CHAPTER 12 RADIATION HEAT TRANSFER. Electromagnetic Radiation – electromagnetic waves (brought upon by accelerated charges or changing electric currents)

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CHAPTER 12 RADIATION HEAT TRANSFER

Electromagnetic Radiation – electromagnetic waves (brought upon by accelerated charges or changing electric currents) They represent energy emitted by matter as a result of changes in electronic configurations of the atoms/molecules Radiation vs. conduction and convection Medium temperature of medium They travel at the speed of light!!! And are characterized by their wave length and frequency! Frequency (depend only on source not medium) Speed of light (Co=3x10 8 m/s)) In a medium it is C c /n, n=1 for air and 1.5 for water and glass

Frequency can range from a few cycles to millions per second! Electromagnetic radiation can be viewed as propagation of a collection of discrete packets of energy called photons or quanta (as proposed by Max Plank-quantum theory) constants Shorter wavelengths have more energy and can be more damaging! We try to avoind gamma and x-rays  harmefull!)

THERMAL RADIATION Different Electromagnetic waves-produced differently Gamma  nuclear reactions X-rays- bombardment of metals with high energy electrons Thermal radiation emitted as a result of vibration and rotational motion of molecules/atoms/electrons. But temperature is also dependent on these!  thermal radiation increases with increase in temperature  Emitted by all matter above 0K

Radiation is a volumetric phenomena However for opaque solids  surface phenomena Since internal radiation cannot reach the surface! Incident radiation is absorbed within a few microns

BLACKBODY RADIATION Blackbody= idealized body =perfect emitter and absorber of radiation! Radiation energy emitted by blackbody/time (Emissive power) = (takes into account sum of radiation over all wavelengths) Stefan-Boltzmann law What about for one wave length= Spectral black body emissive power)

Wien’s displacement law :

We maybe interested in radiation over some wavelength band! Analysis is complicated need numerical solution Therefore use the following dimensionless quantity, Called, blackbody radiation function: Represents the fraction of radiation emitted from a black body at temperature T in the wavelength band 0-

Irradiation, G Radiation energy incident on a surface per unit surface area per unit time Fraction absorbed by surface Fraction reflected by surface Fraction transmitted by surface

First Law of Thermo tells us : The above is for total hemispherical properties, i.e. for all directions and wavelengths incident. Can also consider the same definitions for specific wavelength or direction: