared spectrum is adjacent to the visible light, and the long-wave portions of the waves go into the ultrashort radio wave. Infrared radiation is characterized by rectilinear propagation, it is able to refract, reflect and polarize. Capable of penetrating a range of materials that are opaque to visible light.
In other words, radiant heat transfer can be characterized as transferheat in the form of electromagnetic wave energy, while the process proceeds between surfaces that are in the process of mutual radiation.
The intensity index is determined by the mutual arrangement of surfaces, the emissive and absorbing abilities of bodies. Radiant heat transfer between bodies differs from convection and heat conduction processes in that heat can be sent through a vacuum. The similarity of this phenomenon with others is due to the transfer of heat between bodies with different temperature indices.
Radiation flux
Radiant heat transfer between bodies has a certain number of radiation fluxes:
- The intrinsic radiation flux - E, which depends on the temperature index T and the optical characteristics of the body.
- Flows of incident radiation.
- Absorbed, reflected and transmitted types of radiation fluxes. In sum, they equal Epad.
The environment in which heat exchange occurs can absorb radiation and introduce its own.
Radiant heat exchange between a certain number of bodies is described by an effective radiation flux:
EEF=E+EOTR=E+(1-A)EFAD. Bodies, at any temperature, having indicators L=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 equilibrium 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 lawsBoltzmann
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 operate with information, improving it over the years. Consider its wording.
The Stefan-Boltzmann law is an integral law that describes some features of absolutely black bodies. It allows you to determine the dependence of the radiation power density of a blackbody on its temperature index.
Obeying the law
The laws of radiant heat transfer obey the Stefan-Boltzmann law. The level of intensity of heat transfer through heat conduction and convection is proportional to temperature. The radiant energy in the heat flux is proportional to the temperature to the fourth power. It looks like this:
q=σ A (T14 – T2 4).
In the formula, q is the heat flux, A is the surface area of the body radiating energy, T1 and T2are the temperatures emitting bodies and the environment that absorbs this radiation.
The above law of heat radiation exactly describes only the ideal radiation created by an absolutely black body (a.h.t.). There are practically no such bodies in life. However, flat black surfaces approach the A. Ch. T. Radiation from light bodies is relatively weak.
There is an emissivity factor introduced to take into account the deviation from ideality of numerousamount of s.t. into the right component of the expression explaining the Stefan-Boltzmann law. The emissivity index is equal to a value less than one. A flat black surface can bring this coefficient up to 0.98, while a metal mirror will not exceed 0.05. Therefore, the absorbances are high for black bodies and low for specular bodies.
About the gray body (s.t.)
In heat transfer, there is often a mention of such a term as a gray body. This object is a body that has a spectral type absorption coefficient of electromagnetic radiation less than one, which is not based on the wavelength (frequency).
Emission of heat is the same according to the spectral composition of the radiation of a black body with the same temperature. A gray body differs from a black one by a lower indicator of energy compatibility. To the spectral blackness level of s.t. wavelength is not affected. In visible light, soot, coal and platinum powder (black) are close to the gray body.
Fields of application of heat transfer knowledge
Emission of heat is constantly happening around us. In residential and office premises, you can often find electric heaters that are engaged in heat radiation, and we see it in the form of a reddish glow of a spiral - such heat belongs to the visible, it “stands” at the edge of the infrared spectrum.
Heating the room, in fact, is engaged in an invisible component of infrared radiation. Night vision device appliesa source of heat radiation and receivers sensitive to infrared radiation, which allow you to navigate well in the dark.
Sun Energy
The sun is rightfully the most powerful emitter of energy of a thermal nature. It heats our planet from a distance of one hundred and fifty million kilometers. The intensity of solar radiation, which has been recorded for many years and by various stations located in various parts of the earth, corresponds to approximately 1.37 W/m2.
It is the energy of the sun that is the source of life on planet Earth. Currently, many minds are busy trying to find the most effective way to use it. Now we know solar panels that can heat residential buildings and provide energy for everyday needs.
In conclusion
Summing up, the reader can now define radiant heat transfer. Describe this phenomenon in life and nature. Radiant energy is the main characteristic of the transmitted energy wave in such a phenomenon, and the listed formulas show how to calculate it. In the 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.
