Scribd
Upload
52 views4 pages

Emissivity

The document discusses the concept of emissivity, defined as the ratio of a surface's emissive power to that of a black surface, with values ranging from 0 to 1. It includes practical applications of emissivity in thermal radiation heat exchange and provides a table of emissivity values for various materials. The document also outlines experimental observations and calculations related to measuring emissivity under different conditions.

Uploaded by

Ram Bhandari
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
52 views4 pages

Emissivity

The document discusses the concept of emissivity, defined as the ratio of a surface's emissive power to that of a black surface, with values ranging from 0 to 1. It includes practical applications of emissivity in thermal radiation heat exchange and provides a table of emissivity values for various materials. The document also outlines experimental observations and calculations related to measuring emissivity under different conditions.

Uploaded by

Ram Bhandari
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
You are on page 1/ 4

INTRODUCTION :

Emissivity of a surface is defined as the ratio of emissive power of a surface (e) to emissive
power of a black surface (eb)
e
e = emissivity = -----
eb
THEORY
Emissivity of a perfect black surface is 1 and Emissivity of a perfect surface is 0. All
the practical surface are having emissivity between 0 to 1.

The knowledge of emissivity is necessary when one want to find out heat exchange by
Thermal radiation. In the design of appliances based on sciar Energy, the knowledge of
emissivities of collector surface and reflecting surfaces is a must for the concerned engineer.

At a particular wavelength, over the spectrum of wavelengths the monochromatic


EMISSIVITY is defined as

e
 = --------- Where e is monochromatic emissive power
eb and eb is monochromatic emissive power of black surface

Table given below gives the values of Emissivities of some concerned material surfaces
for ready reference.

Sr.No Surface Temperature Emissivity


1. Metals-polished copper 20-5000C 0 -1.0
Steel,- stinless- steel, nickel
2. Oxidised-copper, steel, Upto 10000C 0.6 -1.0
Stainless- steel, nickel
3. Non Metals : 20-1000C 0.8 – 1.0
Brick, wood, mable, water

OBSERVATION :
To be recorded under study state conditions.
1. Diameter of the dise (d) = 0.15 m
2. Thickness of disc assembly (t) = 0.12 m
3. Ares of disc surface (A)Exposed. = ( d2 / 4 ) m2
4. Heater input to black dise (w1) = Watts
V2 = Volts, A2 Amps.
W2 = V2 x A2
0
5. Temperatures of back dise (W2) = C + 273 = K
V2 = Volts, A2 = Amps.
W2 = V2 x A2
6. Heater input non-back dise (w1) = Watts
V1 = Volts, A1 = Amps.
W1 = V1 x A1
7. Temperatures of non-black disc (T1) 0C = 0
C + 273 = K
8. T4 =(T2 + T1) / 2 = K
0 0
9. Temperatures of the enclosure (T3) C = C + 273 = K

CALCULATION :
Under steady condition, heater input to black disc.
W2 = E
b oA (T44 – T34 ) + conduction loss through base
+ Natural convection loss form black disc.

W1 = E
nb oA (T44 – T34 ) + conduction loss through base
+ Natural convection loss form black disc.

Since the disc assemblies are identical in shape and size and she /are placed in a
symmetrical manner inside the enclosure, Conduction and convection loss must be same for
both the discs when their temperatures are identifical

CONCLUSIONS :
1. Commet on the value of emissivity obtained.
2. Conduct the experiment by changing the values of heater inputs and obtain
emissivity values at different temperatures to study its variation with temperature
REFERENCES-
1) A text book on heat transfer by –Dr. S.P. Sukhatme
2) Fundamental of heat transfer by-M.Mikheyev.
3) Heat transfer by- V.P. Isachenko, V.A.Osipova, A.S.Sukomel

QUESTIONS:
Q.1 Define black body, emissivity & its significance

Q.2 comment on the value of emissivity obtained

Q.3 Explain radiation with suitable example & application.

You might also like