1) The document discusses concepts related to radiation heat transfer including radiative properties like reflectivity, transmissivity, and emissivity. 2) It describes how radiation shields can be used to reduce radiation heat transfer between surfaces by increasing the resistance to radiation flow through multiple reflections. 3) An example is given showing the calculation of radiation heat transfer rate between two parallel plates separated by an aluminum radiation shield, and how the rate is reduced compared to the case without the 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.