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!

v

c

Frequency(depend only on source not medium)

Speed of light (Co=3x108m/s))In a medium it is Cc/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)

hc

hve 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 reactionsX-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:

KmT power .2897)( max

We maybe interested in radiation over some wavelength band!

dTETE bb )()(0

0,

Analysis is complicated need numerical solution

Therefore use the following dimensionless quantity,

Called, blackbody radiation function:

40

)()(

T

dTETf

b

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: