Thermal energy emitted by matter as a result of vibrational and rotational movements of molecules, atoms and electrons. The energy is transported by electromagnetic waves (or photons). Radiation requires no medium for its propagation, therefore, can take place also in vacuum. All matters emit radiation as long as they have a finite (greater than absolute zero) temperature. The rate at which radiation energy is emitted is usually quantified by the modified Stefan-Bolzmann law:

where the emissivity, e , is a property of the surface characterizing how effectively the surface radiates compared to a "blackbody" ( ). E=q/A (W/m2) is the surface emissive power. s is the Stefan-Boltzmann constant

(). Tb is the absolute temperature of the surface (in Kelvin).

Blackbody: is an ideal surface which emits the maximum possible thermal radiation at a given temperature.


Irradiation (G): All incoming radiation that is incident on the surface. It can then be transmitted, absorbed or reflected from the surface. G = Gtrans + Gabs + Gref = t G + aG + rG

Where t, a and r represent transmissivity, absorptivity and reflectivity, respectively. t + a + r =1

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Electromagnetic radiation spectrum

Thermal radiation spectrum range: 0.1 to 100 mm

It includes some ultraviolet (UV) radiation and all visible (0.4-0.76 mm) and infrared radiation (IR).

Question: Can microwave-oven heating process be considered one kind of heat transfer mode?

Strictly speaking, it is not. It heats up food through microwave radiation, not thermal radiation. However, when microwave energy (with very high frequency) interacts with water molecules inside the food, it generates heat inside the food. Therefore, it can be modeled as some forms of heat generation but not relate to any modes of heat transfer.

The Planck Distribution

The Planck law describes theoretical spectral distribution for the emissive power of a black body. It can be written as

where C1=3.742 108 (W.mm4/m2) and C2=1.439 104 (mm.K) are two constants. The planck distribution is shown in the following figure as a function of wavelength for different body temperatures.

Spectral blackbody emissive power