Conduction and convection are heat transfer processes that require the presence of a medium. Radiation heat transfer is characteristically different from the other two in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Electromagnetic radiation has some proper characteristics depending on the frequency and wavelengths of the radiation. The phenomenon of radiation is not yet fully understood. Two theories have been used to explain radiation; however neither of them is perfectly satisfactory.
First, the earlier theory which originated from the concept of a hypothetical medium referred as ether. Ether supposedly fills all evacuated or non evacuated spaces. The transmission of light or of radiant heat are allowed by the propagation of electromagnetic waves in the ether. Electromagnetic waves have similar characteristics to television and radio broadcasting waves they only differ in wavelength. All electromagnetic waves travel at the same speed; therefore, shorter wavelengths are associated with high frequencies. Since every body or fluid is submerged in the ether, due to the vibration of the molecules, any body or fluid can potentially initiate an electromagnetic wave. All bodies generate and receive electromagnetic waves at the expense of its stored energy  The second theory of radiation is best known as the quantum theory and was first offered by Max Planck in 1900. According to this theory, energy emitted by a radiator is not continuous but is in the form of quanta. Planck claimed that quantities had different sizes and frequencies of vibration similarly to the wave theory. The energy E is found by the expression E = hν, where h is the Planck's constant and ν is the frequency. Higher frequencies are originated by high temperatures and create an increase of energy in the quantum. While the propagation of electromagnetic waves of all wavelengths is often referred as "radiation," thermal radiation is often constrained to the visible and infrared regions. For engineering purposes, it may be stated that thermal radiation is a form of electromagnetic radiation which varies on the nature of a surface and its temperature. Radiation waves may travel in unusual patterns compared to conduction heat flow. Radiation allows waves to travel from a heated body through a cold nonabsorbing or partially absorbing medium and reach a warmer body again. This is the case of the radiation waves that travel from the sun to the earth.
The emissivity of a given surface is the measure of its ability to emit radiation energy in comparison to a blackbody at the same temperature. The emissivity of a surface varies between zero and one. The emissivity of a real surface varies as a function of the surface temperature, the wavelength, and the direction of the emitted radiation. The fundamental emissivity of a surface at a given temperature is the spectral directional emissivity, which is defined as the ratio of the intensity of radiation emitted by the surface at a specified wavelength and direction to that emitted by a blackbody under the same conditions. The total directional emissivity is defined in the same fashion by using the total intensities integrated over all wavelengths. In practice, a more convenient method is used: hemispherical properties. These properties are spectrally and directionally averaged. The emissivity of a surface at a specified wavelength may vary as temperature changes since the spectral distribution of emitted radiation changes with temperature. Finally the total hemispherical emissivity is defined in terms of the radiation energy emitted over all wavelengths in all directions. Radiation is a complex phenomenon. The dependability of its properties in wavelength and direction makes it even more complicated. Therefore, the gray and diffuse approximation methods are commonly used to perform radiation calculations. A gray surface is characterized by having properties independent of wavelength, and a diffuse surface has properties independent of direction.
Absorptivity (α), reflectivity (ρ) and transmissivity (t)
If the amounts of radiation energy absorbed, reflected, and transmitted when radiation strikes a surface are measured in percentage of the total energy in the incident electromagnetic waves, the total energy will be divided into three groups. They are called absorptivity (α), reflectivity (ρ) and transmissivity (t).
- α + ρ + t = 1 (1)
- Absorption is the fraction of radiation absorbed by a surface.
- Reflectivity is the fraction reflected by the surface.
- Transmissivity is the fraction transmitted by the surface.
A body is considered transparent if it can transmit some of the radiation waves falling on its surface. If electromagnetic waves are not transmitted through the substance it is therefore called opaque. When radiation waves hit the surface of an opaque body, some of the waves are reflected back while the other waves are absorbed by a thin layer of the material close to the surface. For opaque bodies, Equation 1 reduces to:
- α + ρ = 1 (2)
Reflectivity deviates from the other properties in that it is bidirectional in nature. In other words, this property depends on the direction of the incident of radiation as well as the direction of the reflection. Therefore, the reflected rays of a radiation spectrum incident on a real surface in a specified direction forms an irregular shape that is not easily predictable. In practice, surfaces are assumed to reflect in a perfectly specular or diffuse manner. In a specular reflection, the angles of reflection and incidence are equal. In diffuse reflection, radiation is reflected equally in all directions. Reflection from smooth and polished surfaces can be assumed to be specular reflection, whereas reflection from rough surfaces approximates diffuse reflection. In radiation analysis a surface is defined as smooth if the height of the surface roughness is much smaller relative to the wavelength of the incident radiation.
- Heat conduction
- Heat transfer
- Biot number
- Greenhouse effect
- Forced convection
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- Becker, Martin. Heat Transfer a Modern Approach New York: Plenum Publishing Corporation, 1986.
- Yunus, Cengel. Heat and Mass Transfer.New York: Mc Graw Hill, 2007.
- Edwards, D.K. Radiation Heat Transfer. New York: Hemisphere Publishing Corporation, 1981.