1. Define Radiation.
The heat transfer from one body to
another without any transmitting medium is known as radiation. It is an
electromagnetic wave phenomenon.
2. What is black body?
Black body is an ideal surface having
the following properties.
1.A black body absorbs all incident
radiation, regardless of wave length and direction.
2.For a prescribed temperature and wave
length, no surface can emit more energy than black body.
3. State Wien’s
displacement law.
The Wien’s
relationshiplawgivesbetweentemperaturethe and wave length corresponding to the
maximum spectral emissive power of the black body at that temperature.
4. What is meant by gray body?
If a body absorbs a definite percentage
of incident radiation irrespective of their wave length, the body is known as
gray body. The emissive power of a gray body is always less than that of the
black body.
5. State Kirchhoff’slawradiation.
This law states that the ratio of total
emissive power to the absorptive is constant for all surfaces which are in
thermal equilibrium with the surroundings. This can be written as
It also states that the emissivity of
the body is always equal to its absorptive when the body remains in thermal
equilibrium with its surroundings.
a1
= E1; a2
= E2 and so on.
6. Define irradiation (G)
It is defined as the total radiation
incident upon a surface per unit time per unit area. It is expressed in W/m2.
7. What is the purpose of radiation
shield?
Radiation shields constructed from low
emissivity (high reflective) materials. It is used to reduce the net radiation
transfer between two surfaces.
8. Define intensity of radiation (Ib)?
It is defined as the rate of energy
leaving a space in a given direction per unit solid angle per unit area of the
emitting surface normal to the mean direction in space.
9. What is radiosity (J)?
It is used to indicate the total
radiation leaving a surface per unit time per unit area. It is expressed in W/m2.
10. What is meant by shape factor?
The shape factor is defined as the
fraction of the radioactive energy that is diffused from on surface element and
strikes the other surface directly with no intervening reflections. It is
represented by Fij. Other names for radiation shape factor are view factor,
angle factor and configuration factor.
11. State Stefan –Boltzmann law.
The emissive power of a black body is
proportional to the fourth power of absolute temperature.
Where
E b = Emissive
power, w/m2
s = Stefan.
Boltzmann constant
= 5.67 x 10-8 W/m2 K4
T = Temperature, K
12.What are the assumptions made to
calculate radiation exchange between the surfaces?
1. All
surfaces are considered to be either black or gray
2. Radiation
and reflection process are assumed to be diffuse.
3. The
absorptive of a surface is taken equal to its emissivity and independent of
temperature of the source of the incident radiation.
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