Radiative Properties of material adn radiation shield

1. Module -7
Radiation Heat Transfer
By
Faculty: Mr. LALAN KUMAR
Assistant Professor
Department of Mechanical Engineering
Katihar Engineering College Katihar

2. 01 Version: 1, KEC Katihar
Radiative Properties
 When radiation strikes a surface, a portion of it is reflected, and
the rest enters the surface.
 Of the portion that enters the surface, some are absorbed by the
material, and the remaining radiation is transmitted through.
 The ratio of reflected energy to the incident energy is called
reflectivity, ρ.
 Transmissivity (τ) is defined as the fraction of the incident energy
that is transmitted through the object.
 Absorptivity (α) is defined as the fraction of the incident energy that is absorbed by the
object.
 The three radiative properties all have values between zero and 1.
 Furthermore, since the reflected, transmitted, and absorbed radiation must add up to
equal the incident energy, the following can be said about the three properties:
• a + t +r = 1

3. Emissivity
02 Version: 1, KEC Katihar
 A black body is an ideal emitter.
 The energy emitted by any real surface is less than the energy emitted by a black body at the same
temperature.
 At a defined temperature, a black body has the highest monochromatic emissive power at all
wavelengths.
 The ratio of the monochromatic emissive power El to the monochromatic blackbody emissive
power Ebl at the same temperature is the spectral hemispherical emissivity of the surface.
l
l
l
bE
E
)(

4. 03 Version: 1, KEC Katihar
 The total (hemispherical emissive power is, then, given by



00
)( lll ll dEdEE b
 Define total (hemispherical) emissivity, at a defined temperature









0
0
0
0
)(
l
ll
l
l

l
l
l
l
dE
dE
dE
dE
b
b
b
Here, e can be interpreted as either the emissivity of a body,
which is wavelength independent, i.e., el is constant, or as the
average emissivity of a surface at that temperature.
A surface whose properties are independent of the wavelength is known as a gray surface.
The emissive power of a real surface is given by

5. 04 Version: 1, KEC Katihar
Absorptivity a, Reflectivity r, and Transmissivity t
 Consider a semi-transparent sheet that receives incident radiant
energy flux, also known as irradiation, G .
 Let dG represent the irradiation in the waveband l to l + dl.
 Part of it may be absorbed, part of it reflected at the surface,
and the rest transmitted through the sheet.
 We define monochromatic properties,
• Monochromatic Absorptivity :
dG
dGa
la 
• Total Absorptivity :
G
G
d a
l laa  

0

6. 05 Version: 1, KEC Katihar
• Monochromatic reflectivity :
dG
dGr
lr 
• Total reflectivity :
G
G
d r
l lrr  

0
• Monochromatic Transmissivity :
dG
dGt
lt 
• Total Transmissivity :
G
G
d t
l ltt  

0

7.  Radiation Shields And The Radiation Effect
06 Version: 1, KEC Katihar
 Radiation heat transfer between two surfaces can be reduced greatly by inserting a thin,
high-reflectivity (low-emissivity) sheet of material between the two surfaces. Such highly
reflective thin plates or shells are called radiation shields.
 The role of the radiation shield is to reduce the rate of radiation heat transfer by
placing additional resistances in the path of radiation heat flow.
 The lower the emissivity of the shield, the higher the resistance.

8. 07 Version: 1, KEC Katihar
 The radiation shield placed between two parallel plates and the radiation network associated
with it.

9. 08 Version: 1, KEC Katihar
 The resistances are connected in series, and thus the rate of radiation heat transfer is
F13 =F23 =1 and A1 = A2 = A3 = A for infinite parallel platesNote :

10. 09 Version: 1, KEC Katihar
 Then the radiation heat transfer through large parallel plates separated by N radiation shields
becomes
 If the emissivities of all surfaces are equal

11. 10 Version: 1, KEC Katihar
 A thin aluminum sheet with an emissivity of 0.1 on both sides is placed between two very
large parallel plates that are maintained at uniform temperatures T1 = 800 K and T2 = 500
K and have emissivities Ɛ1 = 0.2 and Ɛ2 = 0.7, respectively. Determine the net rate of
radiation heat transfer between the two plates per unit surface area of the plates and
compare the result to that without the shield.