Thermal radiation black surfaces

Blackbody Radiation Engineering calculations of thermal radiation from surfaces are best keyed to the radiation characteristics of the blackbody, or ideal radiator. The characteristic properties of a blackbody are that it absorbs all the radiation incident on its surface and that the quality and intensity of the radiation it emits are completely determined by its temperature. The total radiative fliix throughout a hemisphere from a black surface of area A and absolute temperature T is given by the Stefan-Boltzmann law ... 

The thermal radiation received from the fireball on a target is given by equation 9.1-31, where Q is the radiation received by a black body target (kW/m ) r is the atmospheric transmissivity (dimensionless), E = surface emitted flux in kW/m", and f is a dimensionless view factor. 

Electrically-heated carbide elements, JO mm in diameter and 0.5 m long, radiating essentially as black bodies, are to be used in the construction of a heater in which thermal radiation from the surroundings is negligible. If the surface temperature of the carbide is limited to 1750 K, how many elements are required to provide a radiated thermal output of 500 kW7... 

Since the earth has temperature, it emits radiant energy called thermal radiation or planetary infrared radiation. Measurements by satellites show an average radiant emission from the earth of about 240 watts per square meter. This is equivalent to the radiation that a black body would emit if its temperature is at -19°C (-3°F). This is also the same energy rate as the solar constant averaged over the earth s surface minus the 30% reflected radiation. This shows that the amount of radiation emitted by the earth is closely balanced by the amount of solar energy absorbed and since the earth is in this state of balance, its temperature will change relatively slowly from year to year. 

Accuracy of Pyrometers Most of the temperature estimation methods for pyrometers assume that the object is either a gray body or has known emissivity values. The emissivity of the non-black body depends on the internal state or the surface geometry of the objects. Also the medium through which the thermal radiation passes is not always transparent. These inherent uncertainties of the emissivity values make the accurate estimation of the temperature of the target objects difficult. Proper selection of the pyrometer and accurate emissivity values can provide a high level of accuracy. 

Time-Resolved Laser-Induced Incandescence (by Prof. Alfred Leipertz et al.) introduces an online characterization technique (time-resolved laser-induced incandescence, TIRE-LII) for nano-scaled particles, including measurements of particle size and size distribution, particle mass concentration and specific surface area, with emphasis on carbonaceous particles. Measurements are based on the time-resolved thermal radiation signals from nanoparticles after they have been heated by high-energetic laser pulse up to incandescence or sublimation. The technique has been applied in in situ monitoring soot formation and oxidation in combustion, diesel raw exhaust, carbon black formation, and in metal and metal oxide process control. 

It is important to note at this point that the blackness of a surface-to-thermal radiation can be quite deceiving insofar as visual observations are concerned. A surface coated with lampblack appears black to the eye and turns out to be black for the thermal-radiation spectrum. On the other hand, snow and ice appear quite bright to the eye but are essentially black for long-wavelength thermal radiation. Many white paints are also essentially black for long-wavelength radiation. This point will be discussed further in later sections. 

The surfaces of a two-surface enclosure exchange heat with one another by thermal radiation. Surface 1 has a tempetature of 400 K, an area of 0.2 m, and a total emissivity of 0.4. Surface 2 is black, has a temperature of 600 K. and an area of 0.3 ntl If the view factor is 0.3, the rate of radiation heal transfer between the two surfaces is ( ) 87W (h) I35W (c) 244W... 

The concept of hlackhody is determining the basis for describing the radiation properties of real surfaces. The black body denotes an ideal radiative surface which absorb all incident radiation, being a diffuse emitter and emit a maximum amount of energy as thermal radiation for a given wavelength and temperature. The black body can be considered as a perfect absorber and emitter. 

The equation of radiative transfer will not be solved here since solutions to some approximations of the equation are well known. In photon radiation, it has served as the framework for photon radiative transfer. It is well known that in the optically thin or ballistic photon limit, one gets the heat flux as q = g T[ - T ) from this equation for radiation between two black surfaces [13]. For the case of phonons, this is known as the Casimir limit. In the optically thick or diffusive limit, the equation reduces to q = -kpVT where kp is the photon thermal conductivity. The same results can be derived for phonon radiative transfer [14,15]. 

It was concluded that radiation was not important for their experimental conditions. You [46] measured flame radiation to the surroundings, using commercial heat flux gauges. The radiant flux to a plane surface was also indirectly measured. For these measurements, commercial heat flux gauges were coated with either gold or black foil, to measure the convective or total heat flux, respectively. The radiant flux was determined by subtracting the convective flux from the total. Thermal radiation was... 

Since most solid bodies are opaque to thermal radiation, transmission is negligible in most cases. To account for a body s outgoing radiation, we make a comparison with a perfect body that emits as much radiation as possible, known as a black-body. The ratio of the actual heat flow to the heat flow of a black-body is defined as the surface emissivity e, and ranges from about 0.05 for polished metal surfaces to more than about 0.7 for ice, cast iron, corroded iron, rubber, and brick. The surface emissivity equals the absorption fraction (Kirchhoffs law) ... 

The quantity of thermal energy leaving a surface by radiation varies with the fourth power of the absolute temperature of the surface and certain characteristics of the surface. A perfect emitter of thermal radiation (called a black body) emits energy at a rate Q) according to the Stefan-Boltzmann (1884) equation ... 

The heat flux radiated from a real surface is less than that from an ideal black body surface at the same temperature. The ratio of real to black body flux is the normal total emissivity. Emissivity, like thermal conductivity, is a property which must be determined experimentally. 

The preceding calculation of the thermal energy balance of a planet neglected any absorption of radiation by molecules within the atmosphere. Radiation trapping in the infrared by molecules such as CO2 and H20 provides an additional mechanism for raising the surface temperature - the greenhouse effect. The local temperature of a planet can then be enhanced over its black body temperature by the atmosphere. 

An average of temperature records on the earth s surface over a year indicates that the earth s average surface temperature is about 14°C (57°F). But, the earth s 240 watts per square meter of thermal infrared radiation as measured by satellite is equivalent to the radiation emitted by a black body whose temperature is about -19°C (-3°F), not the 14°C (57°F) average measured at the earth s surface. The 33°C (60°F) difference between the apparent temperature of the earth as seen in space and the actual temperature of the earth s surface is attributed to the greenhouse effect. 

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