The object is shooting outwardradiant energyAt the same time, it will also continuously absorb the radiant energy emitted by other objects around it and convert it into heat energy again. The heat transfer process of mutual emission and absorption of radiant energy between such objects is called radiation heat transfer.If the radiation heat transfer is carried out between two objects with different temperatures, the result of heat transfer is that the high temperature object transfers heat to the low temperature object. If the two objects have the same temperature, the radiation heat transfer between objects is equal to zero, but the radiation and absorption process between objects is still in progress.
Also called thermal radiation.A basic way of heat transfer.
Radiation is a phenomenon in which energy is transmitted by electromagnetic waves.Objects emit radiant energy for various reasons.The process of emitting radiant energy due to heat is called thermal radiation.Transmitted in the form of electromagnetic waves and propagated in space, when encountering another object, it will be partially or completely absorbed and again transformed into heat energy.Heat radiation is different from heat conduction and convection heat transfer. Radiation is not only the transfer of energy but also the transformation of energy forms.In addition, radiant energy can be transmitted in vacuum without any material as a medium.The most important thermal radiation in industry is the mutual radiation between solids, and only at high temperatures can radiation become the main mode of heat transfer.Liquid and gas can also transfer heat in the form of radiation, but only a small part of the total heat transfer.[1]
Chinese name
Radiant heat transfer
Foreign name
radiant heat transfer
Alias
thermal radiation
Interpretation
An object absorbs the radiant energy emitted by its surrounding objects
Realize between hot and cold objects by electromagnetic wave radiationHeat transferIt is a non-contact heat transfer process that can also be carried out in vacuum.The electromagnetic wave emitted by objects is theoretically distributed in the whole spectrum range, but in the temperature range encountered in industry, it is of practical significance that the wavelength is between 0.38 and 1000 umthermal radiationAnd most of them are located in infrared (also called thermalradial）0.76~20um in the section.The so-called infrared heating is to use the heat radiation in this section.[1]
Detailed introduction
While any object emits radiant energy, it also continuously absorbs radiant energy from surrounding objects.The difference between the energy radiated and absorbed by an object is the net energy transmitted by it.The radiant capacity of an object (that is, the energy radiated from a unit surface in a unit time) increases rapidly with the increase of temperature.
If the energy of the thermal radiation reaching the surface of the object is completely absorbed, the object is called an absolute black body, or black body for short;
If the energy of thermal radiation reaching the surface of the object is completely reflected;When the reflection is regular, the object is called a mirror;
If the energy of the thermal radiation reaching the surface of the object passes through the object completely, the object is called a diathermy.
Actually notAbsolute blackbodyWith absolute white body, only some objects are close to absolute black body or absolute white body.For example, the black paint surface without gloss is close toBlackbodyAnd its absorption rate is 0.97-0.98;The polished copper surface is close to white body, and its reflectivity can reach 0.97.The absorption and reflection properties of solid surface are mainly affected by the surface condition and color, and the influence of surface condition is often greater than that of color.Solids and liquids are generally impermeable to heat.The energy of thermal radiation is completely absorbed after passing through the surface of solid or liquid only for a short distance (generally less than 1mm, and passing through the metal surface only for 1m).The gas has almost no ability to reflect the thermal radiation energy, and the single atom and symmetric doubleAtomic gas(such as Ar, He, H, N, O, etc.), can be regarded as a diathermy, and polyatomic gases (such as CO, HO, SO, NH, CH, etc.) have considerable absorption capacity within a specific wavelength range.[2]
research meaning
Announce
edit
The study of heat radiation law is very important for the reasonable design of heat transfer in the furnace, and also has positive significance for the labor protection of high-temperature furnace operators.When a system needs insulation, even if the temperature of the system is not high, the impact of radiation heat transfer cannot be ignored.For example, the silver plating of thermos bottle is to reduce the heat loss caused by radiation heat transfer.
Absorptive capacity
Announce
edit
Theoretical research has proved that the formula for calculating the radiation capacity of blackbody is called Stefan Boltzmann law.The constant is the radiation constant of the blackbody (or Stefan Boltzmann constant), whose value is 5.669 × 10 W/(mK).This equation shows that the temperature is relative to thethermal radiationHas a great impact.The thermal radiation is often negligible at low temperature (such as in ordinary heat exchangers);At high temperature (such as in the furnace), it becomes the main way of heat transfer.
The wavelength distribution of the radiant energy of the actual object varies with the object and temperature.Assume that the radiation ability of the actual object to radiate any wave [kg1] [kg1] isBlackbody radiationThe capacity of the same wavelength is [152-0ru];If/[152-0ru]=constant, that is, the radiation ability of the object is independent of the wavelength, then this object is calledGray body。Most engineering materials are close to gray body in the wavelength range of thermal radiation.The radiation ability of gray body is related to the surface condition and temperature of the object.
The ratio of the radiant capacity of an object to the radiant capacity of a blackbody at the same temperature is equal to the ratio of their respective radiant coefficients, called blackness, which represents the relative radiant capacity of an object.G. R. Kirchhoff found that the ratio of radiant capacity to absorptivity of any object is the same, and is identical to that at the same temperatureAbsolute blackbodyThe radiation ability of, namely: Kirchhoff's law.It indicates that theabsorptivityIt is equal to the density in numerical value, that is, the greater the radiation capacity of the object, the greater the absorption capacity.[3]
Correlation theorem
Announce
edit
The rate of radiant heat transfer between two objects can be expressed as the surface temperature of two objects respectively;1 surface area of an object;The angular coefficient of the benchmark represents the fraction of the energy radiated from one object projected onto the surface, which depends on the shape, size and relative position of two objects;Is the total radiation coefficient, whose value is related to the blackness, size, shape and relative position of the two objects.[4]
Basic concepts
The object that can fully absorb the radiant energy, that is, the absorptivity A=1, is called black body or absolute black body.
The object that can fully reflect the radiant energy, that is, the reflectivity R=1, is called a mirror or an absolute white body.
(1) The absorptivity A of the gray body does not change with the wavelength of the radiation line.
(2) Grey body is heat impermeable, that is, A+R=1.
Where, the radiation constant of h blackbody is 5.67 * 10W/(m.K)
C-radiation coefficient of black body, which is 5.67W/(m.K)
It should be pointed out that the quartic law can also be extended to the gray body, in which case, C is the radiation coefficient of the gray body, W/(m · K).
As long as the blackness of the object is known, the radiation capacity of the object can be calculated from the above equation.
Kirchhoff's law
Kirchhoff's lawIt reveals the positive relationship between the radiation capacity of the object and the absorptivity A.
q=E1-A1Eb
Where q - heat flux of radiant heat transfer between two plates, W/m.
When the two plates reach thermal balance, i.e. T1=T2, q=0, so
E1=A1Eb
Other interpretations
Announce
edit
The process of an object transmitting energy in the form of electromagnetic waves is called radiation, and the transmitted energy is called radiant energy.Objects can generate electromagnetic waves for different reasons. The electromagnetic wave radiation caused by heat isthermal radiation。In the process of thermal radiation, the heat energy of the object is transformed intoradiant energyAs long as the temperature of the object is constant, the radiant energy emitted is also constant.While an object radiates energy outward, it may also continuously absorb the radiant energy emitted by other objects around it.The so-called radiation heat transfer is a comprehensive process of mutual radiation and energy absorption between different objects.Obviously, the net result of radiative heat transfer is that high temperature objects transfer energy to low temperature objects.
The essence of thermal radiation and optical radiation is completely the same, except for the range of wavelength.Theoretically, the electromagnetic wave length of thermal radiation ranges from zero to infinity, but the wavelength range of practical significance is 0.4~20 μ m, in which the wavelength range of visible light is about 0.4~0.8 μ m, and the wavelength range of infrared light is 0.8-20/μ m.Visible light and infrared light are collectively referred to as thermal rays.But the thermal radiation of infrared lightthermal radiationIt plays a decisive role. Only at a very high temperature can the thermal effect of visible light be detected.
Thermal rays, like visible rays, obey the laws of reflection and refraction and can propagate in a straight line in a homogeneous medium.stayvacuumAnd most gases（inert gasAnd symmetric doubleAtomic gas）Medium, thermal rays can be completely transmitted, but for most solids and liquids, thermal rays cannot be transmitted.Therefore, only objects that can see each other can conduct radiation heat transfer.[5]
