1. CHAPTER 12 RADIATION HEAT TRANSFER

2. 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

3. 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!)

4. 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

5. 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

6. 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)

8. Wien’s displacement law :

11. 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-

15. 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

16. 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: