In the previous sections, we have discussed the transfer of heat through conduction and convection, the two processes requiring presence of a medium. The means by which energy is transmitted between bodies without contact and in the absence of intervening medium is known as radiation. Transmission of energy through radio waves, visible light, X-rays, cosmic rays, etc., all belong to this category, having Heat Radiation different frequencies in the spectrum of electromagnetic radiation.
Here we are concerned with the type of radiation which is principally dependent on the temperature of the body, known as thermal radiation and belonging mostly to the infrared and to a small extent to the visible portion of the electromagnetic radiation spectrum. The heat transferred into or out of an object by thermal radiation is a function of several components. These include its surface re ectivity, emissivity, surface area, temper-
ature and geometric orientation with respect to other thermally participating objects.
In turn, an object’s surface re ectivity and emissivity is a function of its surface conditions (roughness, nish, etc.) and composition.
To account for a body’s outgoing radiation (or its emissive power, de ned as the heat ux per unit time), one makes a comparison to a perfect body, which absorbs the entire amount of heat radiation falling on its surface as well as emits the maximum possible thermal radiation at any given temperature. Such an object is known as a black body. The concept of black body is important in understanding the radiation of heat. According to Stefan–Boltzmann’s law, heat emitted by a black body at any given temperature, qb (W m 2 ), is expressed as follows for a unit area in a unit time:
qb ¼ sT4
where qb is the heat ow through radiation from the surface of a black body, T the temperature, and s a constant known as the Stefan–Boltzmann constant, with a theoretical value of 5.67 10 8 Wm 2 K 4 . Because no material ideally ful lls the properties of absorption and emission of the theoretically de ned black body, for practical purposes a new constant of emissivity, e, is de ned for real surfaces as
Here we are concerned with the type of radiation which is principally dependent on the temperature of the body, known as thermal radiation and belonging mostly to the infrared and to a small extent to the visible portion of the electromagnetic radiation spectrum. The heat transferred into or out of an object by thermal radiation is a function of several components. These include its surface re ectivity, emissivity, surface area, temper-
ature and geometric orientation with respect to other thermally participating objects.
In turn, an object’s surface re ectivity and emissivity is a function of its surface conditions (roughness, nish, etc.) and composition.
To account for a body’s outgoing radiation (or its emissive power, de ned as the heat ux per unit time), one makes a comparison to a perfect body, which absorbs the entire amount of heat radiation falling on its surface as well as emits the maximum possible thermal radiation at any given temperature. Such an object is known as a black body. The concept of black body is important in understanding the radiation of heat. According to Stefan–Boltzmann’s law, heat emitted by a black body at any given temperature, qb (W m 2 ), is expressed as follows for a unit area in a unit time:
qb ¼ sT4
where qb is the heat ow through radiation from the surface of a black body, T the temperature, and s a constant known as the Stefan–Boltzmann constant, with a theoretical value of 5.67 10 8 Wm 2 K 4 . Because no material ideally ful lls the properties of absorption and emission of the theoretically de ned black body, for practical purposes a new constant of emissivity, e, is de ned for real surfaces as
¼ q
qbq being the radiant heat from a real surface
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